JP4926496B2 - Magnesium alloy for die casting with excellent heat resistance, castability and corrosion resistance - Google Patents

Description

  The present invention relates to a magnesium alloy for die casting used for die casting and the like.

  For the purpose of achieving both heat resistance and castability, a heat-resistant magnesium alloy for die casting and various magnesium alloys for die casting (Patent Documents 1 to 3) have been proposed. These magnesium alloys for die casting contain Al, rare earth elements (RE), etc., and have improved heat resistance to some extent, contributing to the expansion of the application range of magnesium alloys. However, die castability comparable to that of general-purpose alloys is required for application to parts with higher heat load and cost reduction. The above heat-resistant Mg alloy for die casting cannot sufficiently cover the required heat resistance and castability.

  The biggest problem with magnesium alloys for die casting is cracking during die casting. The occurrence of defective products due to casting cracks leads to an increase in manufacturing costs, and at the same time, the inherent heat resistance of the material cannot be expressed due to minute casting crack defects. In the prior art, this casting crack cannot be sufficiently suppressed. Therefore, development of a magnesium alloy for die casting having excellent castability and heat resistance and further excellent corrosion resistance is desired.

JP-A-9-291332 Japanese Patent Laid-Open No. 2001-316752 JP 2002-129272 A

  The present invention has been made in view of such conventional problems, and an object of the present invention is to provide a magnesium alloy for die casting excellent in heat resistance, castability, and corrosion resistance.

The present invention is a magnesium alloy containing Al, a rare earth element (hereinafter referred to as RE), and Mn and having excellent heat resistance, castability, and corrosion resistance,
The Mn content is 0.05 to 1.0 % by mass ,
The content of Al is a mass%, and the R.I. E. When the content of is b mass%, a ≧ 3.9 , 2.5 ≧ b / a ≧ 1.38 , and 31.3 / a-4.3 ≧ b / a,
The balance consists of Mg and inevitable impurities,
When the predetermined area in the cross section of the steady portion is 100%, the magnesium alloy for die casting is characterized in that the area ratio of the Mg—Al compound in the predetermined area is 0.1% or less (Claim 1).

  A magnesium alloy excellent in heat resistance, castability and corrosion resistance can be obtained by optimizing the components and the content of the magnesium alloy for die casting of the present invention (hereinafter appropriately referred to as a magnesium alloy).

That is, the magnesium alloy includes Al, R.I. E. And Mn.
The Al has the effect of improving castability and corrosion resistance in the magnesium alloy.
R. above. E. Is partly dissolved in the magnesium alloy, so that the corrosion resistance can be greatly improved.

Al and R.I. E. Are thermally stable Al-R. E. A system compound is produced. The Al-R. E. Since the system compound incorporates impurities such as Fe and Ni that cause corrosion into the compound, the magnesium alloy can obtain excellent corrosion resistance.
Moreover, corrosion resistance can be improved by containing said Mn.

In addition, the content of Al is a mass%, the R.I. E. When the content of b is b mass%, a ≧ 3.9 , 2.5 ≧ b / a ≧ 1.38 , and 31.3 / a-4.3 ≧ b / a.
By satisfying the above range, Al and R.I. E. The content of is optimized. In this case, the Al-R. E. Al is consumed during the crystallization of the system compound, and the crystallization of the thermally unstable Mg—Al compound (hereinafter referred to as “β phase”) can be suppressed. Therefore, when the predetermined area in the cross section of the steady portion is 100%, the area ratio of the thermally unstable β phase is 0.1% or less, and excellent heat resistance can be obtained. Furthermore, the magnesium alloy exhibits quasi-two-dimensional solidification by suppressing the crystallization of the β phase. Therefore, the solid-liquid coexistence temperature range becomes small and the temperature balance during solidification becomes good, so that casting cracks can be suppressed.

As described above, the magnesium alloy for die casting of the present invention contains Mn, and its content is 0.05 to 1.0 % by mass .
When the Mn content is less than 0.05% by mass, there is a problem that the corrosion resistance of the magnesium alloy is lowered. On the other hand, when the Mn content exceeds 1.0% by mass, Mn, Al, R. E. Forms a compound with a large specific gravity and settles, so that the amount of sludge generated is increased, the amount of Al in the alloy and R.I. E. There is a problem of reducing the amount.

In addition, the content of Al is a mass%, the R.I. E. When the content of b is b mass%, a ≧ 3.9 , 2.5 ≧ b / a ≧ 1.38 , and 31.3 / a-4.3 ≧ b / a.
In the case of a < 3.9, the effect of improving castability due to the Al content cannot be ensured, and stable Al—R. E. There is a problem that the absolute amount of the compound-based compound phase is insufficient and sufficient heat resistance cannot be obtained.
The above Al-R. E. Examples of the system compound include an Al ₂ Ce compound, an Al ₁₁ Ce ₃ compound, an Al ₂ La compound, an Al ₁₁ La ₃ compound, an Al ₂ Nd compound, an Al ₁₁ Nd ₃ compound, an Al ₂ Pr compound, and an Al ₁₁ Pr ₃ compound. Etc.

When b / a> 2.5, that is, b> 2.5a, Mg—R. E. Since the amount of the system compound is relatively increased, there is a problem that the castability is greatly lowered and the corrosion resistance is impaired. The Mg-R. E. Examples of the system compound include an Mg ₉ Ce compound, an Mg ₁₂ Ce compound, an Mg ₃ Ce compound, an Mg ₉ La compound, an Mg ₁₂ La compound, an Mg ₃ La compound, and an Mg ₉ Pr compound. On the other hand, in the case of b / a < 1.38 , that is, b < 1.38a , a thermally unstable β phase is crystallized, resulting in a problem that heat resistance is lowered. Further, when b / a> 31.3 / a-4.3, that is, b> 31.3-4.3a, only the amount of sludge increases, and high Al, high R.V. E. There is a problem that a magnesium alloy cannot be obtained.

The balance consists of Mg and inevitable impurities.
Essential elements in the magnesium alloy for die casting of the present invention include Al, R.I. E. , Mn. Other elements are basically contained as inevitable impurities.

Further, when the predetermined area in the cross section of the stationary part is 100%, the area ratio of the Mg—Al compound is 0.1% or less.
Examples of the Mg—Al compound include an Mg ₁₇ Al ₁₂ compound (β layer).
When the area ratio exceeds 0.1%, there is a problem that the heat resistance of the magnesium alloy is lowered due to the thermally unstable β phase.

In addition, the above R.I. E. At least 50 wt% is Ce der Ru misch metal (hereinafter referred to as Mm) as it is preferable to use (claim 2).
R. above. E. Is an expensive alloying element, so adding a large amount leads to an increase in cost. Therefore, it is economical and effective to use inexpensive Mm .

The magnesium alloy for die casting is further selected from Sr: 1.5 % by mass or less, Ca: 1.0 % by mass or less, Si: 1.0 % by mass or less, and Sn: 2.0 % by mass or less. It is preferable to contain 1 type (s) or 2 or more types (Claim 3).
In this case, the Al-R. E. Although it is a phase different from the system compound phase, the Al-R. E. Since a phase showing the same effect as the system compound phase is formed, the heat resistance can be improved.

When the Sr is added, the corrosion resistance of the casting can be further improved.
When the Sr content exceeds 1.5% by mass, the Mg—Sr compound to be crystallized increases, and the castability may be greatly reduced.
Moreover, when adding said Ca, the flame-proof effect of a molten metal can be acquired further.
When the Ca content exceeds 1.0% by mass, the Mg—Ca-based compound to be crystallized increases, so that castability may be greatly reduced.

Moreover, when adding said Si and Sn, castability can be improved further.
When the Si content exceeds 1.0% by mass, Si and R.I. E. Form a compound having a large specific gravity and settle, so that R. If the Sn content exceeds 2.0% by mass, Sn and R.E may be reduced in the molten metal. E. Form a compound having a large specific gravity and settle, so that R. E. May reduce the amount.

Further, assuming that the total area of the crystallized compound in the cross section of the stationary part is 100%, Al—R. E. It is preferable that the area ratio of the system compound is 95% or more.
The above Al-R. E. Since the system compound is thermally stable, the heat resistance of the magnesium alloy can be improved. The Al-R. E. Since the system compound incorporates impurities in the magnesium alloy into the compound, the corrosion resistance of the magnesium alloy for die casting can be improved. Therefore, Al-R. E. It is preferable that the area ratio of the compound is 95% or more.
The Al-R. E. If the area ratio of the system compound is less than 95%, the heat resistance and corrosion resistance of the magnesium alloy for die casting may be reduced.

In addition, the content of Al is a mass%, the R.I. E. When the content of b is b mass%, it is preferable that a ≧ 3.9 , 2.3 ≧ b / a ≧ 1.38 , and 31.3 / a-4.3 ≧ b / a (claims) 5).
In this case, Al and R.I. E. The content of is further optimized. The thermally unstable Mg—Al compound does not crystallize, and the thermally stable Al—R. E. The system compound crystallizes, and excellent heat resistance can be obtained. Moreover, since the solid-liquid coexistence temperature range is further reduced and the temperature balance during solidification is improved, casting cracks can be suppressed.

In the case of b / a> 2.3, Mg—R. E. Since the relative amount of the compound increases, castability may decrease. On the other hand, when b / a < 1.38 , a thermally unstable β phase is crystallized or precipitated, Heat resistance may be reduced.

Example 1
Examples of the magnesium alloy for die casting of the present invention will be described.
In this example, casting was performed using a magnesium alloy having the composition shown in Table 1, and a composition analysis was performed.
In the casting, first, a molten metal was prepared using a melting crucible made of SUS430. Pure Mg (99.9%) ingot, pure Al (99.9%) ingot, and Mg-3.3Mn alloy were put into the crucible, and then a small amount of SF ₆ gas was sprayed as a flameproof gas in the furnace. It melt | dissolved in the state and heated until the molten metal temperature reached 750 degreeC. Further, a predetermined amount of Mm (52% Ce-25% La-16% Nd-6% Pr alloy) was put into the crucible and the molten metal was stirred. After the molten metal temperature was 720 ° C., the molten metal was kept calm for 30 minutes in the furnace. Thereafter, the crucible was taken out from the furnace and cast using a solid die casting machine to produce a die casting.

<Casting conditions>
Injection speed: 0.3m / s
Injection pressure: 28MPa
Pressurization time: 5s
Mold temperature: Room temperature Injection melt temperature: Liquidus temperature + 30 ° C
Mold shape: 20mm x 20mm x 48mm (hereinafter A shape)

  In the composition analysis, a sample was cut out from a part of the die casting obtained by the casting, and the composition analysis of the die casting was performed by induction plasma (ICP) emission analysis. The results are shown in Table 1. Further, FIG. 1 shows the relationship between the analysis value and the blending value of Al and Mm (Ce + La + Nd + Pr) of the die cast casting. In FIG. 1, the horizontal axis represents the amount of Al, the vertical axis represents the amount of Mm, and shows the blending value (◯ mark) and the analysis value (● mark).

  As is known from FIG. 1, when the blending amount of Al and Mm exceeds the broken line K, Al and Mm are Al—R. E. It was crystallized as a system compound, settled and detected as sludge at the bottom of the crucible, and the analytical values of Al and Mm in the die-casting were greatly reduced compared to the blending amount. Thereby, the Mg-Al-Mm-Mn alloy has a broken line K (31.3 / a-4.3 = b / a shown in FIG. It can be seen that the addition of Al and Mm (content of b mass%) or more only increases the amount of sludge, and a high Al, high Mm magnesium alloy casting cannot be obtained.

(Example 2)
In this example, casting was performed using a magnesium alloy having the composition shown in Table 2, and the structure observation, β phase area ratio, axial force retention, and castability were measured.
First, in the same manner as in Example 1, a magnesium alloy having the composition shown in Table 2 was melted to produce the A-shaped die casting. About the sample which cut out a part of obtained die-casting casting, structure | tissue observation was performed, the area of (beta) phase was measured with the structure | tissue photograph, and (beta) phase rate (area ratio) was computed.

FIG. 2 shows the contents of Al and Mm in Samples E1 to E8 and Samples C1 to C10. In the figure, the horizontal axis represents the Al content (a, mass%), and the vertical axis represents the Mm content (b, mass%). The straight line L represents a = 2.5, the straight line M represents b / a = 2.5, the straight line N represents b / a = 1.5−2.5 / a, and the straight line O represents b / a = 2.5. 31.3 / a-4.3 . Also, in FIG. 2, reference numeral P (○ mark) indicates the Examples and Reference Examples in the present invention, the code Q (× mark) shows a comparison example of the present invention. Of the above circles, the reference examples are (Al amount, Mm amount) = (2.8, 2), (2.9, 4.1), (2.9, 7.1), (3 ., 9, 4.1).
The point A is the sample E5, the point B is the sample C4, the point C is the sample C7, and the point D is the sample C8.

  The structure observation was performed using a scanning electron microscope (SEM) after the sample was polished and etched using a 10% aqueous acetic acid solution. The compounds were classified based on the crystallization site and crystallization form. Analysis by an energy dispersive X-ray analyzer (EDX) was also performed to confirm the β phase.

An example of the structure of the die casting is shown in FIGS.
3, FIG. 4, FIG. 5 and FIG. 6 are sample E5 (point A in FIG. 2), sample C4 (point B in FIG. 2), sample C7 (point C in FIG. 2), sample E8 (FIG. 2), respectively. It is a photograph of the structure | tissue of (D point in).

As can be seen from FIG. 3, sample E5, which is an example in the present invention, is composed of Al-R. E. System compound 3 was observed.
As can be seen from FIG. 4, sample C4, which is a comparative example of the present invention, has Al-R. E. Compound 3, and Mg-R. E. System compound 4 was observed.
Further, as is known from FIG. 5, sample C7, which is a comparative example of the present invention, is Al—R. E. System compound 3 and Mg—Al compound 5 were observed.
Further, as is known from FIG. 6, sample C8, which is a comparative example of the present invention, is Al—R. E. System compound 3 and Mg—Al compound 5 were observed.

  Next, the β phase ratio was measured by taking a tissue photograph using an SEM and an optical microscope, and determining the area ratio of the β phase (β phase ratio) using image analysis software (ImagePro). The β phase ratio was an average value of 5 fields of view. The results are shown in Table 2.

Next, the axial force holding test will be described. Casting was performed under the conditions described below to produce a U-shaped magnesium die cast casting having two rod-shaped portions. Then, a ring-shaped sample 11 (see FIG. 8) having an outer diameter of 20 mm, an inner diameter (bolt through hole) of 9 mm, and a height of about 10 mm was cut out from one rod-shaped portion of the die cast casting.
<Casting conditions>
Injection speed: (plunger moving speed) 0.3 to 0.35 m / s
Injection pressure: 28MPa
Mold temperature: room temperature to 40 ° C

  As shown in FIG. 8, the axial force is measured in a state in which the sample 11 is made of steel in a state in which an aluminum alloy washer 82 is inserted into an aluminum alloy block 83 that has been screwed so as to match the bolt 81. The bolt 81 was used. At this time, the axial force (initial axial force) applied to the bolt 81 is 8 kN. The axial force was measured using a strain gauge 84 attached to the bolt 81. Two strain gauges 84 were attached to the shaft surface (two places on the front and back sides) of the bolt 81. The gauge lead wire 85 is pulled out through a hole provided in the head of the bolt 81.

  While the sample 11 was fastened to the block 83, the sample 11 was inserted into a high-temperature thermostat set at 175 ° C., held for 300 hours, then taken out from the high-temperature thermostat and cooled to room temperature. Thereafter, the axial force of the bolt 81 was measured again, and this was taken as the residual axial force. The ratio of the residual axial force to the initial axial force was calculated and used as the axial force retention rate. The axial force retention was calculated as an average value of 4-10.

Thereafter, for the calculated axial force retention rate, the ratio (ADC12 alloy ratio) when the axial force retention rate of the aluminum alloy ADC12 for die casting obtained by the same axial force retention test was set to 100 was calculated, and the heat resistance was evaluated. . Those with an evaluation of ○ were accepted, and those with an evaluation of × were rejected. The results are shown in Table 2.
(Evaluation criteria)
○: Ratio of axial force retention ratio (ADC12 alloy ratio) exceeds 70 x: Ratio of axial force retention ratio (ADC12 alloy ratio) is 70 or less

Next, castability will be described. Casting was performed under the following conditions to produce a die cast casting having a shape shown in FIG. 7 (hereinafter referred to as B shape). The castability was evaluated by observing the presence or absence of cracks generated in the die-cast casting during casting. The results are shown in Table 2.
<Casting conditions>
Injection speed: 1m / s
Injection pressure: 64 MPa
Pressurization time: 5s
Mold temperature: 200 ℃
Injection molten metal temperature: liquidus temperature + 30 ° C
Mold shape: Thickness part of thickness z (3mm), width w (50mm), height (30mm) on the top of the plate of thickness v (2mm), width w (50mm), height x (80mm) Shape

As is known from Table 2, Sample E5 to Sample E8, which are examples of the present invention, showed good results in any evaluation. Samples E1 to E4 are reference examples.
Sample C1 and Sample C2, which are comparative examples of the present invention, have a Al content lower than the lower limit of the present invention, so that the thermally stable Al-R. E. Crystallization of the system compound was insufficient, the axial force retention decreased, and the heat resistance was unacceptable.

Samples C3 and C4, which are comparative examples of the present invention, have an Al content below the lower limit of the present invention, and b / a exceeds the upper limit of the present invention, b / a = 2.5, Mg-R. E. Since the compound was generated and cracking occurred during casting, the castability was unacceptable.
Samples C5, C7, and C9, which are comparative examples of the present invention, are thermally unstable β-phase crystals because b / a is lower than the lower limit of the present invention, b / a = 1.38. The output could not be suppressed, and the β phase ratio exceeded the upper limit of the present invention, so the axial force retention rate was reduced and the heat resistance was unacceptable.

Sample C6, which is a comparative example of the present invention, has b / a higher than the upper limit of the present invention, b / a = 2.5, and Mg—R. E. Since the compound was crystallized and cracked during casting, the castability was unacceptable.
Sample C8, which is a comparative example of the present invention, has a b / a exceeding the upper limit of the present invention, b / a = 31.3 / a-4.3. The output could not be suppressed, and the β phase ratio exceeded the upper limit of the present invention, and the axial force retention rate decreased, so the heat resistance was unacceptable.
In addition, in the sample C10 which is a comparative example of the present invention, since b / a exceeds b / a = 31.3 / Al-4.3 which is the upper limit of the present invention, the axial force retention ratio is reduced, and the heat resistance Was rejected.

(Example 3)
In this example, casting was performed using a magnesium alloy having the composition shown in Tables 3 to 5, and the axial force retention was measured in the same manner as in Example 2 to evaluate the heat resistance.
A magnesium alloy having the composition shown in Tables 3 to 5 in which the Al amount, Mm amount, SR amount, Ca amount, Si amount, Sn amount, and Mn amount were changed was melted in the same manner as in Example 1 above. A die cast casting of shape A was cast in the same manner as in Example 1. Moreover, the axial force retention was obtained by the same method as in Example 2 above, and the heat resistance was evaluated. The results are shown in Tables 3 to 5. Those with an evaluation of ○ are passed, and those with an evaluation of × are rejected.
(Evaluation criteria)
○: Ratio of axial force retention (ADC12 ratio) exceeds 70 x: Ratio of axial force retention (ADC12 ratio) is 70 or less

As can be understood from Tables 3 5, specimen E9~ sample E51 are in any of the evaluation showed good results. Of the samples E9 to E51, the sample E9, the sample E14, the sample E19, the sample E24, the sample E29, the sample E34, the sample E39, the sample E44, and the sample E49 are reference examples. It is an example.
Sample C11, Sample C12, and Sample C13, which are comparative examples of the present invention, have an axial force retention ratio of b / a below the lower limit of the present invention, b / a = 1.5-2.5 / a. The heat resistance decreased.

Example 4
In this example, casting was performed using a magnesium alloy having the composition shown in Table 6, and the corrosion rate was measured.
Magnesium alloys having various compositions with different amounts of Al and Mm shown in Table 6 were melted in the same manner as in Example 1 above, and a B-shaped die casting was cast in the same manner as in Example 2 above. did. A test piece was cut out from the thick portion and subjected to an immersion corrosion test in a 5% NaCl aqueous solution under the following conditions to determine the corrosion rate and evaluate the corrosion resistance.
<Immersion corrosion test conditions>
Test piece shape: 20 mm × 20 mm × 3 mm
Measurement temperature: 24 ° C
Measurement time: 300h

Specifically, as shown in FIG. 9, a test piece holding base 92 is disposed on a beaker 91 containing a 5% NaCl aqueous solution 93, and the sample 12 is placed on the test piece holding base 92. The test piece 12 undergoes a reaction represented by the following formula 1 in a 5% NaCl aqueous solution 93, and the corrosion of the sample 12 proceeds with the generation of hydrogen 94.
Formula 1: Mg + 2H ₂ O = Mg (OH) ₂ + H ₂

A funnel 95 was placed on the test specimen holder 92, and the bullet 97 with a stopcock was held by a stand 96 so that the side having the smaller diameter of the funnel 95 entered the tube. Hydrogen 94 generated in the burette 97 with stopcock was collected. From the following formula 2, the corrosion weight loss of magnesium was calculated as the corrosion rate and the corrosion resistance was evaluated. Those with an evaluation of ○ were accepted, and those with an evaluation of × were rejected.
Formula 2: Y = X × 24.32 / (22.4 × 10 ³ )
(Y: eluted Mg amount (g), X: generated hydrogen gas amount (mL))

(Evaluation criteria)
Since the general-purpose die-cast alloy AZ91D alloy is excellent in corrosion resistance and practically has no problem of corrosion resistance, the corrosion rate of the AZ91D alloy determined by the same immersion corrosion test was used as a criterion for evaluation.
○: When the corrosion rate is less than or equal to the corrosion rate of AZ91D ×: When the corrosion rate is greater than the corrosion rate of AZ91D The results are shown in Table 6.

As can be understood from Table 6, specimen E52~ sample E56 are in any of the evaluation showed good results. Of the samples E52 to E56, the sample E52 and the sample E54 are reference examples, and the remaining samples are examples of the present invention.
Sample C14, which is a comparative example of the present invention, has an Al content lower than the lower limit of the present invention, and b / a exceeds the upper limit of the present invention, b / a = 2.5. Met.
Sample C15, which is a comparative example of the present invention, failed in corrosion resistance because b / a exceeded b / a = 2.5, which is the upper limit of the present invention.

  Samples C16, C17, and C18, which are comparative examples of the present invention, are R.D. Since E was not contained, corrosion resistance fell and it was a failure.

The graph figure which shows the relationship between the compounding quantity of Al and Mm of a Mg-Al-Mm alloy in Example 1, and the analytical value of Al and Mm of a casting. The graph which shows content of Al and Mm in Example 2. FIG. The figure which shows the structure | tissue of the sample E5 in Example 2. FIG. The figure which shows the structure | tissue of the sample C4 in Example 2. FIG. The figure which shows the structure | tissue of the sample C7 in Example 2. FIG. The figure which shows the structure | tissue of the sample C8 in Example 2. FIG. Explanatory drawing which shows the metal mold | die shape in Example 2. FIG. Explanatory drawing which shows the axial force holding | maintenance test in Example 2. Explanatory drawing which shows the immersion corrosion test in Example 4. FIG.

Explanation of symbols

2 Magnesium 3 Al-R. E. Compounds

Claims (5)

A magnesium alloy containing Al, a rare earth element (hereinafter referred to as RE), and Mn and having excellent heat resistance, castability, and corrosion resistance,
The Mn content is 0.05 to 1.0 % by mass ,
The content of Al is a mass%, and the R.I. E. When the content of is b mass%, a ≧ 3.9 , 2.5 ≧ b / a ≧ 1.38 , and 31.3 / a-4.3 ≧ b / a,
The balance consists of Mg and inevitable impurities,
A magnesium alloy for die casting, wherein the area ratio of the Mg-Al compound in the predetermined area is 0.1% or less, assuming that the predetermined area in the cross section of the stationary part is 100%. The R.I. E. At least 50 mass% as the Ce der Ru misch metal (hereinafter referred to as Mm) die casting magnesium alloy, which comprises using a. In Claim 1 or 2, Furthermore, 1 type chosen from Sr: 1.5 mass% or less, Ca: 1.0 mass% or less, Si: 1.0 mass% or less, and Sn: 2.0 mass% or less Or the magnesium alloy for die-casting characterized by containing 2 or more types.   In any one of Claims 1-3, when the total area in the stationary part cross section of the compound to crystallize is set to 100%, Al-R. E. A magnesium alloy for die casting, wherein the area ratio of the compound is 95% or more. In any one of Claims 1-4, content of the said Al is a mass%, said R.I. E. The die casting is characterized in that a ≧ 3.9 , 2.3 ≧ b / a ≧ 1.38 , and 31.3 / a-4.3 ≧ b / a when the content of b is mass%. Magnesium alloy.