JP6257030B2 - Mg alloy and manufacturing method thereof - Google Patents

Description

  The present invention relates to an Mg alloy and a manufacturing method thereof.

  Magnesium alloys (referred to as Mg alloys) are known as the lightest metals among practical metals. Currently, Mg alloys are being studied for application to railways, aircraft, automobiles, and the like as lightweight materials that replace aluminum alloys (referred to as Al alloys). In order to expand the applications of Mg alloys, development of wrought materials and the like has been promoted, but the strength and workability are inferior to those of Al alloys. Therefore, various researches have been conducted to improve them.

  Non-Patent Document 1 reports a Mg-Al-Zn-based commercial alloy that has been refined by strong strain processing.

  Patent Document 1 discloses an Mg alloy in which Mg, Zn, Al, and heavy rare earth element Gd (gadolinium) is contained, and has obtained strength exceeding 300 MPa.

JP 2013-79436 A

W.J.Kim, I.B.Park, S.H.Han, Scripta Materialia, 66, pp.590-593, 2012

  Since the Mg alloy of Non-Patent Document 1 has improved strength by refining crystal grains by strong strain processing, the strength is improved because it is already work-hardened, but the workability is inferior.

  The Mg alloy of Patent Document 1 has a strength of 300 MPa because heavy rare earth metal is added as an alloy element, but the cost is increased and the use is limited.

  An object of the present invention is to provide an Mg alloy that can be used in a wide range of applications with improved strength and workability compared to conventional Mg alloys, and a method for producing the same.

The Mg alloy of the present invention is
Following formula (1):
Mg _a Sn _b Zn _c Al _d (1)
(In the formula (1), a, b, c and d are a + b + c + d = 100,
92.0 ≦ a ≦ 96.0, 0.5 ≦ b, 2.0 ≦ c ≦ 3.0, and 1.5 ≦ d. )
Mg alloy represented by
The Mg alloy includes Mg ₂ Sn precipitates and MgZn ₂ precipitates,
The Mg ₂ Sn precipitates are dispersed in the crystal grains and in the crystal grain boundaries,
The MgZn ₂ precipitate is dispersed in crystal grains.
The method for producing the Mg alloy of the present invention includes:
In obtaining the Mg alloy,
Step 1 of dissolving Mg and metal to obtain a cast solid;
Step 2 of homogenizing the cast solid to obtain a homogenized solid;
Step 3 of hot processing the homogenized solid to obtain a tangible solid;
Step 4 of solution treatment of the tangible solid to obtain a cooled solid;
Step 5 of aging the cooling solid to obtain an Mg alloy;
With
In the step 4, solution treatment is performed at 310 ° C. to 400 ° C.,
In step 5, after aging treatment at a temperature T ₁ of 60 to 80 ° C. , two-stage aging treatment is performed at a temperature T ₂ that is 70 ° C. or more higher than the temperature , and an Mg alloy composed of Mg, Sn, Zn, and Al To do.

  The present invention can provide an Mg alloy having excellent strength, workability, and low cost, and a method for producing the Mg alloy.

It is a flowchart which shows the manufacturing method of Mg alloy of this invention. It is a figure which shows the fine structure of the tangible solid (Extruded material) extruded of Example 1. FIG. 2 is a view showing a microstructure of a cooling solid obtained by the solution treatment of Example 1. FIG. 1 is a diagram showing an age hardening curve of an Mg alloy produced in Example 1. FIG. It is a figure which shows the tensile stress-strain curve of the as-extruded material of Example 1, and Mg alloy material. It is a figure which shows the age hardening curve in Example 2. FIG. It is a figure which shows the age hardening curve of Mg alloy of Example 3. It is a figure which shows the stress-strain curve of the as-extruded material of Example 3, and Mg alloy. 4 is a diagram showing a bright-field transmission electron microscope (TEM) image of the Mg alloy after the two-stage aging treatment of Example 3. FIG. It is a figure which shows the microstructure of the as-extruded material of Reference Example 1. It is a figure which shows the age hardening curve of the sample after the solution treatment in the Mg alloy of the reference example 1. It is a figure which shows the stress-strain curve of the as-extruded material in Mg alloy of the reference example 1, and Mg alloy. It is a figure which shows the age hardening curve of Mg alloy of the reference example 2. It is a figure which shows the tensile stress-strain curve of the as-extruded material in Mg alloy of the reference example 2, and Mg alloy. It is a figure of the optical microscope image which shows the fine structure after the solution treatment of the reference example 3. It is a figure which shows the TEM image of the bright field of the fine structure in Mg alloy after the aging treatment of the reference example 3. FIG. It is a figure which shows the age hardening curve after the solution treatment in the Mg alloy of the reference example 3. It is a figure which shows the tensile stress-strain curve of the as-extruded material in Mg alloy of the reference example 3, and Mg alloy. It is an age hardening curve at the time of carrying out an aging treatment at 70 degreeC in Mg alloy of the same composition as Example 1. FIG.

The Mg alloy of the present invention can be produced by the following steps.
FIG. 1 is a flowchart showing a method for producing an Mg alloy of the present invention.
As shown in FIG. 1, the Mg alloy of the present invention is
Step 1 of dissolving Mg, Sn (tin), Zn (zinc) and Al (aluminum) to obtain a cast solid;
Step 2 for homogenizing the cast solid to obtain a homogenized solid; Step 3 for hot working the homogenized solid to obtain a tangible solid;
Step 4 of solution treatment of the tangible solid to obtain a cooled solid;
Step 5 of aging the cooling solid to obtain the Mg alloy;
Through
In the step 4, solution treatment is performed at 310 ° C. to 400 ° C.,
In the step 5, after aging treatment at a temperature T ₁ of 60 to 80 ° C., aging treatment is performed at a temperature T ₂ that is 70 ° C. higher than the temperature to obtain an Mg alloy composed of Mg, Sn, Zn, and Al. Can do.

Hereinafter, each step will be described.
[Step 1]
This is a step of obtaining a cast solid, in which Mg, Sn, Zn, and Al are melted in an iron crucible to form a molten metal, which is cast by being poured into a mold or the like and cooled to obtain a cast solid.
A suitable quantity ratio of Mg, Sn, Zn, Al is, for example, an atomic (at.%) Ratio,
Preferably, Mg is 92.0-96.0, Sn is 0.5 or more, Zn is 2.0 to 3.0, Al is 1.5 or more,
More preferably, Mg is 93.0-95.2, Sn is 0.8 or more, Zn is 2.2 to 2.8, Al is 1.8 or more,
More preferably, Mg is 94.0 to 94.6, Sn is 1.2 or more, Zn is 2.3 to 2.8, and Al is 1.9 or more.

[Step 2]
This is a step of obtaining a homogenized solid by homogenizing the cast solid.
In the homogenization treatment, the metal distribution of each component existing in the cast solid is homogenized, and precipitates formed during the cooling of the molten metal are dissolved in the matrix.
In the homogenization treatment, from the viewpoint of suppressing melting of the cast solid, first, the distribution of Zn is homogenized at a low temperature, and then the distribution of Sn is homogenized by heat treatment at a high temperature, followed by cooling to obtain a homogenized solid.
Specifically, preferably,
At a low temperature of 300-350 ° C., preferably 310-350 ° C., more preferably 320-350 ° C., for 10-30 hours to homogenize Zn,
The homogenization of Sn is performed at a high temperature of 400 to 450 ° C. for 10 to 30 hours.

[Step 3]
This is a step of obtaining a tangible solid by hot working the homogenized solid by extrusion or rolling. As hot working, extrusion or rolling can be used. For example, the extrusion process can be performed using an extrusion machine.
The temperature of the extrusion process may be 250 to 350 ° C., for example, and the ram speed (piston moving speed) may be 0.1 mm / s or more, for example. The extruded tangible body is allowed to cool and solidify into a tangible solid. What is necessary is just to select the shape of a tangible solid suitably according to the use of Mg alloy.

[Step 4]
This is a step of obtaining a cooling solid by solution treatment of a tangible solid, and is carried out in order to dissolve precipitates formed during hot working in the matrix. The solution treatment can be performed by heating in an electric furnace, for example.

(Solution treatment)
The solution treatment is a heat treatment step in which a hardened tangible solid obtained by hot working is converted into a supersaturated solid solution in which an alloy element is supersaturated.
In the solution treatment, the precipitated phase composed of Mg and Sn (Mg ₂ Sn phase) does not cause solid solution in the magnesium matrix, and the precipitated phase composed of Mg and Zn (MgZn ₂ phase) dissolves in the magnesium matrix. The temperature range is desirable.
This temperature range is 310 to 400 ° C., preferably 310 to 350 ° C., from the viewpoint of obtaining crystal grains having a suitable particle size.
From the standpoint of preventing excessive crystal grains, the temperature range in the solution treatment is set at a temperature at which the Mg ₂ Sn phase is less likely to dissolve in the mother phase, and the MgZn ₂ phase is more likely to dissolve in the mother phase. It may be considered to be the lower limit temperature.

  For example, as shown in FIGS. 2 and 3 in Example 1, when the solution treatment is performed at 350 ° C., the crystal grains grow to a suitable grain size from 1.9 μm to 8.2 μm.

On the other hand, 2.4 at.% Zn is not completely dissolved in the magnesium matrix during the solution treatment when the solution treatment temperature is 310 ° C. or lower. In this case, even if a sufficient amount of Zn is added, the fine MgZn ₂ phase is hardly dispersed in the crystal grains by the aging treatment.

  The solution treatment is preferably performed for 0.1 hour to 1 hour, more preferably 0.2 to 0.75 hour, and still more preferably 0.25 to 0.5 hour.

(Microstructure after solution treatment)
In the Mg alloy production method of the present invention, from the viewpoint of the strength of the Mg alloy, the average crystal grain size of the magnesium matrix after the solution treatment is preferably 15 μm or less, more preferably 5 to 10 μm, still more preferably 6 to 6 μm. It has a crystal grain structure of 9 μm.
This particle size was measured by an intercept method from an inverse local point map (see FIG. 3) obtained by an electron backscatter diffraction (referred to as EBSD). In the intercept method, five vertical and horizontal straight lines are drawn evenly in FIG. Next, the length of the line segment cut by each crystal grain on each straight line is individually measured, and the average value obtained by dividing the length of the line segment by the total number of particles on the straight line by the total number of particles is The average particle size of the grains is used.

[Step 5]
In this step, the cooling solid is aged to obtain an Mg alloy. The aging treatment is a heat treatment step in which precipitates are dispersed in the solution treatment material to impart strength. In the present invention, the aging treatment is characterized by performing a low-temperature and high-temperature two-stage aging treatment.

(Aging treatment)
In the aging treatment in step 5, the Mg alloy of the present invention having high strength, high ductility and good workability is obtained by heat-treating the cooled solid in which the alloy element obtained by the solution treatment is supersaturated. It is a process to obtain. The aging treatment is performed by a two-stage aging treatment comprising a combination of a low temperature aging treatment and a high temperature aging treatment. The aging treatment can be performed using an oil bath (oil bath). You may control the temperature of an oil bath with a thermostat.

That is, after aging treatment at a temperature _{T 1} of the 60-80 ° C., further, 70 ° C. or higher than temperature _{T 1} of, preferably 120 to 160 ° C., more preferably aged at 130 to 150 ° C., high temperature _{T 2} . For example, from the viewpoint of increasing the strength of the Mg alloy, the low temperature aging treatment is preferably performed at a temperature T ₁ for 10 hours or longer, more preferably 30 hours or longer, and further preferably 30 to 150 hours, followed by a high temperature at a temperature T ₂ . The aging treatment is preferably performed for 10 hours or more, more preferably 30 hours or more, and further preferably 15 to 30 hours. Preferably, a two-stage aging treatment comprising a low temperature aging treatment at 70 ° C. for 10 hours or more and a high temperature aging treatment at 140 ° C. for 30 to 60 hours is performed.

According to two-stage aging treatment is performed to solution treatment, as aging of Zn was supersaturated solid solution alloy at low temperatures, since the degree of supercooling is increased, atom clusters of the core of MgZn ₂ phase, even MgZn ₂ Phases are formed more densely. For this reason, higher peak hardness is obtained with the Mg alloy.

(Microstructure after aging treatment)
In the Mg alloy manufacturing method of the present invention, the Mg alloy after the two-stage aging treatment preferably contains Mg ₂ Sn precipitates and MgZn ₂ precipitates. The Mg ₂ Sn precipitate is preferably dispersed in the crystal grains of the magnesium matrix and in the crystal grain boundaries. The MgZn ₂ precipitate is preferably dispersed in the crystal grains of the magnesium matrix.

From the viewpoint of improving the strength of the Mg alloy, the size of the Mg ₂ Sn precipitate is preferably 100 to 300 nm, more preferably 150 to 250 nm, and still more preferably 150 to 200 nm. Since the strength of the Mg alloy is improved as the size of the Mg ₂ Sn precipitate is smaller, it may be set to 300 nm or less. The size of the MgZn ₂ precipitates, preferably 10 to 40 nm, more preferably 10 to 30 nm, and more preferably from 15 to 20 [mu] m. Since the strength of the Mg alloy is improved as the size of the MgZn ₂ precipitate is smaller, it may be set to 15 to 20 μm or less. The size of the precipitate is the diameter of a circle corresponding to the area of the precipitate.

(Technical significance of grain size and precipitates)
The influence of the crystal grain size of the magnesium matrix on the strength of the metal material is expressed by the following formula (2).
A = B + k / √D (2)
In the formula (2), A is strength, B and k are constants, and D is the crystal grain size of the Mg alloy.

  Equation (2) shows that the strength (A) decreases when the crystal grain size (D) becomes coarse. Thus, it is clear that it is necessary to suppress the crystal grain growth of the magnesium matrix. In order to increase the strength of the Mg alloy of the present invention, the size of the crystal grain size is preferably 10 to 20 μm or less, more preferably 15 μm or less, and more preferably 8.2 μm or less.

  Next, assuming that the shape of the precipitate is spherical, the influence of the dispersion of the precipitate in the crystal grains on the increment Δτ of the strength of the metal material is expressed by the following formula (3).

In Equation (3), G is the rigidity, b is the Burgers vector, ν is the Poisson's ratio, f is the volume fraction of the precipitate, and _dt is the particle size.

  In the formula (3), the increase in strength means that the increase in strength is large as fine precipitates having a high volume fraction are finely dispersed. Therefore, as shown in FIG. 9 of Example 3 to be described later, the dispersion of precipitates in the crystal grains of the magnesium matrix greatly contributes to strengthening of the Mg alloy.

Further, in order not to reduce the influence of the strengthening, the solution treatment is performed at a temperature higher than that in which all MgZn ₂ phases are dissolved in the matrix, and the coarsening of the precipitate is prevented by the two-stage aging treatment. There is a need.

The Mg alloy obtained by the production method of the present invention has the following mechanical characteristics.
The Vickers hardness of the Mg alloy is preferably 95 VHN or more, more preferably 95 to 100 VHN.

  The yield strength of the Mg alloy is preferably 340 MPa or more, more preferably 340 to 400 MPa.

  The tensile strength of the Mg alloy is preferably 340 MPa or more, more preferably 350 to 410 MPa.

  The elongation of the Mg alloy is preferably 10% or more, more preferably 10% to 20%.

  According to the method for producing an Mg alloy of the present invention, after hot working, the solution is preferably formed at a temperature at which the high melting point precipitate phase does not dissolve in the mother phase and only the low melting point precipitate dissolves in the mother phase. By performing the solidification treatment, significant coarsening of the structure during the solution treatment is suppressed. Subsequent two-stage aging treatment allows the low melting point precipitate to be finely precipitated and greatly strengthened. As a result, the Mg alloy of the present invention has excellent strength and ductility with only a simple heat treatment of the drawn alloy without the use of grain refinement by strong strain processing or the use of heavy rare earth metal elements as in the case of conventional alloys. Can be expressed.

  According to the manufacturing method of the Mg alloy of the present invention, a solution treatment is performed after the extension process, and after softening, the difference in melting point between the two types of precipitates precipitated in the alloy is utilized to perform a two-stage aging treatment. By dispersing the precipitate, excellent strength can be imparted. Thereby, both workability, strength, and ductility can be achieved.

  The manufacturing method of Mg alloy of this invention consists of a solution treatment which makes an alloy element form a solid solution in supersaturation in an alloy, and a two-stage aging treatment which disperses and strengthens a precipitate. If this is applied to plastic processed products such as plates and bars, it will soften after solution treatment, so it can give excellent molding processability, and it can give strength to excellent molded products by subsequent two-stage aging treatment. it can. Further, hardening by the two-stage aging treatment can be achieved without using a rare and expensive element group such as a rare earth metal, so that an Mg alloy that can be used in a wide range of applications can be obtained. The Mg alloy of the present invention is preferably a light-weight material that replaces the Al alloy, and is more preferably used for railways, aircrafts, automobiles, railway vehicle structures, etc., more preferably for railways, aircrafts, railway vehicle structures. Can be applied to railway vehicle structures.

Hereinafter, the present invention will be described in more detail with reference to examples.
Example 1
Each process in the manufacturing method of Mg alloy of Example 1 is shown below.
[Step 1]
Mg1214g, Sn76g, Zn82g, and Al28g high-purity materials were melted at 700 ° C. to form a molten metal, held for 10 to 20 minutes, then poured into a mold and allowed to cool naturally to obtain a cast solid.

[Step 2]
The cast solid was homogenized to obtain a homogenized solid.
The homogenization treatment was performed at a low temperature of 350 ° C. for 24 hours to homogenize Zn, and was performed at a high temperature of 450 ° C. for 24 hours to homogenize Sn.
In the homogenization treatment, the distribution of alloy elements present in the cast alloy is homogenized, and precipitates formed during the cooling of the molten metal are dissolved in the matrix.
In particular, in the region where Zn is macrosegregated to a high concentration, the alloy melts when heat treatment is started at 450 ° C., so the Zn distribution is first homogenized at 350 ° C., and the Sn distribution is made uniform by heat treatment at 450 ° C. Turn into.

[Step 3]
The homogenized solid was extruded to obtain a tangible solid. Specifically, the cast material was processed into a bar material, that is, stretched.
For the extrusion process, a 1000 kN hydraulic servo press (CFT2-100 type manufactured by Kawasaki Oil Industries Co., Ltd.) extruder was used. Extrusion conditions are shown below.
Extrusion temperature: 300 ° C
Extrusion ratio = 20
Ram speed: 0.1 mm / s

[Step 4]
The tangible solid was subjected to solution treatment to obtain a cooled solid. An electric furnace was used for the solution treatment. The solution treatment was performed at a low temperature of 350 ° C. for 0.25 hour and quenched with water.

[Step 5]
The cooled solid was aged to obtain the Mg alloy of the present invention. An oil bath was used for the aging treatment.
The composition of the Mg alloy was Mg _94.4 Sn _1.2 Zn _2.4 Al _{2.0 .} The aging treatment in step 5 was performed by a two-stage aging treatment of low temperature and high temperature. After a low temperature aging treatment at 70 ° C. for 150 hours, a high temperature aging treatment at 140 ° C. for 2000 hours was performed.

  Table 1 summarizes the alloy compositions, solution treatment, and aging treatment steps of Examples and Reference Examples.

FIG. 2 is a view showing the microstructure of the tangible solid (hereinafter also referred to as an extruded material) obtained in Example 1.
The microstructure of FIG. 2 is a reverse local map obtained by EBSD. The reverse locality map is a technique for analyzing individual crystal grains by analyzing the crystal orientation of the crystal grains based on the Kikuchi line diffraction pattern generated by electron beam backscattering diffraction that occurs when the sample is irradiated with an electron beam. In order to irradiate the sample of the present invention with an electron beam, a scanning electron microscope (Cross Beam 1540 EsB) manufactured by Carl Zeiss was used. The backscattered electron beam is detected by a detector equipped with a scanning electron microscope (a detector manufactured by Oxford Instruments). EBSD analysis software (HKL, CHANNEL 5) is used for analysis of electron backscatter diffraction. software).
The microstructure shown in FIG. 2 is a reverse local map obtained by the EBSD method. The average crystal grain size of the extruded material sample calculated by the intercept method from the reverse local map was 1.9 μm.

  FIG. 3 is a view showing the microstructure of the cooling solid obtained by the solution treatment of Example 1. The observation of the fine structure was obtained by the EBSD method as in FIG. As shown in FIG. 3, the average crystal grain of the sample after performing the solution treatment at 350 ° C. was 8.2 μm.

4 is a diagram showing an age hardening curve of the Mg alloy produced in Example 1. FIG. The Vickers hardness was measured with an apparatus manufactured by MITSUTOYO (model HM-102). In the figure, the horizontal axis represents the high temperature aging treatment time (h), and the vertical axis represents the Vickers hardness (VHN).
As shown in FIG. 4, the Vickers hardness of the extruded material is 79.4 VHN (sometimes referred to as HV).
The solution treatment reduces the Vickers hardness to 69.2 VHN, but the low temperature aging treatment at 70 ° C. for 150 hours increases the Vickers hardness to 80.7 VHN.
When the subsequent high temperature aging treatment at 140 ° C. is started, the Vickers hardness starts to increase immediately after the high temperature aging starts, and the peak of the Vickers hardness reaches 99.0 VHN in 30 hours. This Mg alloy having a peak Vickers hardness is also called a peak aging material.
In the high temperature aging treatment at 140 ° C., after about 60 hours, the Vickers hardness of the sample starts to decrease, and decreases substantially monotonically up to 2000 hours. From this, it is understood that when the low temperature aging treatment is performed at 70 ° C. for 150 hours, the aging treatment at 140 ° C. may be 30 hours or more.

FIG. 5 is a diagram showing tensile stress-strain curves of the extruded material and the Mg alloy of Example 1. In the figure, the horizontal axis represents strain, and the vertical axis represents stress (MPa). Tensile strength, tensile strength, and elongation were measured using a measuring instrument manufactured by INSTRON (model 5567). Aged Mg alloy is also called an aging material.
As shown in FIG. 5, the yield strength, tensile strength, and elongation of the extruded material were 244 MPa, 351 MPa, and 0.21, respectively. The elongation is 21% in%. Hereinafter, the elongation is displayed not in% but in numbers. The yield strength, tensile strength, and elongation of the Mg alloy of Example 1 obtained in the above process were 345 MPa, 3845 MPa, and 0.15, respectively. Table 2 summarizes the yield strength, tensile strength, and elongation of the extruded material and Mg alloy.

(Necessity and conditions of two-stage aging treatment)
From comparison of FIG. 4 of Example 1 and FIG. 13 of Reference Example 2 described later, even if the composition and solution treatment temperature are the same, the Vickers hardness does not reach 98 VHN unless two-stage aging is performed.

(Example 2)
In Example 2, a Mg alloy having the same composition as that of Example 1 was produced under the same conditions as Example 1 except that low temperature aging treatment was performed at 70 ° C. for 30 hours and 10 hours.

  The microstructures before and after the solution treatment in Example 2 were the same as those in FIGS. The as-extruded material had a crystal grain size of 1.9 μm, and the crystal grain size after solution treatment at 350 ° C. was 8.2 μm.

6 is a graph showing an age hardening curve in Example 2. FIG. In the figure, the horizontal axis represents the high temperature aging treatment time (h), and the vertical axis represents the Vickers hardness (VHN).
As shown in FIG. 6, the Vickers hardness of the as-extruded material was 79.4 VHN. Although the Vickers hardness decreased to 69.2 VHN by the solution treatment, a low temperature aging treatment at 70 ° C. for 10 hours and a low temperature aging treatment at 70 ° C. for 30 hours were performed.
After each low temperature aging treatment, the Vickers hardness increases to 69.2 VHN and 71.9 VHN. When the high temperature aging treatment at 140 ° C. was started, the Vickers hardness started to increase with the start of the high temperature aging treatment, and reached a peak in the high temperature aging treatment of the sample with a Vickers hardness of 60 hours. The Vickers hardnesses at a high temperature aging treatment time of 140 ° C. for 30 hours and 60 hours were 97.9 VHN and 98.3 VHN, respectively.
Therefore, the high temperature aging treatment time is preferably 10 to 100 hours, more preferably 10 to 60 hours, and further preferably 30 to 60 hours from the viewpoint of productivity.

  As for the time of the low-temperature aging treatment, for example, in an alloy that was performed at 70 ° C. for 10 hours or more from Example 2, a peak hardness of 98 VHN was obtained. Therefore, the same result can be obtained by performing low temperature aging treatment at 70 ° C. for 10 hours or more.

As shown in the results of Example 2, the low temperature aging treatment may be performed until an increase is observed as compared with the Vickers hardness after the solution treatment.
By performing such low-temperature aging treatment and high-temperature aging treatment, it is possible to disperse clusters serving as nuclei of high-density precipitates compared to the case of only high-temperature aging treatment.
From the viewpoint of improving the strength of the Mg alloy, the low temperature aging treatment preferably has a hardness of 90 VHN or more, more preferably 95 VHN or more, and even more preferably 98 VHN or more as a peak of the Vickers hardness of the Mg alloy during the low temperature aging treatment. It is necessary to carry out at the temperature obtained.
In other words, the low temperature aging treatment is performed in a temperature range in which the peak of Vickers hardness can exceed 90 VHN, more preferably 95 VHN, and even more preferably 98 VHN.
Specifically, the low temperature aging treatment is preferably performed at 60 to 80 ° C. for 10 hours or longer, more preferably 20 hours or longer, still more preferably 30 hours or longer, further preferably 40 hours or longer, more preferably 60 hours or longer. .

(Example 3)
Unlike the Mg alloy of Example 1, an alloy having a composition of Mg _94.1 Sn _1.5 Zn _2.4 Al _2.0 was produced under the same conditions as in Example 1.
FIG. 7 is a diagram showing an age hardening curve of the Mg alloy of Example 3. In the figure, the horizontal axis represents the high temperature aging treatment time (h), and the vertical axis represents the Vickers hardness (VHN).
As shown in FIG. 7, the Vickers hardness of the as-extruded material of Example 3 is 77.3 VHN. The solution treatment decreased the Vickers hardness to 65.5 VHN, but the Vickers hardness increased to 77.5 VHN after a low temperature aging treatment at 70 ° C. for 150 hours. When the high temperature aging treatment at 140 ° C. was started, the Vickers hardness started to increase with the start of the high temperature aging treatment, and reached a peak hardness of 98.3 VHN in 16 hours.

FIG. 8 is a diagram showing stress-strain curves of the extruded material and Mg alloy of Example 3. In the figure, the horizontal axis represents strain, and the vertical axis represents stress (MPa).
As shown in FIG. 8, the yield strength, tensile strength, and elongation of the as-extruded material of Example 3 were 244 MPa, 343 MPa, and 0.12, respectively. The yield strength, tensile strength, and elongation of the Mg alloy of Example 3 obtained in the above process were 345 MPa, 387 MPa, and 0.12, respectively. Table 3 summarizes the yield strength, tensile strength, and elongation of the extruded material and Mg alloy.

FIG. 9 is a diagram showing a transmission electron microscope (TEM) image of a bright field in the Mg alloy after the two-stage aging treatment of Example 3. As shown in FIG. 9, coarse Mg ₂ Sn precipitates and fine MgZn ₂ precipitates are observed. These coarse Mg ₂ Sn precipitates and fine MgZn ₂ precipitates are factors that improve the strength of the Mg alloy of the present invention.

Next, a reference example will be described.
(Reference Example 1)
In Reference Example 1, an Mg alloy composed of Mg _95.1 Sn _1.2 Zn _1.7 Al _2.0 was produced in the same manner as in Example 1 except that the raw materials were Mg 1236g, Sn 76g, Zn 60g, and Al 28g. Obtained. Solution treatment was performed at 350 ° C. for 0.25 hours, aging treatment was performed at 70 ° C. for 150 hours, and then isothermal aging treatment at 140 ° C. was performed.
10 is a view showing the microstructure of the as-extruded material of Reference Example 1. FIG. The observation of the fine structure was obtained by the EBSD method as in FIG.
As shown in FIG. 10, the crystal grain size of the extruded material is 2.4 μm, and the microstructure of the extruded material does not depend on the Zn concentration.

FIG. 11 is a diagram showing an age hardening curve of a sample after solution treatment in the Mg alloy of Reference Example 1. In the figure, the horizontal axis represents the high temperature aging treatment time (h), and the vertical axis represents the Vickers hardness (VHN).
As shown in FIG. 11, the Vickers hardness of the as-extruded material of Reference Example 1 is 74.2 VHN. Although the Vickers hardness decreased to 67.6 VHN by the solution treatment, the Vickers hardness hardly increased to 68.5 VHN even when the low temperature aging treatment was performed at 70 ° C. for 150 hours. The Vickers hardness started to increase 3 hours after starting the aging treatment at 140 ° C., and reached a peak hardness of 86.8 VHN in 100 hours. In the Mg alloy of Reference Example 1, the Vickers hardness was lower than that of the Mg alloy of Example 1. This is different from the Mg alloy composed of Mg _94.4 Sn _1.2 Zn _2.4 Al _{2.0 in} Example 1 in comparison with Mg _95.1 Sn _1.2 Zn _1.7 Al _{2 in} Reference Example 1. _This is due to the small Zn composition of the Mg alloy consisting of _0.0 .

FIG. 12 is a diagram showing a stress-strain curve of an as-extruded material and an Mg alloy in the Mg alloy of Reference Example 1. In the figure, the horizontal axis represents strain, and the vertical axis represents stress (MPa).
As shown in FIG. 12, the yield strength, tensile strength, and elongation of the as-extruded material of Reference Example 1 were 182 MPa, 314 MPa, and 0.19, respectively. The yield strength, tensile strength, and elongation of the Mg alloy of Reference Example 1 obtained in the above process were 239 MPa, 315 MPa, and 0.16, respectively. Table 4 summarizes the yield strength, tensile strength and elongation of the extruded material and Mg alloy.

(Reference Example 2)
An Mg alloy having the same composition as in Example 1 was produced. The solution treatment was performed at 350 ° C., and the aging treatment was not a two-stage aging treatment but a one-stage aging treatment at temperatures of 140 ° C., 160 ° C., and 200 ° C.
The microstructure before and after the solution treatment was the same as in Example 1. The average crystal grain size of the sample of the extruded material was 1.9 μm (see FIG. 2), and the crystal grain size after solution treatment at 350 ° C. was 8.2 μm (see FIG. 3).

FIG. 13 is a diagram showing an age hardening curve of the Mg alloy of Reference Example 2. The horizontal axis in the figure is the aging treatment time (h), and the vertical axis is the Vickers hardness (VHN).
As shown in FIG. 13, in the aging treatment material at 200 ° C., the Vickers hardness started to increase with the start of the aging treatment, and the peak of the Vickers hardness reached 79.4 VHN in 10 hours. The aging treatment material at 160 ° C. starts to increase in Vickers hardness as the aging treatment starts, and reaches a peak hardness of 81.2 VHN in 30 hours. In the aging treatment material at 140 ° C., the Vickers hardness starts to increase with the start of the aging treatment, and the peak of the Vickers hardness reaches 86.2 VHN after 160 hours.

FIG. 14 is a diagram showing a tensile stress-strain curve of an as-extruded material and an Mg alloy in the Mg alloy of Reference Example 2. In the figure, the horizontal axis represents strain, and the vertical axis represents stress (MPa).
As shown in FIG. 14, the yield strength, tensile strength, and elongation of the as-extruded material of Reference Example 2 were 244 MPa, 351 MPa, and 0.21, respectively. The yield strength, tensile strength, and elongation of the Mg alloy of Reference Example 2 obtained in the above process were 179 MPa, 298 MPa, and 0.141, respectively. Table 5 summarizes the yield strength, tensile strength, and elongation of the as-extruded material and Mg alloy of Reference Example 2.

(Reference Example 3)
An Mg alloy having the same composition as that of Reference Example 1 was produced, but the solution treatment was performed at 450 ° C., and the aging treatment was not a two-stage aging treatment but a one-stage aging treatment at 200 ° C.
The structure immediately after extrusion was the same as in Reference Example 1 (see FIG. 10), and the average crystal grain size was 2.4 μm.
FIG. 15 is an optical microscopic image showing the microstructure after the solution treatment of Reference Example 3. As shown in FIG. 15, the crystal grain size after the solution treatment at 450 ° C. was 160 μm.

  FIG. 16 is a diagram showing a bright field TEM image of the microstructure in the Mg alloy after the aging treatment of Reference Example 3. FIG. As shown in FIG. 16, in the fine structure of the Mg alloy after the aging treatment in Reference Example 3, it can be seen that coarse precipitates are sparsely dispersed.

  FIG. 17 is a view showing an age hardening curve after solution treatment in the Mg alloy of Reference Example 3. The horizontal axis in the figure is the aging treatment time (h), and the vertical axis is the Vickers hardness (VHN). As shown in FIG. 17, the Vickers hardness starts to increase with the start of the aging treatment, and the peak of the Vickers hardness reaches 64.6 VHN at 160 hours.

FIG. 18 is a diagram showing a tensile stress-strain curve of the as-extruded material and the Mg alloy in the Mg alloy of Reference Example 3. In the figure, the horizontal axis represents strain, and the vertical axis represents stress (MPa).
As shown in FIG. 18, the yield strength, tensile strength, and elongation of the as-extruded material of Reference Example 3 were 267 MPa, 340 MPa, and 0.12, respectively. The yield strength, tensile strength, and elongation of the Mg alloy of Reference Example 3 obtained in the above process were 195 MPa, 266 MPa, and 0.05, respectively. Table 6 summarizes the yield strength, tensile strength, and elongation of the as-extruded material and Mg alloy of Reference Example 3.

  As shown in the results of Example 1 and Reference Example 3, the solution treatment can be performed at a low temperature to suppress the growth of crystal grains of the magnesium matrix.

(Reference Example 4)
Reference Example 4 is an age hardening curve when the Mg alloy having the same composition as in Example 1 is subjected to an aging treatment at 70 ° C. for 3000 hours.
FIG. 19 is an age hardening curve when an aging treatment is performed at 70 ° C. for 3000 hours in the Mg alloy having the same composition as in Example 1. The horizontal axis in the figure is the aging treatment time (h), and the vertical axis is the Vickers hardness (VHN).
As shown in FIG. 19, when aging treatment is performed at 70 ° C., the diffusion of atoms becomes slow, and it can be seen that it takes 3000 hours to reach 98 VHN, which is the peak of Vickers hardness.

  According to the above Examples 1 to 3, the tensile strength of the Mg alloy of the present invention is 385 MPa to 387 MPa, which is larger than 343 MPa to 351 MPa of the extruded material, and the elongation at break is 10% or more. The characteristic which processing becomes easy in the stage after the chemical conversion treatment is obtained. In the Mg alloy of the present invention, a Vickers hardness of 98 VHN or more can be easily obtained.

  The present invention is not limited to the above embodiment, and various modifications are possible within the scope of the invention described in the claims, and it goes without saying that these are also included in the scope of the present invention. Nor.

Claims (9)

Following formula (1):
Mg _a Sn _b Zn _c Al _d (1)
(In the formula (1), a, b, c and d are a + b + c + d = 100,
92.0 ≦ a ≦ 96.0, 0.5 ≦ b, 2.0 ≦ c ≦ 3.0, and 1.5 ≦ d. )
Mg alloy represented by
The Mg alloy includes Mg ₂ Sn precipitates and MgZn ₂ precipitates,
The Mg ₂ Sn precipitate is dispersed in the crystal grains and in the crystal grain boundaries,
The MgZn ₂ precipitate is dispersed in crystal grains, and is a method for producing the Mg alloy,
Step 1 of dissolving Mg, Sn, Zn and Al to obtain a cast solid;
Step 2 for homogenizing the cast solid to obtain a homogenized solid; Step 3 for hot working the homogenized solid to obtain a tangible solid;
Step 4 of solution treatment of the tangible solid to obtain a cooled solid;
Step 5 of aging the cooling solid to obtain an Mg alloy;
With
In the step 4, solution treatment is performed at 310 ° C. to 400 ° C.,
Production of Mg alloy composed of Mg, Sn, Zn and Al, which is subjected to two-stage aging treatment at a temperature T ₂ which is 70 ° C. or more higher than the temperature after aging treatment at a temperature T ₁ of 60 to 80 ° C. in the step 5 Method. The performed temperature T ₁ of the aging treatment more than 10 hours, said temperature T ₂ of the aging 10 hours or more, the production method of the Mg alloy according to claim 1.   The manufacturing method of Mg alloy of Claim 1 or 2 which performs the solution treatment of the said process 4 for 0.25 to 0.5 hour.   The manufacturing method of Mg alloy in any one of Claims 1-3 whose Vickers hardness of the said Mg alloy is 95VHN or more.   The manufacturing method of Mg alloy in any one of Claims 1-4 whose yield strength of the said Mg alloy is 340-400 Mpa.   The manufacturing method of Mg alloy in any one of Claims 1-5 whose tensile strength of the said Mg alloy is 350-410 Mpa.   The manufacturing method of Mg alloy in any one of Claims 1-6 whose elongation of the said Mg alloy is 10 to 20%. Following formula (1):
Mg _a Sn _b Zn _c Al _d (1)
(In the formula (1), a, b, c and d are a + b + c + d = 100,
92.0 ≦ a ≦ 96.0, 0.5 ≦ b, 2.0 ≦ c ≦ 3.0, and 1.5 ≦ d. )
Mg alloy represented by
The Mg alloy includes Mg ₂ Sn precipitates and MgZn ₂ precipitates,
The Mg ₂ Sn precipitates are dispersed in the crystal grains and in the crystal grain boundaries,
The MgZn ₂ precipitate is an Mg alloy dispersed in crystal grains. The Mg alloy according to claim 8 , wherein an average grain size of the crystal grains is 15 μm or less.