JP4852082B2 - Magnesium alloy - Google Patents

Description

  The present invention relates to a magnesium alloy having excellent heat resistance and castability.

  In industries such as the automobile industry, magnesium alloys are applied for the purpose of weight reduction. In the future, it is expected that the range of application of magnesium alloys will be further expanded, and in particular, application to engine peripheral parts and the like that are effective in reducing weight is considered. Since engine peripheral parts are required to have high heat resistance, development of magnesium alloys having excellent heat resistance is required. For example, a magnesium-aluminum-silicon alloy, a magnesium-aluminum-rare earth element alloy, and the like have been developed with the aim of improving heat resistance with respect to conventional magnesium-aluminum alloys. However, these magnesium alloys are not always sufficient in terms of corrosion resistance, castability, and cost. With respect to these magnesium alloys, magnesium-aluminum-calcium alloys having excellent heat resistance, corrosion resistance, and castability have been developed.

For example, Patent Document 1 discloses a magnesium alloy (Mg—Al—Ca—Mn alloy) containing aluminum (Al), calcium (Ca), and manganese (Mn). Al improves castability, corrosion resistance, and mechanical properties at room temperature, particularly die cast castability, in a magnesium alloy. However, at the same time, a thermally unstable compound phase (Mg ₁₇ Al ₁₂ phase) crystallizes, so that the heat resistance of the magnesium alloy is greatly reduced. Crystallization of the Mg ₁₇ Al ₁₂ phase is suppressed by adding Ca that forms a compound with Al, and the heat resistance of the magnesium alloy is improved. On the other hand, when the content of Ca is too large, coarse Mg 2 _Ca (Al) phase to lower the castability tends to crystallize as a primary crystal. Therefore, by defining the mass ratio of aluminum and calcium contained, a magnesium alloy having both excellent heat resistance and castability can be obtained.

Patent Document 1 describes that silicon (Si) and / or tin (Sn) may be added to the Mg—Al—Ca—Mn alloy. However, in Patent Document 1, Si and Sn are not essential alloy elements, and the total content of Si and Sn is not disclosed.
JP 2007-197796 A

  When Si or Sn is added to the Mg—Al alloy, a Mg—Si compound or a Mg—Sn compound is generated, and the heat resistance is improved. Therefore, simultaneous addition of Ca and Si or Ca and Sn to the Mg—Al alloy can be expected to improve heat resistance due to an increase in the amount of the compound. However, it is found that when Ca and Si or Ca and Sn are added simultaneously to the Mg—Al alloy, a coarse Ca—Si—Mg compound or a coarse Ca—Sn—Mg compound is formed, and the heat resistance is not improved. It was. It was also found that the formation of a coarse compound reduces the ductility and easily causes casting cracks.

  The present inventors have conceived that the heat resistance and castability of the magnesium alloy can be further improved by simultaneously adding a predetermined amount of Si and Sn to the Mg—Al—Ca—Mn alloy. That is, an object of the present invention is to provide a novel magnesium alloy excellent in heat resistance and castability.

  The magnesium alloy of the present invention has an aluminum (Al) content of 6 mass% to 10 mass%, calcium (Ca) content of 2.2 mass% to 8.0 mass%, silicon (100% by mass), silicon ( Si) is 0.1 mass% or more and 1.0 mass% or less, tin (Sn) is 0.1 mass% or more and 1.0 mass% or less, and manganese (Mn) is 0.05 mass% or more and 1.0 mass% or less. The remainder is composed of magnesium (Mg) and inevitable impurities, the Al content is a mass%, the Ca content is b mass%, the Si content is c mass%, and the Sn content is d mass%. In this case, 0.35 ≦ b / (a + c + d) ≦ 0.75 and 0.2 ≦ (c + d) ≦ 1.4.

  In order to improve the heat resistance of the magnesium alloy, it is important to disperse as much of the thermally stable high melting point compound as possible in the crystal grain boundaries and / or crystal grains of the magnesium alloy. When Ca is added to the Mg—Al alloy, the Al—Ca—Mg compound is crystallized in the form of a network at the grain boundary, so that the grain boundary sliding at a high temperature and the movement of dislocations at the grain boundary are suppressed, and heat resistance Will improve.

  Further, as described above, even when Ca and Si or Ca and Sn are simultaneously added to the Mg—Al alloy, a coarse compound is formed, and neither heat resistance nor castability is improved. However, when Ca, Si and Sn are added at the same time, fine Ca—Si—Sn—Mg compounds are dispersed and generated, so that the heat resistance is improved by increasing the amount of the compound and casting cracks are increased by increasing the ductility. Reduced.

  Moreover, since crystallization of a coarse compound can be suppressed by simultaneously adding Si and Sn, the ductility of the magnesium alloy is improved, and solidification cracking during casting, particularly during die casting can be suppressed.

The Al—Ca—Mg compound is, for example, Mg ₂ Al ₄ Ca ₃ or (Mg, Al) ₂ Ca. Further, the Ca—Si—Sn—Mg compound is a metastable phase.

  The best mode for carrying out the magnesium alloy of the present invention will be described below.

  In the magnesium alloy of the present invention, when the whole is 100 mass%, Al is 6 mass% or more and 10 mass% or less, Ca is 2.2 mass% or more and 8.0 mass% or less, and Si is 0.1 mass% or more. 1.0 mass% or less, Sn is 0.1 mass% or more and 1.0 mass% or less, Mn is 0.05 mass% or more and 1.0 mass% or less, and remainder consists of Mg and an unavoidable impurity.

  Al contributes to improvement of castability, particularly hot water flowability, and also contributes to strengthening of the alloy and improves mechanical properties. The Al content is 6% by mass or more and 10% by mass or less. If it is less than 6% by mass, the absolute amount of the Al—Ca—Mg compound effective for improving heat resistance is insufficient, and sufficient heat resistance cannot be obtained. On the other hand, when the amount of Al is small, the melting point is increased and the flowability of the molten metal is lowered or casting cracks are increased, so that the castability is lowered. The lower limit of the amount of Al is desirably 7.0% by mass or more. On the other hand, if the Al content exceeds 10% by mass, the absolute amount of the Al—Ca—Mg compound is excessive and the ductility is lowered, so that it is difficult to suppress casting cracks even if Si and Sn are added. In addition, when the amount of Al is large, coarse primary crystal Al—Ca—Mg compound and coarse primary crystal Al—Mn compound are generated, and precipitation of the compound occurs in the molten metal. Will increase. These coarse compounds are not effective in improving heat resistance and greatly reduce castability. The upper limit of the amount of Al is desirably 9.5% by mass or less, and more desirably 9.0% by mass or less.

Ca forms a compound effective for improving heat resistance together with Al, and further suppresses crystallization of the Mg ₁₇ Al ₁₂ phase, which lowers heat resistance, so that the heat resistance of the magnesium alloy is improved. The content of Ca is 2.2% by mass or more and 8.0% by mass or less. When the Ca content is less than 2.2% by mass, the absolute amount of the Al—Ca—Mg compound effective for improving heat resistance is insufficient, and the Mg ₁₇ Al ₁₂ phase tends to appear. The lower limit of the Ca content is desirably 2.5% by mass or more, and more desirably 3.0% by mass or more. When the amount of Ca exceeds 8.0% by mass, a coarse Mg ₂ Ca (Al) phase that lowers castability is easily crystallized. The upper limit of the amount of Ca is desirably 7.5% by mass or less, more desirably 7.0% by mass or less, and 6.0% by mass or less.

Further, as described above, in order to obtain a magnesium alloy having high heat resistance and castability, it is necessary to suppress crystallization of the Mg ₁₇ Al ₁₂ phase, and further, a fine Ca—Si—Sn—Mg system is required. Since it is necessary to form a compound, the Ca content is set to Ca amount ≧ 0.35 × (Al amount + Si amount + Sn amount). Moreover, the casting work of a magnesium alloy is made easy by the flame-retardant effect of Ca. On the other hand, excess Ca leads to the formation of Mg-Ca compounds sensitive to cracking, and casting cracks occur frequently. Therefore, the content of Ca is set to Ca amount ≦ 0.75 × (Al amount + Si amount + Sn amount). That is, when the Al content is a mass%, the Ca content is b mass%, the Si content is c mass%, and the Sn content is d mass%, 0.35 ≦ b / (a + c + d ) ≦ 0.75. The lower limit of b / (a + c + d) is desirably 0.40 or more. Moreover, the upper limit of b / (a + c + d) is desirably 0.7 or less.

  By adding Si and Sn at the same time, fine Ca—Si—Sn—Mg compounds are dispersed and generated, so that heat resistance can be improved and casting cracks can be reduced. The contents of Si and Sn are 0.1% by mass or more and 1.0% by mass or less, respectively, and (Si amount + Sn amount) ≦ 1.4 [% by mass]. When at least one of the Si content and the Sn content exceeds 1.0% by mass and the total content of both exceeds 1.4% by mass, the coarse primary crystal Ca—Si—Mg-based compound or the coarse primary crystal Ca -Sn-Mg-based compounds, and further coarse primary crystal Ca-Si-Sn-Mg-based compounds are easily generated, resulting in an increase in casting cracks and an increased amount of sludge generated. The upper limit of the amount of Si is desirably 0.8% by mass or less, and more desirably 0.5% by mass or less.

  When the amount of Si or Sn is less than 0.1% by mass, a coarse primary crystal Ca—Si—Mg-based compound or a coarse primary crystal Ca—Sn—Mg-based compound is likely to be generated. Further, even if both the Si amount and the Sn amount are 0.1% by mass or more, if the total of both is less than 0.2% by mass, a fine Ca—Si—Sn—Mg compound is not sufficiently generated. In addition, castability may be reduced. The lower limit of the Sn amount is desirably 0.2% by mass or more, and more desirably 0.5% by mass or more.

  That is, 0.2 ≦ (c + d) ≦ 1.4, where the Si content is c mass% and the Sn content is d mass%. The lower limit of (c + d) is desirably 0.5 or more, and more desirably 1.0 or more. The upper limit of (c + d) is desirably 1.2 or less.

  Mn contributes to the improvement of corrosion resistance. However, if it is too small, the corrosion resistance is lowered and a sufficient effect on the corrosion resistance cannot be obtained. The Mn content is 0.05% by mass or more and 1.0% by mass or less. If it is less than 0.05% by mass, the corrosion resistance of the magnesium alloy is significantly reduced. The lower limit of the amount of Mn is desirably 0.15% by mass or more, and more desirably 0.25% by mass or more. On the other hand, if it exceeds 1.0% by mass, Mn in the melt forms a coarse compound such as Al and precipitates, so that the generation of sludge is increased and the amount of Al in the obtained magnesium alloy is reduced to reduce heat resistance. Sex is reduced. The upper limit of the amount of Mn is desirably 0.8% by mass or less, and more desirably 0.5% by mass or less.

  The magnesium alloy of the present invention may further contain one or more selected from the group consisting of strontium (Sr) and rare earth elements (RE). However, in the magnesium alloy of the present invention, Sr and RE may not be contained unless particularly required.

Sr is advantageous for improving heat resistance. However, if the Sr content is excessive, the amount of Mg—Al—Sr compound or Al ₄ Sr produced may increase and ductility may decrease. The upper limit of the amount of Sr is desirably 1.0% by mass or less, and more desirably 0.7% by mass or less. Further, if the lower limit of the Sr amount is defined, it is preferably 0.1% by mass or more, and more preferably 0.3% by mass or more.

  The rare earth element (RE) is dissolved in the primary α-magnesium matrix (Mg crystal grains) and contributes to improvement of heat resistance by solid solution strengthening. Further, RE forms a compound phase at the crystal grain boundary, suppresses grain boundary sliding, and contributes to improvement in heat resistance. However, when RE is excessive, ductility and strength, hot water flowability, and corrosion resistance tend to decrease. Therefore, RE preferably contains 1.0% by mass or less of one or more elements selected from RE in total, and more preferably 0.8% by mass or less. In addition, if the lower limit of the amount of RE is stipulated, the total amount of one or more REs is preferably 0.2% by mass or more, and more preferably 0.3% by mass or more.

  The magnesium alloy of the present invention having the above-described composition includes Mg crystal grains, an Al—Ca—Mg-based compound crystallized in the form of a network at the grain boundaries of the Mg crystal grains, and within and / or the crystal grains. And a metal structure composed of a Ca—Si—Sn—Mg compound dispersed in the boundary. The Al—Ca—Mg compound is a plate-like compound, and crystallizes at a crystal grain boundary to constitute a network. Therefore, grain boundary sliding at a high temperature and dislocation movement at the grain boundary are suppressed, and heat resistance is greatly improved. Furthermore, the Ca—Si—Sn—Mg compound finely dispersed in the crystal grain boundaries and crystal grains is a granular compound, and is effective in further improving heat resistance by suppressing dislocation slip.

  The granular Ca—Si—Sn—Mg compound preferably has a maximum particle size of 10 μm or less, more preferably 5 μm or less. In the magnesium alloy of the present invention having the above composition, the size of the Ca—Si—Sn—Mg compound can be kept small regardless of the production method as long as it is a normal production method. In this specification, the area of the Ca—Si—Sn—Mg compound on the photograph is determined by image analysis software (such as ImagePro) from the structure photograph obtained with an optical microscope, and the equivalent circle diameter is calculated from the area. This is adopted as the particle size of the compound.

  An example of a method for producing a magnesium alloy according to the present invention will be briefly described as follows. That is, this alloy is obtained by adding each element to molten Mg in the form of pure metal, alloy, chloride or fluoride (RE is misch metal, didymium, etc.) and casting by a general method such as die casting. A magnesium alloy casting is obtained. The as-cast magnesium alloy casting mainly has the above-described metal structure composed of Mg crystal grains, an Al—Ca—Mg compound, and a Ca—Si—Sn—Mg compound.

  The magnesium alloy according to the present invention has good castability, and die casting, die gravity casting, sand casting, thixo molding, high pressure casting, low pressure casting and the like are suitable as a casting method. The die casting may be a cold chamber method or a hot chamber method.

  The casting made of the magnesium alloy of the present invention can be applied to parts that require both weight reduction and heat resistance in various fields such as automobiles and electrical equipment, including the fields of space and aviation. Examples include, but are not limited to, a cylinder head cover, a cylinder block, a piston, a transmission case, an oil pan, an intake manifold, a chain cover, and a crankcase of a vehicle.

  As mentioned above, although embodiment of the magnesium alloy of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the present invention will be specifically described with reference to examples of the magnesium alloy of the present invention.

[Production of Mg-Al-Ca-Si-Sn-Mn alloy castings]
In order to have the composition shown in Table 1, the contents of Al, Ca, Si and Sn (a, b, c and d in this order) are changed, and the values of b / (a + c + d) and (c + d) are changed. The raw materials were blended so as to change, and Mg—Al—Ca—Si—Sn—Mn alloy castings (# 01 to # 10: Examples, # C01 to C10: equivalent to comparative examples) were prepared by the following procedure. . Further, Sr or misch metal (Mm: including cerium, lanthanum, praseodymium and neodymium) was added as necessary. In addition, the composition shown in Table 1 is a target value.

  Two types of castings were produced: a shape A casting for producing a test piece for measuring the axial force retention rate, and a shape B casting for evaluating castability. Each production procedure is shown below.

[Production of casting with shape A]
The raw material was dissolved using a melting crucible made of stainless steel (SUS430). The dissolution was performed using an electric furnace in a state where a small amount of SF ₆ gas was sprayed as a flameproof gas. After the molten metal temperature was 720 ° C., the molten metal was kept calm in the furnace for 30 minutes. The crucible was taken out of the furnace, and a 20 mm × 20 mm × 48 mm die casting was produced using a vertical die casting machine. The casting conditions were as follows: injection speed 0.3 m / s, injection pressure: 28 MPa, pressurization time: 5 seconds, mold temperature: room temperature, injection molten metal temperature: liquidus temperature + 30 ° C. The obtained die casting is shown in FIG.

  In addition, the measurement of the axial force holding | maintenance rate demonstrated later used the ring-shaped test piece (right figure) cut out from the center part of the die-casting casting (left figure) shown in FIG. The ring-shaped test piece had an outer diameter of 20 mm, an inner diameter of 9 mm, and a height of 10 mm.

[Production of casting with shape B]
The raw material was dissolved using a melting crucible made of stainless steel (SUS430). The dissolution was performed using an electric furnace in a state where a small amount of SF ₆ gas was sprayed as a flameproof gas. After the molten metal temperature was 720 ° C., the molten metal was kept calm in the furnace for 30 minutes. The crucible was taken out from the furnace, and a die cast casting having a thick part at both ends and a thin part (thickness 3 mm) at the center part was produced using a vertical die casting machine. The casting conditions were an injection speed of 1 m / second, injection pressure: 64 MPa, pressurization time: 1 second, mold temperature: 80 ° C., injection molten metal temperature: liquidus temperature + 50 ° C. FIG. 2 shows a front view and a side view of the obtained die casting.

[Evaluation]
For # 01 to # 10 and # C01 to # C10, the axial force retention rate was measured and the casting cracks were observed to evaluate the heat resistance and castability. The results are shown in Table 1.

[Measurement of axial force retention]
A method for measuring the axial force retention will be described with reference to FIG. For the measurement of the axial force retention rate, the ring-shaped test piece 10, the bolt 20 and the aluminum block 30 are used.

  As the bolt 20, a steel M8 × 25 (strength division 10.9) hexagon bolt was used. The aluminum block 30 was an aluminum die cast alloy member of ADC12 (JIS standard). The aluminum block 30 is formed with a screw hole 31 having a female screw that is screwed with a male screw formed on the outer peripheral surface of the shaft portion 21 of the bolt 20.

  In the measurement, the shaft portion 21 of the bolt 20 is inserted into the insertion hole of the test piece 10 through the washer 15 (outer diameter φ18 mm, thickness 3 mm, A6061-T6), and the screw hole 31 of the aluminum block 30 is inserted. And an initial axial force of 8 kN. At this time, the strain gauge 40 for measuring the axial force was fixed to the bolt 20, and the initial axial force of the bolt 20 was measured by the strain gauge 40.

  The test piece 10 fastened by the bolt 20 and the aluminum block 30 was placed in a high-temperature thermostatic bath, kept at a high temperature at 180 ° C. for 300 hours, and then cooled to room temperature. Thereafter, the axial force (residual axial force) of the bolt 20 was measured. And the ratio of the residual axial force with respect to the above-mentioned initial axial force was calculated, and the axial force retention was obtained. In addition, the axial force retention was determined as a plurality of average values.

  The value of the axial force retention shown in Table 1 is the ratio (ADC12 ratio) of the axial force retention of each alloy when the axial force retention of the ADC12 alloy, which is a general-purpose aluminum alloy, is 100. The ratio of the axial force retention rate was evaluated as “◯” when 80% or more and “×” when less than 80%.

[Observation of casting cracks]
A die-cast casting having the shape shown in FIG. 2 was produced, and a casting crack around the thin portion at the center of the casting, which was generated by restraint from the mold at the upper and lower portions, was observed. The observation of the casting crack was performed immediately after the mold was opened and before the casting was taken out from the mold, and the presence or absence of a crack generated in the casting was visually confirmed.

  # 01 to # 10 in which the contents of Al, Ca, Si, Sn, and Mn were predetermined values were magnesium alloys having excellent heat resistance and castability. In addition, if Mn content exists in the range of 0.25 mass%-0.5 mass%, the characteristic equivalent to # 01- # 10 will be shown.

  # C01 having an Al content of 4.5% by mass had very low axial force retention and low heat resistance. This is because the crystallization of the Al—Ca—Mg compound was insufficient. As the Al content increases, the heat resistance tends to be improved, and when 7.0 mass% or more of Al is included, the axial force retention is 85% or more (# 05, # 06 and # 06). 10). However, # C10 containing 12% by mass of Al caused casting cracks because the crystallization amount of the Al—Ca—Mg compound was too large and the ductility was lowered.

  Even if the Al content is in a desirable range, the relationship between the contents of Al, Ca, Si and Sn, # C02 having a b / (a + c + d) value of 0.33 is Al-Ca-Mg which affects the heat resistance. The production of the compound was insufficient and the heat resistance was low. As the value of b / (a + c + d) is larger, the heat resistance tends to be improved. The axial force retention exceeds 80% at 0.40 or more, and 85% or more at 0.55 or more. However, casting cracks were observed in # C07, which is 0.79. Also, the axial force retention rate was not so large.

  Further, # C06 and # C08 in which the total content of Si and Sn (c + d) was 1.5 had low castability. When (c + d) was 1.4 or less, casting cracks were reduced, but # C03 to 05 containing no Si and / or Sn had low castability.

[Tissue observation]
Metal structures were observed for # 04 and # C03 to # C05. The observation results are shown in FIGS. In addition, the structure observation was performed using the optical microscope with respect to the cross section of each die-casting.

  In # 04 which is the magnesium alloy of the present invention, Mg crystal grains (the brightest part in FIG. 4), an Al—Ca—Mg-based compound crystallized at the crystal grain boundary, and Ca—Si— finely dispersed throughout. A Sn—Mg compound (the darkest part in FIG. 4) was observed. These granular Ca—Si—Sn—Mg compounds did not exceed 10 μm in particle size, and the average particle size was about 3.2 μm.

  In other words, # 04 in which fine Ca—Si—Sn—Mg compound is dispersed and crystallized by simultaneously adding Si and Sn to the Mg—Al—Ca alloy within a predetermined range has excellent heat resistance. And a magnesium alloy having castability. Although not shown in the drawings, no Ca—Si—Sn—Mg compound having a particle size exceeding 10 μm was observed in all the structural observations in # 01 to # 03 and # 05 to # 10.

  In # C03, which is an Mg—Al—Ca alloy, only Mg crystal grains and Al—Ca—Mg compounds crystallized at the crystal grain boundaries were observed (FIG. 5). On the other hand, a coarse Ca—Si—Mg-based compound was observed in # C04 in which only Si was added to the Mg—Al—Ca alloy (FIG. 6). In # C05 in which only Sn was added to the Mg—Al—Ca alloy, coarse Ca—Sn—Mg based compounds were observed (FIG. 7). The formation of coarse compounds caused not only castability degradation but also heat resistance degradation.

  That is, it was found that a magnesium alloy obtained by simultaneously adding a predetermined amount of Si and Sn to an Mg—Al—Ca—Mn alloy is a magnesium alloy having excellent heat resistance and castability.

It is explanatory drawing which shows the cutout form of the test piece used for the measurement of axial force retention. It is the front view (left figure) and side view (right figure) which show the shape of the test piece which evaluates heat resistance. It is explanatory drawing which shows the measuring method of an axial force retention, Comprising: It is an axial sectional view of each member. It is a drawing substitute photograph which shows the metal structure of # 04 magnesium alloy. It is a drawing substitute photograph which shows the metal structure of the magnesium alloy of # C03. It is a drawing substitute photograph which shows the metal structure of the magnesium alloy of # C04. It is a drawing substitute photograph which shows the metal structure of the magnesium alloy of # C05.

Explanation of symbols

10: Test piece (magnesium alloy)
20: Bolt 30: Aluminum block 40: Strain gauge

Claims (8)

  When the total is 100 mass%, aluminum (Al) is 6 mass% to 10 mass%, calcium (Ca) is 2.2 mass% to 8.0 mass%, and silicon (Si) is 0.1 mass. % To 1.0% by mass, tin (Sn) 0.1% to 1.0% by mass, manganese (Mn) 0.05% to 1.0% by mass, with the balance being magnesium ( Mg) and unavoidable impurities, and when the Al content is a mass%, the Ca content is b mass%, the Si content is c mass%, and the Sn content is d mass%, it is 0. Magnesium alloy characterized in that 35 ≦ b / (a + c + d) ≦ 0.75 and 0.2 ≦ (c + d) ≦ 1.4.   The magnesium alloy according to claim 1, wherein the contents of Si and Sn are 1.0 ≦ (c + d) ≦ 1.4.   The magnesium alloy according to claim 1, wherein the contents of Al, Ca, Si and Sn are 0.40≤b / (a + c + d) ≤0.75.   Mg crystal grains, an Al—Ca—Mg-based compound crystallized in a network at the grain boundaries of the Mg crystal grains, and Ca—Si— dispersed in the Mg crystal grains and / or at the crystal grain boundaries The magnesium alloy according to any one of claims 1 to 3, which has a metal structure composed of an Sn-Mg compound.   The magnesium alloy according to claim 4, wherein the Ca—Si—Sn—Mg compound has a maximum particle size of 10 μm or less.   Furthermore, the magnesium alloy in any one of Claims 1-5 containing 1.0 mass% or less of strontium (Sr).   Furthermore, the magnesium alloy in any one of Claims 1-5 containing 1.0 mass% or less of 1 or more types of elements chosen from rare earth elements (RE) in total.   A magnesium alloy casting comprising the magnesium alloy according to any one of claims 1 to 7.