JP7785366B2 - Aged magnesium alloy material and its manufacturing method - Google Patents

Description

The present invention relates to an aged magnesium alloy material and a method for producing it.

Magnesium alloys are known as the lightest metal among practical metals, and are currently being considered for use in railways, aircraft, automobiles, and other applications as a lightweight material to replace aluminum alloys.
However, wrought magnesium alloys have inferior workability at room temperature compared to aluminum alloys. Therefore, when wrought magnesium alloys are processed into their final shape, they must be processed at temperatures above 200°C, which increases processing costs. For these reasons, magnesium alloys are currently rarely used as transportation equipment materials. To overcome this issue and expand the uses of magnesium alloys, new wrought magnesium alloys with excellent room-temperature workability must be developed.

In particular, much research has been conducted on improving workability at room temperature. As a result, there have been reports of alloys being developed that have excellent workability at room temperature comparable to that of aluminum alloys through the addition of alloying elements and improvements to the rolling process (see Patent Documents 1 and 2). However, a summary of these reports shows that strength tends to decrease as room temperature workability improves (see Patent Documents 1 and 2, and Non-Patent Document 1).

To develop alloys for applications such as automotive body panels, alloys that exhibit both excellent room-temperature workability and high strength are required. However, since it is not possible to create alloys with both strength and workability using the above alloys, it is difficult to develop alloys with the required mechanical properties of 160 MPa 0.2% yield strength and Erichsen value of approximately 8 mm for application in automotive materials.

One way to solve the above problems is to develop heat-treatable alloys. For example, if a heat treatment process called T6 treatment is used, which consists of a solution treatment (T4 treatment) that supersaturates the alloy elements in the alloy, and an aging treatment that disperses precipitates and strengthens the alloy to its maximum hardness, the alloy will soften after T4 treatment, imparting excellent formability, and the subsequent aging treatment can impart excellent strength to the formed product. Therefore, T6 treatment has the potential to achieve both excellent room-temperature formability and strength.

Several alloys that can be strengthened by aging after solution treatment have been reported (see Patent Documents 3 and 4). In these reports, Mg-Ca-Al-based alloys, which do not contain expensive rare earth metal elements, exhibit excellent room-temperature formability after solution treatment. Furthermore, aging after solution treatment forms nano-sized precipitates known as the single-layer ordered Guinier Preston Zone (single-layer ordered G.P. zone), which strengthens the alloy. Therefore, these alloys are considered promising alloys that can solve the above-mentioned problems (see Patent Document 4). In fact, Mg-Al-Ca-Zn alloys that precipitate the G.P. zone have been disclosed that exhibit excellent room-temperature formability after solution treatment and achieve high strengths exceeding 200 MPa when solution-treated materials are peak-aged (see Non-Patent Document 2).

JP 2007-83261 A JP 2010-13725 A Japanese Patent Application Laid-Open No. 2002-266044 JP 2016-169427 A

B.C. Suh, M.S. Shim, K.S. Shin, N.J. Kim, Scripta M aterialia, 84-85 (2014) 1-6 M.Z. Bian, T.T. Sasaki, B.C. Suh, T. Nakata, S. Kam ado, K. Hono, Scripta Materialia 138 (2017) 151-155 J.C. Oh, T. Ohkubo, T. Mukai, K. Hono, Scripta Mate realia, 53 (2005) 675-679 K. Oh-ishi, R. Watanabe, C.L. Mendis, K. Hono, Mate rials Science and Engineering A 526 (2009) 177-184 J. Jayaraj, C.L. Mendis, T. Ohkubo, K. Oh-ishi, K. Hono, Scripta Materialia, 63 (2010) 831-734 J.F. Nie, Metallurgical and Materials Transactions A, 43 (2012) 3891-3939

However, in actual automobile manufacturing processes, bake hardenability is required because low-temperature, short-time aging treatment, or so-called bake painting treatment, is performed after forming. The bake hardenability here refers to the property of increasing strength by low-temperature, short-time heat treatment after introducing a certain amount of deformation into a sheet material.
As explained above, the report on magnesium alloys in Non-Patent Document 2 previously conducted aging treatment immediately after solution treatment, and did not mention whether strengthening could be achieved by forming after solution treatment and then performing low-temperature, short-time aging treatment (baking finish treatment). In particular, as in Comparative Example 7 described below, the strength of the commercially available Mg-3Al-1Zn (AZ31) alloy decreases when aging treatment is performed after forming.

In light of the above-mentioned issues, the present invention aims to provide a magnesium alloy aging material and a method for producing it. The material is a sheet made of inexpensive alloy elements that does not contain expensive rare earth metals. The material is formed after solution treatment through a simple combination of heat treatments, and its strength is improved by aging treatment.

To achieve the above objective, the aged magnesium alloy material of the present invention contains 0.3% by mass or more and 1% by mass or less, preferably 0.3% by mass or more and 0.7% by mass or less, and more preferably 0.3% by mass or more and 0.55% by mass or less, of Ca, and one or more alloying elements selected from at least 0.5% by mass or more and less than 3.5% by mass of Zn and 0.1% by mass or more and less than 3% by mass of Al, with the balance consisting of Mg and unavoidable impurities. The magnesium alloy has bake hardenability and a 0.2% yield strength of 150 MPa or more.

In the above structure, preferably, Mn or Zr is further contained.
Preferably, the bake hardening amount is 15 MPa or more. Preferably, the bake hardening amount is 25 MPa or more and the 0.2% proof stress is 190 MPa or more.
Preferably, the precipitates comprising Mg, Ca, and Al are GP zones or atomic clusters that serve as precursors of the GP zones, the number density of the GP zones being 3×10 ²² /m ³ or more and the size being 3 to 10 nm, and the number density of the atomic clusters being 3×10 ²⁴ /m ³ or more and the size being 1 to 5 nm.
Furthermore, it is preferable that any one of the solute elements Ca, Zn, and Al has a structure that is fixed to the dislocation lines.

In order to achieve the above object, the method for producing an aged magnesium alloy of the present invention comprises the steps of:
Step 1: Melting Mg, Ca, and at least one alloy element selected from Zn and Al to obtain a cast solid;
Step 2: homogenizing the cast solid to obtain a homogenized solid;
Step 3: hot or warm processing the homogenized solid to obtain a shaped solid;
Step 4: solution treating the tangible solid to obtain a cooled solid;
Step 5 of inducing strain in the cooled solid;
and step 6 of aging the cooled solid into which strain has been introduced to obtain an aged magnesium alloy material.

In the above-described configuration, preferably, in step 2, homogenization treatment is performed at 400° C. or higher and 500° C. or lower for a predetermined time.
In step 5, the strain is preferably set to 1 to 10%.

The present invention provides a general-purpose aged magnesium alloy material that combines excellent strength and workability and can be obtained at low cost, as well as a method for manufacturing the same.

1 is a diagram schematically showing the tensile stress-strain curves of a solution-treated magnesium alloy aged material of the present invention and an aged material obtained by subjecting the solution-treated material to a strain of, for example, 2%. 1 is a flow chart showing a method for producing an aged magnesium alloy according to the present invention. FIG. 1 is a graph showing the age-hardening curve at 170° C. of the Mg-1.2Al-0.5Ca-0.4Mn alloy of Example 1 when aging treatment was performed without applying pre-strain. FIG. 1 is a diagram showing tensile stress-strain curves of a solution-treated material of Example 1 and an aged material that was subjected to aging treatment after introducing 2% strain. FIG. 1 is a graph showing age-hardening curves at 170° C. when solution-treated magnesium alloys of Examples 1 to 5 were subjected to aging treatment without applying pre-strain. FIG. 1 is a graph showing tensile stress-strain curves of solution-treated magnesium alloys of Examples 1 to 5 and aged magnesium alloys that were subjected to aging treatment after introducing 2% strain. FIG. 10 is a graph showing the tensile stress-strain curves of a solution-treated magnesium alloy of Example 6 and an aged magnesium alloy that was subjected to aging treatment after introducing 2% strain. FIG. 1 is a graph showing tensile stress-strain curves of solution-treated magnesium alloys of Examples 4 and 6 to 8 and aged magnesium alloys subjected to aging treatment after introducing 2% strain. FIG. 10 is a graph showing the tensile stress-strain curves of a solution-treated magnesium alloy of Example 9 and an aged magnesium alloy that was subjected to aging treatment after introducing 2% strain. FIG. 1 is a graph showing tensile stress-strain curves of solution-treated magnesium alloys of Examples 4 and 9 to 12 and aged magnesium alloys that were subjected to aging treatment after introducing 2% strain. FIG. 10 is a graph showing the age-hardening curve at 170° C. of the Mg-0.5Ca-0.4Zr-0.8Zn alloy of Example 13 when aging treatment was performed without applying pre-strain. FIG. 10 is a graph showing the tensile stress-strain curves of a solution-treated magnesium alloy of Example 13 and an aged magnesium alloy that was subjected to aging treatment after introducing 2% strain. FIG. 10 is a graph showing age hardening curves at 170° C. when the magnesium alloys of Examples 13 to 15 were subjected to aging treatment without applying pre-strain. FIG. 1 is a graph showing tensile stress-strain curves of solution-treated magnesium alloys of Examples 13 to 15 and aged magnesium alloys that were subjected to aging treatment after introducing 2% strain. FIG. 10 is a graph showing the tensile stress-strain curves of a solution-treated material of the Mg-0.5Ca-0.4Zr-1.6Zn alloy of Example 16 and an aged material that was subjected to aging treatment after introducing 2% strain. FIG. 10 is a graph showing the tensile stress-strain curves of the solution-treated Mg-0.5Ca-0.4Zr-1.6Zn alloys of Examples 14, 16, and 17, and the aged Mg-0.5Ca-0.4Zr-1.6Zn alloys after introducing 2% strain. FIG. 10 is a graph showing age-hardening curves at 170° C. of the Mg-1.2Al-0.5Ca-0.4Mn alloys of Examples 13 and 18 when aging treatment was performed without applying pre-strain. FIG. 1 is a graph showing tensile stress-strain curves of the solution-treated materials of Examples 13 and 18 and the aged materials that were subjected to aging treatment after introducing 2% strain. FIG. 10 is a graph showing the tensile stress-strain curves of the solution-treated material of Example 19 and the aged material that was subjected to aging treatment after introducing 2% strain. FIG. 1 is a graph showing tensile stress-strain curves of the solution-treated materials of Examples 19 and 20 and the aged materials that were subjected to aging treatment after introducing 2% strain. FIG. 10 is a graph showing the age-hardening curve at 170° C. of the Mg-1.3Al-0.5Ca-0.7Mn-0.8Zn alloy of Example 21 when aging treatment was performed without applying pre-strain. FIG. 10 is a graph showing the tensile stress-strain curves of the solution-treated material of Example 21 and the aged material that was subjected to aging treatment after introducing 2% strain. FIG. 1 is a graph showing tensile stress-strain curves of the solution-treated materials of Examples 21 to 23 and the aged materials that were subjected to aging treatment after introducing 2% strain. FIG. 10 is a graph showing the age-hardening curve at 170° C. of the Mg-1.0Zn-0.3Zr-0.3Ca alloy of Example 24 when aging treatment was performed without applying pre-strain. FIG. 10 is a graph showing the tensile stress-strain curves of the solution-treated material of Example 24 and the aged material that was subjected to aging treatment after introducing 2% strain. FIG. 10 is a graph showing age hardening curves at 170° C. when aging treatment was performed without applying pre-strain in Examples 24 and 25. FIG. 10 is a graph showing the tensile stress-strain curves of the solution-treated materials of Examples 24 and 25 and the aged materials that were subjected to aging treatment after introducing 2% strain. FIG. 1 is a diagram showing an age hardening curve at 170° C. when aging treatment was performed without applying pre-strain in Comparative Example 1. FIG. 1 is a diagram showing tensile stress-strain curves of a solution-treated material of Comparative Example 1 and an aged material that was subjected to aging treatment after introducing 2% strain. FIG. 10 is a graph showing the age hardening curve at 170° C. when aging treatment was performed without applying pre-strain in Comparative Example 2. FIG. 1 is a graph showing tensile stress-strain curves of a solution-treated material of Comparative Example 2 and an aged material that was subjected to aging treatment after introducing 2% strain. FIG. 10 is a diagram showing an age hardening curve at 170° C. when aging treatment was performed without applying pre-strain in Comparative Example 3. FIG. 1 is a graph showing tensile stress-strain curves of a solution-treated material of Comparative Example 3 and an aged material that was subjected to aging treatment after introducing 2% strain. FIG. 1 is a graph showing age hardening curves at 170° C. when aging treatment was performed without applying pre-strain in Comparative Examples 3 to 6. FIG. 1 is a graph showing tensile stress-strain curves of the solution-treated materials of Comparative Examples 3 to 6 and the aged materials that were subjected to aging treatment after introducing 2% strain. FIG. 10 is a graph showing the age hardening curve at 170° C. when aging treatment was performed without applying pre-strain in Comparative Example 7. FIG. 1 is a diagram showing tensile stress-strain curves of a solution-treated material of Comparative Example 7 and an aged material that was subjected to aging treatment after introducing 2% strain. FIG. 1 is a graph showing tensile stress-strain curves of a solution-treated material of Comparative Example 8 and an aged material that was subjected to aging treatment after introducing 2% strain. The Mg-1.3Al-0.5Ca-0.7Mn-0.8Zn alloy of Example 21 was solution treated and then aged without pre-straining to the peak aging temperature. (a) is a dark-field transmission electron microscope image (referred to as a DF-STEM image), (b) is a three-dimensional elemental map obtained by a three-dimensional atom probe, and (c) is the result of elemental analysis in the longitudinal direction of (b). FIG. 10 is a bright-field TEM image of the Mg-5.0Zn-0.3Zr-0.3Ca alloy of Comparative Example 5, which was solution-treated and then aged to peak aging. 1 shows the microstructure of a sample of the Mg-1.3Al-0.5Ca-0.7Mn-0.8Zn alloy of Example 21, which was subjected to 2% strain and then aged at 170°C for 20 minutes. (a) is a bright-field transmission electron microscope image of the sample for 3D atom map analysis, (b) is a 3D atom map of (a), (c) is a diagram obtained by superimposing (a) and (b), (d) is a 3D atom map of Ca, Al, and Zn, and (e) is a diagram showing the positions of atomic clusters identified in (d) by cluster analysis.

The present invention will now be described in detail with reference to several examples.
The aged magnesium alloy material of the present invention contains 0.3% by mass or more and 1% by mass or less of Ca (calcium), one or more alloying elements selected from at least 0.5% by mass or more and less than 3.2% by mass of Zn (zinc) and 0.1% by mass or more and less than 3% by mass of Al (aluminum), with the balance being Mg (magnesium) and unavoidable impurities, and has bake hardenability and a 0.2% yield strength of 150 MPa or more.
Furthermore, the Ca content is preferably 0.3 mass % or more and 0.7 mass % or less, and more preferably 0.3 mass % or more and 0.55 mass % or less.

The aged magnesium alloy material of the present invention is obtained by subjecting a solution-treated material to a strain of, for example, 2% and then aging treatment, and therefore has increased 0.2% proof stress and tensile strength.
FIG. 1 is a diagram showing a schematic diagram of tensile stress-strain curves of a solution-treated magnesium alloy aged material of the present invention, and an aged material obtained by simulating bake hardening by introducing a pre-strain of, for example, 2% into the solution-treated material and then aging the material at a predetermined temperature and time, as described below.
As shown in Figure 1, a tensile test is performed on the test piece after aging treatment, and the difference between the maximum stress value when strain is introduced and the 0.2% proof stress value of the aged material can be evaluated as the amount of strengthening. The amount of strengthening is also called the amount of bake hardening.

Furthermore, the composition of the aged magnesium alloy may further contain Mn (manganese) or Zr (zirconium).
The addition of Mn is effective in refining crystal grains. The amount of Mn added is 0.1 mass % or more, and is approximately 1 mass %. If the amount of Mn added is small, a sufficient amount of Al-Mn compounds, which play a role in suppressing the coarsening of crystal grain precipitates, is not formed, which is undesirable. Conversely, if the amount of Mn added is more than 1 mass %, a large amount of Al is used to form the Al-Mn compounds, which is undesirable because age hardening is not exhibited.
The addition of Zr is effective in refining crystal grains. The amount of Zr added is preferably 0.2 mass% or more and 0.8 mass% or less. If the amount of Zr added is less than 0.2 mass%, a sufficient amount of Zn-Zr compound, which plays a role in suppressing the coarsening of crystal grain precipitates, is not formed, which is undesirable. Conversely, if the amount of Zr added is more than 0.8 mass%, a large amount of Zn is consumed in the formation of Zn-Zr compound, and age hardening is not exhibited, which is undesirable.

The bake hardening amount is preferably 15 MPa or more. Furthermore, the bake hardening amount is preferably 25 MPa or more.

The 0.2% yield strength of the aged magnesium alloy material of the present invention is preferably 190 MPa or more.

The precipitates formed after aging in the magnesium alloy of the present invention are precipitates consisting of Mg, Ca, and Zn. The precipitates consisting of Mg, Ca, and Zn are nano-sized precipitates called G.P. zones (Guinier Preston Zones) dispersed on the (0001) plane of the magnesium matrix. The formation of the precipitates consisting of Mg, Ca, and Zn during aging treatment can improve the strength of the alloy.
The term "dispersed precipitates" refers to a state in which many fine nano-order precipitates are precipitated. The G.P. zone, which is a precipitate consisting of Mg, Ca, and Zn observed in an aged magnesium alloy, may be, but is not limited to, a plate-like precipitate.
In addition to G.P. zones, atomic clusters that serve as precursors to the G.P. zones are observed in the precipitates after aging, which can improve the strength of the alloy. It is preferable that the number density of the G.P. zones is 3 x ¹⁰ / ^m or more and the size is 3 to 10 nm, and that the number density of the atomic clusters is ³ x ¹⁰ /m or more and the size is 1 to 5 nm.
Furthermore, in the aged structure of the magnesium alloy of the present invention, any of the solute elements Ca, Zn, and Al is fixed or segregated at dislocation lines. In addition, in the aged structure of the magnesium alloy of the present invention, all of the solute elements Ca, Zn, and Al may be segregated at dislocation lines. This structure also contributes to improving the strength of the alloy.

When Ca, an element with an atomic radius larger than that of Mg, is not included, as in Comparative Example 7 described below, age hardening is not exhibited, and therefore bake hardening is also not exhibited.

Comparative Example 2 and Non-Patent Documents 3 to 5 show that when Al and Zn, elements with atomic radii smaller than that of Mg, are not included, the G.P. zone seen in the bake-hardenable alloys of the Examples is not formed. Furthermore, it is presumed that the behavior of aging precipitation is significantly slower, resulting in no bake hardening.

The features of the magnesium alloy of the present invention will be described.
(1) It is an age-hardening alloy that is strengthened by GP zones or atomic clusters, and hardening begins immediately after the start of aging, for example, within 0.1 hours.
(2) Solution treatment is carried out at a temperature higher than 350°C and lower than 550°C, so that the alloying elements are supersaturated in the matrix before strain introduction and aging treatment.
(3) In addition to Ca, the alloy contains at least one alloy element selected from Zn and Al, and the amount of Ca added is 0.3% by mass to 1% by mass, preferably 0.3% by mass to 0.7% by mass, and more preferably 0.3% by mass to 0.55% by mass. If the amount of Ca added is less than 0.3% by mass, it is difficult to obtain useful precipitates (G.P. zones) as described below, which is undesirable. Conversely, if the amount of Ca added is more than 1% by mass, precipitates consisting of Mg and Ca are formed, which is undesirable as it reduces formability and ductility.

(4) The amount of Zn and Al added is 0.5 mass % or more and less than 3 mass %, and 0.1 mass % or more and less than 3.2 mass %, respectively.
If the amount of Zn added is less than 0.5 mass%, the age hardenability is reduced and significant bake hardenability cannot be obtained, which is undesirable. Conversely, if the amount of Zn added is more than 3 mass%, the precipitate phase changes from the G.P. zone to the _MgZn2 phase, which significantly slows down the kinetics of age hardening, which is undesirable.
If the amount of Al added is less than 0.1 mass %, the age hardenability decreases and significant bake hardenability cannot be obtained, which is not preferred. Conversely, if the amount of Al added is more than 3 mass %, Al and Mn form Al-Mn particles, which is not preferred because the amount of Al that contributes to age hardening decreases.
(5) The material to be bake hardened must contain, as alloying elements that form precipitates such as the G.P. zone, both Al and Zn, which have an atomic radius smaller than that of Mg, and Ca, which has an atomic radius larger than that of Mg.

In the past, attempts to impart excellent room-temperature formability only resulted in magnesium alloys with low strength. However, the aged magnesium alloy material of the present invention has a fine structure achieved by combining relatively inexpensive alloying elements, and by subjecting it to a short aging treatment after forming, it is possible to provide an aged magnesium alloy material with excellent room-temperature strength and elongation that meet the properties required for automotive applications.

The aged magnesium alloy material of the present invention can provide an aged magnesium alloy material with bake hardenability. Bake hardenability is the property in which strength increases after a certain amount of deformation (strain) is introduced into a sheet material and then heat-treated at a low temperature for a short period of time. This is a property that could not be obtained with conventional aged magnesium alloy materials.

The aged magnesium alloy material of the present invention has strength and ductility comparable to the 6000 series aluminum alloys currently used as automotive materials, and therefore has the potential to replace bake-hardenable steel materials and aluminum alloys that have been used in automotive applications to date.

The aged magnesium alloy material of the present invention can exhibit bake paintability that was not possible with conventional commercial aged magnesium alloy sheet materials.

With the aged magnesium alloy material of the present invention, while conventional commercial aged magnesium alloy materials lose strength when they are heat-treated after forming, the strength of the material can be significantly improved by performing heat treatment after strain introduction in the present invention.

(Manufacturing method)
The aged magnesium alloy material of the present invention can be produced by the following steps.
2 is a flow diagram showing the method for producing an aged magnesium alloy material of the present invention. As shown in FIG. 2, the aged magnesium alloy material of the present invention is
Step 1: Melting Mg, Ca, and at least one alloy element selected from Zn and Al to obtain a cast solid;
Step 2: homogenizing the cast solid to obtain a homogenized solid;
Step 3: hot or warm processing the homogenized solid to obtain a tangible solid;
Step 4: solution treating the tangible solid to obtain a cooled solid;
Step 5: inducing strain in the cooled solid;
Step 6: aging the cooled solid into which strain has been introduced to obtain an aged magnesium alloy material;
It can be produced by a process including the steps of:

Each step will be described in more detail below.
(Step 1: Melting and casting)
In the step of obtaining a cast solid, Mg and at least alloy elements Al and/or Zn, and Ca are melted in an iron crucible to obtain a molten metal, which is then poured into a mold or the like and cooled for casting to obtain a cast solid.
Specifically, for example, an alloy having the above composition can be melted in a high-frequency induction melting furnace and cast in an iron mold. In Examples 24 to 26 described below, samples were prepared by rapid solidification casting.
The melting furnace used for melting is not limited to a high-frequency induction melting furnace, and any other device may be used as long as it can produce an alloy of the desired composition. The cast solid may be obtained by any of rapid solidification casting, gravity casting, and vacuum casting.

(Step 2: Homogenization treatment)
This is a process in which the cast solid is homogenized to obtain a homogenized solid. The homogenization process homogenizes the distribution of each metal component present in the cast solid and dissolves precipitates formed during cooling of the molten metal into the matrix. The homogenization process is a heat treatment that dissolves the precipitates formed during cooling of the molten metal in the magnesium matrix in step 1 and eliminates solidification segregation.
In particular, in the region where Zn is highly segregated, the alloy melts when heat treatment is started at 450° C. Therefore, for example, the alloy is first heat-treated at 300° C. for 24 hours to suppress the initial melting of the Mg—Zn phase formed during casting, and then heat-treated at 450° C. to homogenize the Zn distribution.
Here, the conditions for the homogenization treatment are not limited to the above conditions (24 hours at 350°C + 4 hours at 450°C), and it is sufficient to perform the heat treatment under the conditions of predetermined temperature and time so that the alloying elements are dissolved in the magnesium matrix.

(Step 3: Rolling)
This is the process of hot working the homogenized solid by rolling, extrusion, or the like to obtain a tangible solid.
The conditions for rolling include sample temperature, roll temperature, reduction ratio, roll peripheral speed, and whether or not intermediate heat treatment is performed.
Since bake hardenability and rolling conditions are not significantly related, it is sufficient if the material can be processed into a plate. Hot processing can be rolling, extrusion, or forging. Therefore, the processing method and its conditions are not important in order to achieve this effect. In extreme cases, it is also possible to simply cut the ingot into a plate.

(Step 4: Solution treatment)
This is a process in which a tangible solid is solution-treated to obtain a cooled solid, and is a heat treatment process carried out to dissolve precipitates formed during hot working into the matrix and form a recrystallized structure.
Here, the solution treatment is performed by heat treatment at a predetermined temperature for a predetermined time so that precipitates formed during the rolling process in step 3 are dissolved in the matrix and a recrystallized structure is formed. In the solution treatment, a sufficient amount of solute elements must be dissolved in a supersaturated state. Therefore, the solution treatment must be performed at 400°C or higher. The solution treatment can be performed at 450°C for about one hour, but a longer heat treatment time leads to an increase in manufacturing costs, so the solution treatment time should be kept to the minimum necessary.

(Step 5: strain introduction)
In a process for introducing a certain amount of deformation into a sheet material, tensile test specimens were prepared from the sheet material, and various amounts of tensile strain were introduced through tensile testing. A pre-strain range of 1 to 15% is preferable. A pre-strain range of less than 1% is undesirable because the density of dislocations introduced into the material is low, preventing strengthening through the pinning of dislocation lines by the segregation of solute elements. Conversely, a pre-strain range greater than 15% is undesirable because the number density of introduced dislocations is very high, causing recovery during aging and softening the material itself. Note that the strain introduction process for achieving the effects of strain introduction is not limited to tensile testing. Strain introduction may also be achieved by applying deformation using known methods such as compression or bending.

(Step 6: Aging treatment)
This is a heat treatment process that disperses precipitates in solution-treated material to give it strength.

Next, each step will be described in detail.
The method of producing the ingot used for rolling does not affect the bake hardenability. For example, whether the ingot is produced by melting and casting using a high-frequency melting furnace or by rapid solidification casting, bake hardenability will be exhibited as long as the requirements for alloy elements and structure are met. Comparison of Examples 21 to 23 with other Examples will be described later, showing that bake hardening occurs even when the ingot production method is changed.

In rolling, the rolling conditions, such as the rolling temperature and whether or not the sample is reheated during rolling, do not affect the bake hardenability. Comparison of Examples 4 and 6 to 8 will show that bake hardening occurs even when the rolling conditions are changed for alloys of the same composition, as will be described later.
The method of preparing a plate-shaped sample is not limited to rolling, and the effects of the present invention can be achieved even if the plate-shaped sample is prepared by extrusion, forging, or other methods, as long as the desired structure can be obtained. In extreme cases, even if the sample is a cast sample, an alloy that exhibits bake hardening can be prepared by the heat treatments shown in (A), (D) to (F) in Figure 2.
The solution treatment must be carried out at a temperature of 400°C or higher in order to dissolve a sufficient amount of solute elements in a supersaturated state.
Even if the alloy has a composition that exhibits bake hardening, such as Comparative Example 1 described below, a low solution treatment temperature is not preferable because the amount of bake hardening decreases.

The amount of strain should be such that the test specimen does not break. The amount of strain is also called the pre-strain amount.

The temperature and time conditions for aging treatment should be such that, if aging is performed without introducing strain after solution treatment, rapid age hardening occurs within 0.1 hours, or 6 minutes, and a hardness at least higher than that of solution-treated material is obtained.

(Microstructure after aging treatment)
The structure of the bake-hardened material can be described as follows:
G.P. zones or their precursor atomic clusters are precipitated.
Alloying elements segregate at the dislocations introduced during pre-strain.

According to the method for producing an aged magnesium alloy material of the present invention, it is possible to produce, at low cost, an aged magnesium alloy material having excellent room temperature strength and formability required for automotive applications by using relatively inexpensive alloying elements and a production method that combines simple rolling, heat treatment, and strain introduction.
Next, an embodiment of the present invention will be described in detail.

Examples in which bake hardenability was exhibited are shown as Examples, and examples in which bake hardenability was not exhibited are shown as Comparative Examples.
Examples 1 to 5
Examples 1 to 5 are examples in which the allowable amount of Zn added to the Mg-1.2Al-0.5Ca-0.4Mn-xZn alloy was investigated.
Example 1
A magnesium alloy having the following composition was prepared as the aged magnesium alloy material of Example 1. The numbers written before Al, Ca, and Mn, which are additives to the magnesium alloy, indicate mass %.
The homogenization conditions were the same as those in Example 1 in Examples 23, 5 to 8, and Comparative Examples 1 to 9, which will be described later.

Alloy composition: Mg-1.2Al-0.5Ca-0.4Mn (mass%)
Wrought processing: The temperature of the plate material is 100°C, the roll temperature is 100°C, and the sample is reheated at 450°C for 5 minutes between passes. After reheating, the sample temperature is lowered to 100°C before rolling.
Solution treatment: After heat treatment at 300°C for 4 hours, the temperature was increased to 450°C at a rate of 7.5°C/h (hours) and maintained at that temperature for 6 hours, followed by water cooling.
Pre-strain amount and aging conditions: After introducing 2% strain, aging treatment was carried out at 170°C for 20 minutes.

Table 1 shows the alloy composition (mass%) of the examples and comparative examples, the temperature and time of the homogenization treatment, the rolling state in the rolling process and whether or not intermediate heat treatment such as sample reheating was performed, the temperature and time of the solution treatment, the amount of strain introduced, and the temperature and time of the aging treatment.

The stretching process was carried out using a rolling mill (custom-made, serial number: H9132) manufactured by Uenotex Co., Ltd. As shown in Table 1, the roll temperature during rolling was 100°C, the plate temperature was 100°C, and the intermediate heat treatment was performed at 450°C for 5 minutes.

Fig. 3 shows the age-hardening curve at 170°C of the Mg-1.2Al-0.5Ca-0.4Mn alloy of Example 1 when aging was performed without pre-straining. The vertical axis of Fig. 3 represents Vickers hardness (HV), and the horizontal axis represents aging time (h (hours)).
As shown in FIG. 3, the Vickers hardness of the solution-treated material was 49.4±0.9 HV, and increased to a peak hardness of 60.1±0.8 HV after 4 hours of aging, with the amount of age hardening being 10.7 HV.

Fig. 4 shows the tensile stress-strain curves of the solution-treated material of Example 1 and the aged material that was subjected to aging treatment after introducing 2% strain. The vertical axis of Fig. 4 represents stress (MPa), and the horizontal axis represents strain (%).
As shown in Figure 4, the 0.2% yield strength of the solution-treated material is 147 MPa, and the strength at 2% strain is 167 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increases to 197 MPa, resulting in a bake hardening of 30 MPa, a tensile strength of 241 MPa, and an elongation of 27%. The 0.2% yield strength is also known as the yield strength.
The 0.2% yield strength, tensile strength, elongation and bake hardenability obtained from the stress-strain curve of FIG.
Here, the test direction shown in Table 2 as the RD direction indicates that the tensile test was carried out in the rolling direction.
As shown in Table 2, the mechanical properties of the cooled solid obtained in Example 1 were measured, and the Erichsen value, which is an index of formability evaluated by an Erichsen test (testing apparatus: Model 111 manufactured by Erichsen), was 6.1 mm. In the following examples and comparative examples, the Erichsen value was measured in the same manner as in Example 1.

(Examples 2 to 5)
As shown in Table 1, Examples 2 to 5 differ from Example 1 in the amount of Zn added in the alloy composition, but the rolling conditions and heat treatment conditions other than the homogenization treatment are all the same as those of Example 1. This allowed us to investigate the effect of the amount of Zn added to the Mg-1.2Al-0.5Ca-0.4Mn alloy.
The alloy compositions of Examples 2 to 5 are shown below.
Example 2: Mg-1.2Al-0.5Ca-0.4Mn-0.3Zn (mass%)
Example 3: Mg-1.2Al-0.5Ca-0.4Mn-0.8Zn (mass%)
Example 4: Mg-1.2Al-0.5Ca-0.4Mn-1.6Zn (mass%)
Example 5: Mg-1.2Al-0.5Ca-0.4Mn-3.2Zn (mass%)

The homogenization treatment in Examples 2 and 3 was carried out in the same manner as in Example 1.
The homogenization treatment in Examples 4 and 5 was carried out in the following steps.
Solution treatment: After heat treatment at 300°C for 4 hours, the sample was heated to 450°C at a rate of 7.5°C/h (hours) and held at that temperature for 6 hours. Thereafter, the sample was air-cooled until the temperature reached 300°C, and then water-cooled.

Fig. 5 shows age-hardening curves at 170°C for the magnesium alloys of Examples 2 to 5 when aging was performed without pre-straining, and Fig. 6 shows tensile stress-strain curves for solution-treated magnesium alloys of Examples 2 to 5 and aged materials aged after introducing 2% strain. The vertical and horizontal axes in Fig. 5 and Fig. 6 are the same as those in Fig. 3 and Fig. 4, respectively.
As shown in Fig. 5 and Table 2, the time to reach the maximum hardness was 2 hours for each of the magnesium alloys of Examples 2 to 5. The age hardening amounts of the magnesium alloys of Examples 2 to 5 were 9.4 HV, 9.9 HV, 8.4 HV, and 7.9 HV, respectively.

The mechanical properties of the cooled solid obtained in Example 2 were measured, and the Erichsen value was 7.2 mm. As shown in Figure 6 and Table 2, the 0.2% yield strength of the solution-treated magnesium alloy of Example 2 was 142 MPa, and the strength at 2% strain was 171 MPa. Aging at 170°C for 20 minutes increased the 0.2% yield strength to 210 MPa, and the sample exhibited a bake hardening amount of 39 MPa, a tensile strength of 249 MPa, and an elongation of 29%.
The mechanical properties of the cooled solid obtained in Example 3 were measured, and the Erichsen value was 7.7 mm. The solution-treated magnesium alloy of Example 3 had a 0.2% yield strength of 142 MPa and a strength of 179 MPa at 2% strain. Aging at 170°C for 20 minutes increased the 0.2% yield strength to 218 MPa, and the sample exhibited a bake hardening amount of 39 MPa, a tensile strength of 260 MPa, and an elongation of 24%.
The mechanical properties of the cooled solid obtained in Example 4 were measured, and the Erichsen value was 8.1 mm. The solution-treated magnesium alloy of Example 4 had a 0.2% yield strength of 145 MPa and a strength of 185 MPa at 2% strain. Aging at 170°C for 20 minutes increased the 0.2% yield strength to 220 MPa, and the sample exhibited a bake hardening amount of 35 MPa, a tensile strength of 266 MPa, and an elongation of 25%.
The mechanical properties of the cooled solid obtained in Example 5 were measured, and the Erichsen value was 5.2 mm. The solution-treated magnesium alloy of Example 5 had a 0.2% yield strength of 137 MPa and a strength of 183 MPa at 2% strain. Aging at 170°C for 20 minutes increased the 0.2% yield strength to 204 MPa, and the sample exhibited a bake hardening amount of 21 MPa, a tensile strength of 255 MPa, and an elongation of 24%.

(Examples 6 to 8)
In Examples 6 to 8, the alloy composition was the same as in Example 4, ie, Mg-1.2Al-0.5Ca-0.4Mn-1.6Zn alloy, but the rolling conditions were changed.
Example 6
Alloy composition: Mg-1.2Al-0.5Ca-0.4Mn-1.6Zn alloy. Wrought processing: The sheet temperature was 300°C, the roll temperature was 300°C, and the sample was reheated at 450°C for 5 minutes between passes. After reheating, the sample temperature was lowered to 100°C before rolling.
Solution treatment: 1 hour at 450°C. Pre-strain amount and aging conditions: After introducing 2% strain, aging treatment at 170°C for 20 minutes.

Fig. 7 shows the tensile stress-strain curves of the solution-treated magnesium alloy of Example 6 and the aged material which was subjected to aging treatment after introducing 2% strain, and Fig. 8 shows the tensile stress-strain curves of the solution-treated magnesium alloy of Examples 4 and 6 to 8 and the aged material which was subjected to aging treatment after introducing 2% strain. The vertical and horizontal axes in Fig. 7 and Fig. 8 are the same as those in Fig. 4.
The mechanical properties of the cooled solid obtained in Example 6 were measured, and the Erichsen value was 6.2 mm. As shown in Figure 7 and Table 2, the 0.2% yield strength of the solution-treated material of Example 6 was 133 MPa, and the strength at 2% strain was 170 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 210 MPa, and the material exhibited a bake hardening amount of 40 MPa, a tensile strength of 260 MPa, and an elongation of 28%.

The mechanical properties of the cooled solid obtained in Example 7 were measured, and the Erichsen value was 6.1 mm. As shown in Figure 7 and Table 2, the 0.2% yield strength of the solution-treated material of Example 7 was 156 MPa, and the strength at 2% strain was 195 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 234 MPa, and the material exhibited a bake hardening amount of 39 MPa, a tensile strength of 271 MPa, and an elongation of 22%.
The mechanical properties of the cooled solid obtained in Example 8 were measured, and the Erichsen value was 5.8 mm. As shown in Figure 7 and Table 2, the 0.2% yield strength of the solution-treated material of Example 8 was 145 MPa, and the strength at 2% strain was 176 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 217 MPa, and the material exhibited a bake hardening amount of 41 MPa, a tensile strength of 262 MPa, and an elongation of 26%.

Example 9
Examples 9 to 12 are examples in which the composition is the same as that of Example 4, but the amount of Al added is changed.
As shown in Table 1, Examples 9 to 12 differ from Example 4 in the amount of Al added in the alloy composition, but the rolling conditions and heat treatment conditions other than the homogenization treatment are all the same as those of Example 4. This allowed us to investigate the effect of the amount of Al added to the Mg-xAl-0.5Ca-0.4Mn-1.6Zn alloy.
The alloy compositions of Examples 9 to 12 are shown below.
Example 9: Mg-0.8Al-0.5Ca-0.4Mn-1.6Zn (mass%)
Example 10: Mg-0.3Al-0.5Ca-0.4Mn-1.6Zn (mass%)
Example 11: Mg-0.5Ca-0.4Mn-1.6Zn (mass%)
Example 12: Mg-0.5Ca-0.4Zr-1.6Zn (mass%)

Stretching: The temperature of the plate material was 100°C, the roll temperature was 100°C, and the sample was reheated at 450°C for 5 minutes between passes. After reheating, the sample temperature was lowered to 100°C before rolling.
Solution treatment: 1 hour at 450°C. Pre-strain amount and aging conditions: After introducing 2% strain, aging treatment at 170°C for 20 minutes. In Example 12, the Mn added as a grain refiner to the sample of Example 11 was replaced with Zr, and all other experimental conditions were the same as in Example 4.

Fig. 9 is a diagram showing the tensile stress-strain curves of the solution-treated magnesium alloy of Example 9 and the aged material which was subjected to aging treatment after introducing 2% strain, and Fig. 10 is a diagram showing the tensile stress-strain curves of the solution-treated magnesium alloy of Examples 4 and 9 to 12 and the aged material which was subjected to aging treatment after introducing 2% strain. The vertical and horizontal axes in Fig. 9 and Fig. 10 are the same as those in Fig. 4.
The mechanical properties of the cooled solid obtained in Example 9 were measured, and the Erichsen value was 7.5 mm. As shown in Figure 9 and Table 2, the 0.2% yield strength of the solution-treated material of Example 9 was 171 MPa, and the strength at 2% strain was 194 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 236 MPa, and the material exhibited a bake hardening amount of 42 MPa, a tensile strength of 276 MPa, and an elongation of 28%.

The mechanical properties of the cooled solid obtained in Example 10 were measured, and the Erichsen value was 7.1 mm. As shown in Figure 10 and Table 2, the 0.2% yield strength of the solution-treated material of Example 10 was 180 MPa, and the strength at 2% strain was 193 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 239 MPa, and the material exhibited a bake hardening amount of 46 MPa, a tensile strength of 282 MPa, and an elongation of 28%.
The mechanical properties of the cooled solid obtained in Example 11 were measured, and the Erichsen value was 5.6 mm. As shown in Figure 10 and Table 2, the 0.2% yield strength of the solution-treated material of Example 11 was 124 MPa, and the strength at 2% strain was 159 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 184 MPa, and the material exhibited a bake hardening amount of 25 MPa, a tensile strength of 237 MPa, and an elongation of 14%.
As shown in Fig. 10 and Table 2, the 0.2% yield strength of the solution-treated material of Example 12 was 163 MPa, and the strength at 2% strain was 193 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 217 MPa, and the material exhibited a bake hardening amount of 44 MPa, a tensile strength of 265 MPa, and an elongation of 25%.

(Examples 13 to 15)
In this example, the composition of the magnesium alloy is Mg-0.5Ca-0.4Zr-xZn alloy, and the amount of Zn added is changed.
The alloy compositions of Examples 13 to 15 are shown below.
Example 13: Mg-0.5Ca-0.4Zr-0.8Zn (mass%)
Example 14: Mg-0.5Ca-0.4Zr-1.6Zn (mass%)
Example 15: Mg-0.5Ca-0.4Zr-2.1Zn (mass%)

The following conditions other than the homogenization treatment were the same as those in Example 1.
Stretching: The temperature of the plate material is 100°C, the roll temperature is 100°C, and the sample is reheated at 450°C for 5 minutes between passes. After reheating, the sample temperature is lowered to 100°C before rolling.
Solution treatment: 1 hour at 400°C. Pre-strain amount and aging conditions: After introducing 2% strain, aging treatment at 170°C for 20 minutes.

As shown in Table 1, the samples of Examples 14 and 15 differ from the sample of Example 13 in the amount of Zn added and the homogenization treatment conditions. This is because the samples of Examples 14 and 15 contain 1.6 mass% or more of Zn, and therefore may crack if water-cooled after heat treatment at 450°C. Therefore, the samples were water-cooled after waiting for the temperature to drop to 300°C. However, since the samples were reheated under the same conditions during rolling, there was no impact on the properties. Therefore, the comparison between Examples 13 and 15 essentially compares the effect of the amount of Zn added. All other rolling and heat treatment conditions were the same as those of Example 1, as described above.

11 is a graph showing the age-hardening curve at 170°C when the Mg-0.5Ca-0.4Zr-0.8Zn alloy of Example 13 was aged without pre-straining. The vertical and horizontal axes in FIG. 11 are the same as those in FIG. 3.
As shown in FIG. 11, the Vickers hardness of the solution-treated material was 48.3±1.0 HV, and increased to a peak hardness of 59.3±0.9 HV after 4 hours of aging, with the amount of age hardening being 11 HV.

Fig. 12 shows the tensile stress-strain curves of the solution-treated magnesium alloy of Example 13 and the aged material aged after 2% strain introduction, Fig. 13 shows the age-hardening curves at 170°C of the magnesium alloys of Examples 13 to 15 aged without pre-straining, and Fig. 14 shows the tensile stress-strain curves of the solution-treated magnesium alloys of Examples 13 to 15 and the aged material aged after 2% strain introduction. The vertical and horizontal axes of Fig. 12 and Fig. 14 are the same as Fig. 4, and the vertical and horizontal axes of Fig. 13 are the same as Fig. 3.
The mechanical properties of the cooled solid obtained in Example 13 were measured, and the Erichsen value was 7.7 mm. As shown in Figure 12 and Table 2, the 0.2% yield strength of the solution-treated material of Example 13 was 146 MPa, and the strength at 2% strain was 164 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 197 MPa, and the material exhibited a bake hardening amount of 33 MPa, a tensile strength of 237 MPa, and an elongation of 28%.

The mechanical properties of the cooled solid obtained in Example 14 were measured, and the Erichsen value was 8.2 mm. As shown in Figure 13 and Table 2, the 0.2% yield strength of the solution-treated material of Example 14 was 163 MPa, and the strength at 2% strain was 177 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 212 MPa, and the material exhibited a bake hardening amount of 35 MPa, a tensile strength of 256 MPa, and an elongation of 34%.
The mechanical properties of the cooled solid obtained in Example 15 were measured, and the Erichsen value was 7.8 mm. As shown in Figure 13 and Table 2, the 0.2% yield strength of the solution-treated material of Example 15 was 169 MPa, and the strength at 2% strain was 182 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 213 MPa, and the material exhibited a bake hardening amount of 31 MPa, a tensile strength of 262 MPa, and an elongation of 26%.

Example 16
Example 16 and Example 17 described below are examples in which the aging conditions for the Mg-0.5Ca-0.4Zr-1.6Zn alloy of Example 14 were changed.
Alloy composition: Mg-0.5Ca-0.4Zr-1.6Zn alloy. Wrought processing: The sheet temperature was 100°C, the roll temperature was 100°C, and the sample was reheated at 450°C for 5 minutes between passes. After reheating, the sample temperature was lowered to 100°C before rolling.
Solution treatment: 1 hour at 400°C Pre-strain amount and aging conditions: After introducing 2% strain, aging treatment at 170°C for 5 minutes

Fig. 15 shows the tensile stress-strain curves of a solution-treated Mg-0.5Ca-0.4Zr-1.6Zn magnesium alloy and an aged Mg-0.5Ca-0.4Zr-1.6Zn magnesium alloy of Example 16. The vertical and horizontal axes in Fig. 15 are the same as those in Fig. 4.
The mechanical properties of the cooled solid obtained in Example 16 were measured, and the Erichsen value was 8.2 mm. As shown in Figure 15 and Table 2, the 0.2% yield strength of the solution-treated material of Example 16 was 163 MPa, and the strength at 2% strain was 177 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 205 MPa, and the material exhibited a bake hardening amount of 28 MPa, a tensile strength of 253 MPa, and an elongation of 31%.

(Example 17)
Alloy composition: Mg-0.5Ca-0.4Zr-1.6Zn alloy The wrought processing and solution treatment were the same as in Example 16, but the aging conditions were different from those in Example 16 as follows.
Pre-strain amount and aging conditions: After introducing 2% strain, aging treatment at 170°C for 5 minutes

Fig. 16 shows the tensile stress-strain curves of the solution-treated magnesium alloys Mg-0.5Ca-0.4Zr-1.6Zn alloys of Examples 14, 16, and 17, and the aged magnesium alloys aged after introducing 2% strain. The vertical and horizontal axes in Fig. 16 are the same as those in Fig. 4.
The mechanical properties of the cooled solid obtained in Example 17 were measured, and the Erichsen value was 8.2 mm. As shown in Figure 16 and Table 2, the 0.2% yield strength of the solution-treated material of Example 17 was 163 MPa, and the strength at 2% strain was 177 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 215 MPa, and the material exhibited a bake hardening amount of 38 MPa, a tensile strength of 257 MPa, and an elongation of 27%.

As shown in Table 1, Examples 16 and 17 were obtained by systematically varying the aging treatment time from Example 14, while all other experimental conditions, such as composition and rolling conditions, were the same. A comparison of Examples 14, 16, and 17 shows that the aging treatment time affects the amount of bake hardening, tensile strength, and elongation after aging.

(Example 18)
Example 18 is an example in which, like Example 13, the alloy is Mg-0.5Ca-0.4Zr-1.6Zn, but the solution treatment conditions are changed.
Alloy composition: Mg-0.5Ca-0.4Zr-1.6Zn alloy. Wrought processing: The sheet temperature was 100°C, the roll temperature was 100°C, and the sample was reheated at 450°C for 5 minutes between passes. After reheating, the sample temperature was lowered to 100°C before rolling.
Solution treatment: 1 hour at 500°C. Pre-strain amount and aging conditions: After introducing 2% strain, aging treatment at 170°C for 20 minutes. In other words, in Example 18, the solution treatment conditions of Example 13, which were 1 hour at 500°C, were changed to 1 hour at 500°C.

17 is a graph showing age-hardening curves at 170°C when the Mg-1.2Al-0.5Ca-0.4Mn alloys of Examples 13 and 18 were aged without pre-straining. The vertical and horizontal axes in Fig. 17 are the same as those in Fig. 3.
As shown in FIG. 17, the Vickers hardness of the solution-treated material of Example 13 was 48.3±1.0 HV, and increased to a peak hardness of 59.3±0.9 HV after 4 hours of aging.
On the other hand, the Vickers hardness of the solution-treated material of Example 18 was 47.7±1.0 HV, and increased to a peak hardness of 65.7±1.7 HV after 4 hours of aging.
Example 18 differs from Example 13 only in the solution treatment conditions. Therefore, the above results show that in Example 18, in which solution treatment was performed at 500°C for 1 hour, the Vickers hardness of the solution-treated material is almost the same as that of Example 13, but the Vickers hardness of the aging-treated material increases by about 6 HV.

18 is a graph showing the tensile stress-strain curves of the solution-treated materials of Examples 13 and 18 and the aged materials aged after introducing 2% strain. The vertical and horizontal axes in Fig. 18 are the same as those in Fig. 4.
The mechanical properties of the cooled solid obtained in Example 18 were measured, and the Erichsen value was 7.0 mm. As shown in Figure 18 and Table 2, the 0.2% yield strength of the solution-treated material of Example 13 was 146 MPa, and the strength at 2% strain was 164 MPa. Aging at 170°C for 20 minutes increased the 0.2% yield strength to 197 MPa, and the material exhibited a bake hardening amount of 33 MPa, a tensile strength of 237 MPa, and an elongation of 28%.
On the other hand, the 0.2% yield strength of the solution-treated material of Example 18 was 129 MPa, and the strength at 2% strain was 158 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 213 MPa, and the bake hardening amount was 55 MPa, the tensile strength was 259 MPa, and the elongation was 18%.
Since Example 18 differs from Example 13 only in the solution treatment conditions, the above results show that in Example 18, in which solution treatment was performed at 500°C for 1 hour, the 0.2% proof stress and strength at 2% strain introduction of the solution-treated material are smaller than those of Example 13. Furthermore, the 0.2% proof stress, amount of bake hardening, and tensile strength of the aged material of Example 18 are greater than those of Example 13.

(Examples 19 and 20)
Examples 19 and 20 are examples in which the amount of Zr added to the Mg-0.8Ca-xZr-0.8Zn alloy was changed.
The alloy compositions of Examples 19 and 20 are shown below.
Example 19: Mg-0.8Ca-0.4Zr-0.8Zn (mass%)
Example 20: Mg-0.8Ca-0.2Zr-0.8Zn (mass%)

In Examples 19 and 20, the following conditions other than the stretching processing were the same as in Example 1.
Stretching: The sheet and rolls are rolled at temperatures of 300°C.
Solution treatment: 1 hour at 450°C. Strain amount and aging conditions: After introducing 2% strain, aging treatment at 170°C for 20 minutes.

Figure 19 shows the tensile stress-strain curves of the solution-treated material of Example 19 and the aged material aged after introducing 2% strain, and Figure 20 shows the tensile stress-strain curves of the solution-treated material of Examples 19 and 20 and the aged material aged after introducing 2% strain. The vertical and horizontal axes in Figures 19 and 20 are the same as those in Figure 4.

The mechanical properties of the cooled solid obtained in Example 19 were measured, and the Erichsen value was 6.8 mm. From Figure 19 and Table 2, the 0.2% yield strength of the solution-treated material of Example 19 was 138 MPa, and the strength at 2% strain was 170 MPa. Aging at 170°C for 20 minutes increased the 0.2% yield strength to 210 MPa, and the material exhibited a bake hardening amount of 40 MPa, a tensile strength of 251 MPa, and an elongation of 19%.

The mechanical properties of the cooled solid obtained in Example 20 were measured, and the Erichsen value was 7.0 mm. From Figure 20 and Table 2, the 0.2% yield strength of the solution-treated material of Example 20 was 125 MPa, and the strength at 2% strain was 160 MPa. Aging at 170°C for 20 minutes increased the 0.2% yield strength to 195 MPa, and the material exhibited a bake hardening amount of 35 MPa, a tensile strength of 245 MPa, and an elongation of 17%.

From the above results, the magnesium alloys of Examples 19 and 20 were manufactured under the same conditions except for the amount of Zr added. It was found that Example 19, which added 0.4 mass% Zr, had better properties than Example 20, which added 0.2 mass% Zr.

(Examples 21 to 23)
In Examples 21 to 23, the strain introduced into the Mg-1.3Al-0.5Ca-0.7Mn-0.8Zn alloy was set to 2%, 5%, and 10%, respectively.
Other than the amount of strain, the other conditions such as the alloy composition, rolling conditions, and heat treatment conditions were all the same.
Alloy composition: Mg-1.3Al-0.5Ca-0.7Mn-0.8Zn alloy. Wrought processing: Rapidly solidified cast materials with a thickness of 4 mm were produced and then subjected to rolling processing. The plate temperature was 100°C, and the roll temperature was 100°C. Between each pass, the samples were reheated at 450°C for 5 minutes, and after reheating, the samples were rolled after their temperature had cooled to 100°C.
Solution treatment: 1 hour at 450°C. Strain amount and aging conditions: After introducing 2% strain, aging treatment at 170°C for 20 minutes.

21 shows the age-hardening curve at 170° C. when the Mg-1.3Al-0.5Ca-0.7Mn-0.8Zn alloy of Example 21 was aged without pre-straining. The vertical and horizontal axes in FIG. 21 are the same as those in FIG. 3.
As shown in FIG. 21, the Vickers hardness of the solution-treated material of Example 21 was 54.9±0.5 HV, and increased to a peak hardness of 62.4±1.1 HV after aging for 1 hour.

Figure 22 shows the tensile stress-strain curves of the solution-treated material of Example 21 and the aged material aged after introducing 2% strain, and Figure 23 shows the tensile stress-strain curves of the solution-treated material of Examples 21 to 23 and the aged material aged after introducing 2%, 5%, and 10% strain. The vertical and horizontal axes in Figures 22 and 23 are the same as those in Figure 4.

From Figure 22 and Table 2, the 0.2% yield strength of the solution-treated material of Example 21 was 175 MPa, and the strength at 2% strain was 198 MPa. Aging at 170°C for 20 minutes increased the 0.2% yield strength to 238 MPa, and showed a bake hardening amount of 40 MPa, a tensile strength of 272 MPa, and an elongation of 27%.

The mechanical properties of the cooled solid obtained in Example 22 were measured, and the Erichsen value was 7.8 mm. As shown in Figure 23 and Table 2, the 0.2% yield strength of the solution-treated material of Example 22 was 175 MPa, and the strength at 5% strain was 222 MPa. Aging at 170°C for 20 minutes increased the 0.2% yield strength to 256 MPa, and the material exhibited a bake hardening amount of 34 MPa, a tensile strength of 276 MPa, and an elongation of 22%.
23 and Table 2, the 0.2% yield strength of the solution-treated material of Example 23 was 175 MPa, and the strength at 10% strain was 251 MPa. Aging at 170°C for 20 minutes increased the 0.2% yield strength to 277 MPa, and the bake hardening amount was 26 MPa, the tensile strength was 277 MPa, and the elongation was 18%.

As shown in Table 1, Examples 21, 22, and 23 are samples in which the amount of pre-strain introduced was varied. All other conditions, such as alloy composition, rolling conditions, and heat treatment conditions, were the same. From the above results, it can be seen that as the strain increased from Example 21 (pre-strain 2%) to Example 22 (pre-strain 5%) to Example 23 (pre-strain 10%), the 0.2% yield strength and strength at 2% pre-strain of the solution-treated material, and the 0.2% yield strength, amount of bake hardening, and tensile strength of the aged material increased.

(Examples 24 and 25)
Examples 24 and 25 are examples in which the amount of Zn added to the Mg-xZn-0.3Zr-0.3Ca alloy was changed.
The alloy compositions of Examples 24 and 25 are shown below.
Example 24: Mg-1.0Zn-0.3Zr-0.3Ca (mass%)
Example 25: Mg-2.0Zn-0.3Zr-0.3Ca (mass%)

In Examples 24 and 25, the conditions other than the magnesium alloy composition are shown below.
Stretching: Rolling is carried out at a temperature of 300°C and a roll temperature of 300°C.
Solution treatment: 1 hour at 450°C. Strain amount and aging conditions: After introducing 2% strain, aging treatment at 170°C for 20 minutes.

Fig. 24 shows the age-hardening curve at 170°C of the Mg-1.0Zn-0.3Zr-0.3Ca alloy of Example 24 when aging treatment was performed without pre-straining, and Fig. 25 shows the tensile stress-strain curves of the solution-treated material of Example 24 and the aged material aged after introducing 2% strain. The vertical and horizontal axes of Fig. 24 and Fig. 25 are the same as those of Fig. 3 and Fig. 5, respectively.
As shown in FIG. 24, the Vickers hardness of the solution-treated material of Example 24 was 45.0±1.0 HV, and increased to a peak hardness of 58.0±0.8 HV after 20 minutes of aging.
The mechanical properties of the cooled solid obtained in Example 24 were measured, and the Erichsen value was 6.0 mm. As shown in Figure 25, the 0.2% yield strength of the solution-treated material of Example 24 was 172 MPa, and the strength at 2% strain was 191 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 214 MPa, and the material exhibited a bake hardening amount of 30 MPa, a tensile strength of 258 MPa, and an elongation of 20%.

Fig. 26 shows the age-hardening curves at 170°C when aging treatment was performed without applying pre-strain in Examples 24 and 25, and Fig. 27 shows the tensile stress-strain curves of the solution-treated materials and the aged materials aged after introducing 2% strain in Examples 24 and 25. The vertical and horizontal axes in Fig. 26 and Fig. 27 are the same as those in Fig. 3 and Fig. 5, respectively.
As shown in FIG. 26, the Vickers hardness of the solution-treated material of Example 25 was 47.2±1.4 HV, and increased to a peak hardness of 57.9±0.9 HV after 6 hours of aging.
The mechanical properties of the cooled solid obtained in Example 25 were measured, and the Erichsen value was 7.0 mm. As shown in Figure 26 and Table 2, the 0.2% yield strength of the solution-treated material of Example 25 was 172 MPa, and the strength at 2% strain was 191 MPa. After 20 minutes of aging at 170°C, the 0.2% yield strength increased to 207 MPa, and the material exhibited a bake hardening amount of 16 MPa, a tensile strength of 268 MPa, and an elongation of 21%.

Example 25 is a sample with a different amount of Zn than Example 24, but all other conditions such as alloy composition, rolling conditions, and heat treatment conditions are the same, as shown in Table 1. From the above results, in Example 25 with an added amount of Zn of 2.0 mass%, the 0.2% proof stress and strength at 2% strain introduction of the solution-treated material, and the 0.2% proof stress, amount of bake hardening, and tensile strength of the aged material were increased compared to Example 24 with an added amount of Zn of 1.0 mass%.
According to the above-mentioned Examples 3, 13-17, 22, etc., a bake hardening amount of 15 MPa or more, a 0.2% yield strength of 190 MPa or more, and an Erichsen value of 7.7 mm or more were obtained, and low-cost magnesium alloys were obtained that had excellent strength and workability comparable to low-carbon steel and 6000-series aluminum alloys.

Next, a comparative example will be described in comparison with the example.
(Comparative Example 1)
This is a comparative example related to Examples 13 and 18, in which the solution treatment conditions were changed for an Mg-0.5Ca-0.4Zr-1.6Zn alloy. The alloy composition and production conditions are shown below.
Alloy composition: Mg-0.5Ca-0.4Zr-1.6Zn alloy. Wrought processing: The sheet temperature was 100°C, the roll temperature was 100°C, and the sample was reheated at 450°C for 5 minutes between passes. After reheating, the sample temperature was lowered to 100°C before rolling.
Solution treatment: 1 hour at 350°C. Strain amount and aging conditions: After introducing 2% strain, aging treatment at 170°C for 20 minutes.

Fig. 28 shows the age-hardening curve at 170°C when aging treatment was performed without applying pre-strain in Comparative Example 1, and Fig. 29 shows the tensile stress-strain curves of the solution-treated material and the aged material aged after introducing 2% strain in Comparative Example 1. The vertical and horizontal axes in Fig. 28 and Fig. 29 are the same as those in Figs. 3 and 4.
As shown in FIG. 28, the Vickers hardness of the solution-treated material of Comparative Example 1 was 49.9±0.6 HV, and it was found that the hardness increased to a peak value of 51.6±0.5 HV after aging for 2 hours.
The amount of age hardening in Comparative Example 1 was 1.7 HV, which is lower than the 11 HV and 18 HV values in Examples 13 and 18.
This shows that when the solution treatment temperature is as low as 350°C, the amount of age hardening is reduced compared to Examples 13 and 18.

When the mechanical properties of the cooled solid obtained in Comparative Example 1 were measured, the Erichsen value was 7.1 mm. From Figure 29 and Table 2, the 0.2% yield strength of the solution-treated material in Comparative Example 1 was 167 MPa, and the strength at 2% strain was 186 MPa. Aging at 170°C for 20 minutes increased the 0.2% yield strength to 202 MPa, and the material exhibited a bake hardening amount of 16 MPa, a tensile strength of 235 MPa, and an elongation of 27%.

(Comparative Example 2)
Comparative Example 2 relates to Example 14 and is a comparative example in which Ca is not added to the alloy composition of an Mg-0.4Zr-1.6Zn alloy. The alloy composition and production conditions are shown below.
Alloy composition: Mg-0.4Zr-1.6Zn alloy. Wrought processing: The sheet temperature was 100°C, the roll temperature was 100°C, and the sample was reheated at 450°C for 5 minutes between passes. After reheating, the sample temperature was lowered to 100°C before rolling.
Solution treatment: 1 hour at 400°C. Strain amount and aging conditions: After introducing 2% strain, aging treatment at 170°C for 20 minutes.

Fig. 30 shows the age-hardening curve at 170°C when aging treatment was performed without applying pre-strain in Comparative Example 2, and Fig. 31 shows the tensile stress-strain curves of the solution-treated material and the aged material aged after introducing 2% strain in Comparative Example 2. The vertical and horizontal axes in Fig. 30 and Fig. 31 are the same as those in Fig. 3 and Fig. 4.
As shown in FIG. 30, the Vickers hardness of the solution-treated material of Comparative Example 1 was 49.9±0.6 HV, and increased to a peak hardness of 51.6±0.5 HV after aging for 2 hours.
The amount of age hardening in Comparative Example 2 was 1.5 HV, which is lower than that of Example 14, which was 9.4 HV.
From this, it was found that in Comparative Example 2, in which no Ca was added, the amount of age hardening was reduced compared to Example 14.

When the mechanical properties of the cooled solid obtained in Comparative Example 2 were measured, the Erichsen value was 6.9 mm. Figure 31 and Table 2 show that the 0.2% yield strength of the solution-treated material in Comparative Example 2 was 164 MPa, and the strength at 2% strain was 173 MPa. Aging at 170°C for 20 minutes reduced the 0.2% yield strength to 170 MPa, and the material exhibited a bake hardening amount of -2 MPa, a tensile strength of 226 MPa, and an elongation of 34%.

(Comparative Examples 3 to 6)
Comparative Examples 3 to 6 relate to Examples 24 and 25 and are comparative examples regarding the upper limit of the amount of Zn added to an Mg—Zn—Ca-based alloy.
The alloy compositions of Comparative Examples 3 to 6 are shown below.
Comparative example 3: Mg-3.0Zn-0.3Zr-0.3Ca (mass%)
Comparative example 4: Mg-4.0Zn-0.3Zr-0.3Ca (mass%)
Comparative example 5: Mg-5.0Zn-0.3Zr-0.3Ca (mass%)
Comparative example 6: Mg-6.0Zn-0.3Zr-0.3Ca (mass%)

In Comparative Examples 3 to 6, the manufacturing conditions other than the magnesium alloy composition are shown below.
Stretching: Rolling is carried out at a temperature of 300°C and a roll temperature of 300°C.
Solution treatment: 1 hour at 450°C Pre-strain amount and aging conditions: After introducing 2% strain, aging treatment at 170°C for 20 minutes

Fig. 32 shows the age-hardening curve at 170°C when aging treatment was performed without applying pre-strain in Comparative Example 3, and Fig. 33 shows the tensile stress-strain curves of the solution-treated material in Comparative Example 3 and the aged material aged after introducing 2% strain. The vertical and horizontal axes in Fig. 32 and Fig. 33 are the same as those in Fig. 3 and Fig. 4.
As shown in FIG. 32, the Vickers hardness of the solution-treated material of Comparative Example 3 was 47.0±4 HV, and increased to a peak hardness of 57.6±1.6 HV after 6 hours of aging.
The mechanical properties of the cooled solid obtained in Comparative Example 3 were measured, and the Erichsen value was 5.9 mm. As shown in Figure 33 and Table 2, the 0.2% yield strength of the solution-treated material in Comparative Example 3 was 162 MPa, and the strength at 2% strain was 200 MPa. Aging at 170°C for 20 minutes increased the 0.2% yield strength to 205 MPa, and the material exhibited a bake hardening amount of 5 MPa, a tensile strength of 267 MPa, and an elongation of 23%.

Fig. 34 shows the age-hardening curves at 170°C when aging treatment was performed without applying pre-strain in Comparative Examples 3 to 6, and Fig. 35 shows the tensile stress-strain curves of the solution-treated materials and the aged materials aged after introducing 2% strain in Comparative Examples 3 to 6. The vertical and horizontal axes in Fig. 34 and Fig. 35 are the same as those in Fig. 3 and Fig. 4.
As shown in FIG. 34, the Vickers hardness of the solution-treated materials of Comparative Examples 4 to 6 increases as the amount of Zn added increases, compared to Comparative Example 3, and it can be seen that the peak hardness also increases with aging treatment.
As shown in Table 2, when the mechanical properties of the cooled solids obtained in Comparative Examples 3 to 6 were measured, the Erichsen values were 4.4 to 5.9 mm. As shown in Fig. 35 and Table 2, the 0.2% proof stress and tensile strength of the solution-treated and aged materials in Comparative Examples 4 to 6 were almost the same as those in Comparative Example 3, but the elongation was reduced.

As shown in Table 1, Comparative Examples 4 to 6 are samples with different Zn addition amounts than Comparative Example 3, but all other conditions, such as rolling conditions and heat treatment conditions, other than the alloy composition, are the same. This demonstrates that a Zn addition amount of 3 mass% is sufficient for the magnesium alloy composition.

(Comparative Example 7)
Comparative Example 7 is related to Example 3 and Comparative Example 8 described later, and shows that in an Mg—Al—Zn based alloy, the addition of Ca is required to make the alloy an age-hardenable alloy.
Alloy composition: Mg-3.0Al-1.0Zn alloy. Wrought processing: The temperature of the sheet material was 100°C, the roll temperature was 100°C, and the sample was reheated at 450°C for 5 minutes between passes. After reheating, the sample temperature was lowered to 100°C before rolling.
Solution treatment: 1 hour at 450°C Pre-strain amount and aging conditions: After introducing 2% strain, aging treatment at 170°C for 20 minutes

Fig. 36 is a graph showing the age-hardening curve at 170°C when aging treatment was performed without applying pre-strain in Comparative Example 7, and Fig. 37 is a graph showing the tensile stress-strain curves of the solution-treated material in Comparative Example 7 and the aged material which was subjected to aging treatment after introducing 2% strain. The vertical and horizontal axes in Fig. 36 and Fig. 37 are the same as those in Fig. 3 and Fig. 4.
As shown in FIG. 36, the Vickers hardness of the solution-treated material of Comparative Example 7 was 55.1±0.9 HV, and this sample did not exhibit age hardening.
The mechanical properties of the cooled solid obtained in Comparative Example 7 were measured, and the Erichsen value was 2.7 mm. As shown in Figure 37 and Table 2, the 0.2% yield strength of the solution-treated material in Comparative Example 7 was 162 MPa, and the strength at 2% strain was 198 MPa. Aging at 170°C for 20 minutes reduced the 0.2% yield strength to 186 MPa, resulting in a reduction in strength of 1.2 MPa, a bake hardening amount of -12 MPa, a tensile strength of 254 MPa, and an elongation of 30%.
From the above results, it was found that the magnesium alloy of Comparative Example 7 does not exhibit age hardenability. This revealed that in order to obtain an age hardenable alloy, it is necessary to add Ca to Mg in addition to Al and Zn.

(Comparative Example 8)
Comparative Example 8 is related to Examples 4, 9, 10, and Comparative Example 7, and is a comparative example that determines the upper limit of the amount of Al added in an Mg-Al-Zn-Ca alloy, and shows that a refined material is not necessary to exhibit bake hardenability (BH).
Alloy composition: Mg-1Ca-3.0Al-1.0Zn alloy. Wrought processing: As this is a commercially available material, the processing conditions are unknown.
Solution treatment: 1 hour at 450°C. Strain amount and aging conditions: After introducing 2% strain, aging treatment at 170°C for 20 minutes.

Comparative Example 8 is a sample in which Ca has been added to the sample of Comparative Example 7, as shown in Table 1. Since the magnesium alloy of Comparative Example 8 is a commercially available material, the rolling conditions are unknown. However, a comparison with Examples 4 and 6 to 9 shows that the rolling process does not affect bake hardenability, so it is acceptable for comparison purposes.

FIG. 38 is a graph showing the tensile stress-strain curves of the solution-treated material of Comparative Example 8 and the aged material that was subjected to aging treatment after introducing 2% strain, and the vertical and horizontal axes in FIG. 38 are the same as those in FIG. 3.
The mechanical properties of the cooled solid obtained in Comparative Example 8 were measured, and the Erichsen value was 6 mm. As shown in Figure 38 and Table 2, the 0.2% yield strength of the solution-treated material in Comparative Example 8 was 147 MPa, and the strength at 2% strain was 167 MPa. Aging at 170°C for 20 minutes increased the 0.2% yield strength to 176 MPa, resulting in a bake hardening amount of only 9 MPa, a tensile strength of 255 MPa, and an elongation of 25%.
According to the above Comparative Examples 1 to 8, it was found that in all the comparative examples, no bake hardening amount was obtained, or at most 13 MPa or less, and bake hardening amounts of 15 MPa or more as in the examples could not be obtained.

(Comparison between Examples and Comparative Examples)
Figure 39 shows the precipitation structure of the Mg-1.3Al-0.5Ca-0.7Mn-0.8Zn alloy of Example 21, which was solution-treated and then aged to peak aging without pre-straining. (a) is a dark-field transmission electron microscope image (referred to as a DF-STEM image), (b) is a three-dimensional elemental map obtained using a three-dimensional atom probe, and (c) is a diagram showing the results of elemental analysis in the longitudinal direction of (b). The transmission electron microscope used was a scanning transmission electron microscope (Titan, G2 80-200) manufactured by FEI. The transmission electron microscope image is referred to as a TEM image.

The 3D atom probe (also called 3DAP) is a method for measuring the three-dimensional atomic distribution by applying a high voltage to a sample, detecting ions that evaporate from the surface of the sample using a mass spectrometer, continuously detecting each ion in the depth direction, and arranging the ions in the order of detection. The 3D atom probe used was a CAMEKA LEAP5000 XS.
The measurement range of the 3D atom probe in Figure 39(b) was 3 nm x 3 nm x 10 nm, and it was confirmed that the G.P. zone observed in the DF-STEM image in Figure 39(a) was composed of Mg, Ca, and Zn. The number density was 4.5 x 10 ²² m ^-3 to 5 x 10 ²³ m ^-3 .

Figure 40 shows a bright-field TEM image of the Mg-5.0Zn-0.3Zr-0.3Ca alloy of Comparative Example 5, which was solution-treated and then aged to peak aging. The inset in the upper right of Figure 40 is a bright-field TEM image. It can be seen that in alloys that do not exhibit bake hardening, such as Comparative Example 5, a β ₁ ' phase, a MgZn ₂ phase precipitation extending in the [0001] direction of the magnesium matrix, is present, rather than a G.P. zone.

Figure 41 shows the microstructure of a sample of the Mg-1.3Al-0.5Ca-0.7Mn-0.8Zn alloy of Example 21, which was subjected to 2% strain and then aged at 170°C for 20 minutes. (a) is a bright-field transmission electron microscope image of the sample for 3D atom map analysis, (b) is a 3D atom map of (a), (c) is a diagram obtained by superimposing (a) and (b), (d) is a 3D atom map of Ca, Al, and Zn, and (e) is a diagram showing the positions of atomic clusters identified in (d) by cluster analysis.
As shown in Figure 41, atomic clusters were formed in the Mg-1.3Al-0.5Ca-0.7Mn-0.8Zn alloy of Example 21, and a comparison of the bright-field TEM image and the three-dimensional atom map revealed that in Example 21, solute elements Al and Zn were segregated at dislocations introduced during pre-straining.
The microstructure observed in Figure 41(d) is an atomic cluster consisting of Mg, Ca, and Al, which is a precursor of the G.P. zone, and its number density was 2.04 x ¹⁰²⁴ / ^m3 .
The atomic clusters are observed because a short aging treatment of 20 minutes at 170°C is performed to measure the amount of bake hardening. When aging treatment is performed until the maximum hardness is obtained, the atomic clusters become G.P. zones and are observed as G.P. zones.

The development of bake hardenability in aged magnesium alloys was discovered by the present invention, and from the results of the above examples and comparative examples, the following is presumed to be necessary to achieve a large amount of bake hardening and high strength.

From the examples and comparative examples, the requirements for obtaining a large amount of bake hardening and high strength are shown below.
(A) The material must be age-hardenable when aged without pre-strain. Samples that are not age-hardened, such as Comparative Examples 1, 2, and 7, do not exhibit bake hardening.
(B) Materials that exhibit bake hardening are limited to those that, among age-hardening materials, exhibit rapid age hardening, meaning that hardening begins within 0.1 hours when aging is performed without applying pre-strain.
Even among samples that are age hardenable, such as Comparative Examples 3 to 6, samples that begin to harden after an incubation period of several hours do not exhibit bake hardening.

(C) When bake-hardenable materials are subjected to aging treatment without applying pre-strain immediately after solution treatment, precipitates called G.P. zones are formed at peak aging.
For example, as in Example 21, when age hardening is performed immediately after solution treatment and aged to peak aging, a sample in which the G.P. zone precipitates exhibits bake hardening.
However, in alloys that do not exhibit bake hardening, such as Comparative Example 5, a β ₁ ' phase, an MgZn ₂ phase that extends in the [0001] direction of the magnesium matrix, is precipitated instead of the GP zone (see Figure 39).
(D) In particular, among the alloying elements used in the present invention, the concentrations of the alloying elements for exhibiting bake hardening are as follows:
Ca: 0.3 mass% or more, 1 mass% or less (basis) The lower limit is the solid solubility limit of Ca, and the upper limit is the limit when the alloy cannot be produced due to casting cracks, etc.
Zn: 0.5 mass% or more, less than 3 mass% (basis) Experimentally determined Al: 0.1 mass% or more, less than 3 mass% (basis) Experimentally determined

(E) As an element having an atomic radius larger than that of Mg, from (d), Ca can be substituted with yttrium, rare earth metal elements, etc.
Non-Patent Document 6 reveals that similar precipitates are formed when Ca is added to the above elements.

(F) In the examples, Al and Zn were used as elements with atomic radii smaller than that of Mg, but based on (D), it can be inferred that these elements can be substituted with Sn.

The present invention is not limited to the above-described embodiment, and various modifications are possible within the scope of the invention as defined in the claims, and it goes without saying that these modifications are also included within the scope of the present invention.

Claims (8)

0.3% by mass or more and 1% by mass or less of Ca;
0.5% by mass or more and less than 3.5% by mass of Zn;
0.1% by mass or more and less than 3% by mass of Al;
0.1% by mass or more and 1% by mass or less of Mn;
and the balance being Mg and inevitable impurities,
The magnesium alloy has a 0.2% yield strength of 150 MPa or more, and precipitates consisting of Mg, Ca, and Al or precipitates consisting of Mg, Ca, and Zn are dispersed on the (0001) plane of the magnesium matrix.
The precipitates comprising Mg, Ca, and Al or the precipitates comprising Mg, Ca, and Zn are G.P. zones or atomic clusters that serve as precursors of the G.P. zones, the number density of the G.P. zones is 3×10 ²² /m ³ or more and a size of 5 nm or more and 10 nm or less, the number density of the atomic clusters is 2.04×10 ²⁴ /m ³ or more and a size of 1 nm or more and less than 5 nm,
The solute element has a structure in which any one of Ca, Zn, and Al is fixed to the dislocation line.
Aged magnesium alloy material. 2. The aged magnesium alloy material according to claim 1 , wherein the Ca content is 0.3 mass % or more and 0.7 mass % or less. 2. The aged magnesium alloy material according to claim 1 , wherein the Ca content is 0.3 mass % or more and 0.55 mass % or less. A step 1 of melting Mg, Ca, Zn, Al and Mn in the composition ratios set forth in claim 1 to obtain a cast solid;
Step 2: homogenizing the cast solid to obtain a homogenized solid;
Step 3: hot or warm processing the homogenized solid to obtain a shaped solid;
a step 4 of subjecting the tangible solid to a solution treatment and then water-cooling the solution-treated tangible solid to obtain a cooled solid in which the tangible solid is supersaturated;
Step 5 of inducing strain in the cooled solid;
a step 6 of aging the cooled solid into which the strain has been introduced to obtain an aged magnesium alloy material;
The produced magnesium alloy aged material has a 0.2% yield strength of 150 MPa or more, and precipitates consisting of Mg, Ca, and Al or precipitates consisting of Mg, Ca, and Zn are dispersed on the (0001) plane of the magnesium matrix.
The precipitates comprising Mg, Ca, and Al or the precipitates comprising Mg, Ca, and Zn are G.P. zones or atomic clusters that serve as precursors of the G.P. zones, the number density of the G.P. zones is 3×10 ²² /m ³ or more and a size of 5 nm or more and 10 nm or less, the number density of the atomic clusters is 2.04×10 ²⁴ /m ³ or more and a size of 1 nm or more and less than 5 nm,
A method for producing an aged magnesium alloy material, which has a structure in which any one of solute elements Ca, Zn, and Al is fixed to dislocation lines. 5. The method for producing an aged magnesium alloy material according to claim 4 , wherein in step 2, homogenization treatment is carried out at 400° C. or higher and 500° C. or lower for a predetermined time. 6. The method for producing an aged magnesium alloy material according to claim 4 , wherein in step 5, the strain is set to 1 to 10%. 7. The method for producing an aged magnesium alloy material according to claim 4 , wherein in step 6 , the temperature and treatment time of the aging treatment correspond to those of a baking finish treatment. A method for producing an automobile that has been subjected to a baking finish treatment using the aged magnesium alloy material according to any one of claims 1 to 3 .