| Strontium and its master alloy is an effective modification agent, has been widely used in the aluminum-silicon alloy, magnesium alloy has also been some research, but research is not very systematic. Al-Sr master alloy was most widely used strontium-containing master alloy in magnesium alloy. The reported of Mg-Sr master alloy renfinment magnesium alloy was very little. But up to now, only the effects of Mg-Sr binary master alloys on the grain refinement of magnesium alloys are reported. But the grain size and second phase of Mg-Sr master alloy were very bulkly, so the modification effect of Mg-Sr master alloy was restrained.In order to improve the modification efficiency of Mg-10Sr master alloy, in the present work, addition calcium elements in Mg-lOSr master alloy, refined Mg-10Sr master alloy dendrite size, changed the second phase morphology and distribution, enhance its modification effect. Mg-Zn-Al alloy has good high temperature performance, lower cost and better casting performance, is considered to be a promising high-temperature creep resistant magnesium alloy. However, the tensile strength and impact toughness of the alloy are still to be further improved. Addition Mg-10Sr-xCa(x=0,0.5,1.0,1.5) master alloy in ZA125alloys modification, in order to achieve the purpose of improving its mechanical properties. The study results showed that:With the increased of Ca addition, the secondary dendrite arm spacing(SDAS) of Mg-10Sr master alloy was gradually decreased. Because of Ca can efficiency restrained the grain growth of a-Mg. When the amount of Ca is1.5%, the second dendrite arm spacing reaches a minimum,8.08μm than not adding Ca reduced23.2%, refining effect is very obvious.The microstructure of Mg-lOSr master alloy consists of a-Mg matrix and Mg17Sr2phase. When Ca addition in Mg-10Sr master alloy, its microstructure changed significantly, the particle Mg17Sr2phase at grain boundaries is disappeared, the number of block Mg17Sr2gradually increased, reduced number of Mg17Sr2phase, and formed Mg2Ca eutectic phase. Inadditon, when the increase of Ca addition, the segregation of Sr elements at interdendrite area increased gradually. Addition1.5%Ca,the segregation of Sr elements increased23.6%than Mg-10Sr master alloy.The thermal stability test of Mg-10Sr master alloys and Mg-10Sr-1.5Ca by differential scanning calorimetry(DSC), the α-Mg and Mg17Sr2melting point of Mg-10Sr master alloy was624.1℃and591.7℃;The α-Mg and Mg17Sr2phase melting point of Mg-10Sr-1.5Ca was614.1℃and583.1℃, decreased10℃and8.6℃than Mg-10Sr master alloy. The thermal stability of the master alloy was decreased when Ca addition. Inaddition, the melting point of Mg2Ca phase was514.8℃in Mg-10Sr-1.5Ca master alloy.Due to interdendritic segregation of Sr elements increased and thermal stability of Mg17Sr2phase decreased, so the number of free Sr atomic and the diffusion rate of free Sr atomic were increased. According to the mechanism of grain refinement of Sr elements, adding Ca to Mg-lOSr master alloy maybe improved its modification effect.The microstructure of ZA125high zinc magnesium with Mg-lOSr master alloy consist of a-Mg matrix phase, T-Mg32(Al, Zn)49and Mg51Zn2o phase; When addition Ca-containing Mg-10Sr master alloy in ZA125alloy, the microstructure consist of a-Mg matrix phase, τ-Mg32(Al, Zn)49and, Mg51Zn2o phase and a small amount of Ca2Mg6Zn3phase. Addition Mg-10Sr-xCa master alloy in ZA125high zinc magnesium alloy, the microstructure morphology of ZA125alloy changed greatly. The coarse net-like phase at boundaries gradually broken into small semi-continuous shape or long strip, and the number of lamellar eutectic Mg51Zn20phase decreased. When the Mg-10Sr-1.5Ca master alloys, the gray massive phase was formed.With the Ca content of the master alloy increased, the room and high temperature tensile strength of ZA125high zinc magnesium alloy gradually increased. When the master alloy containing Ca to1.5wt%, the room and high temperature tensile strength of the ZA125alloy reached the maximum197MPa and170MPa than the master alloys without Ca increased of21.4%and25.9%, while the elongation of alloy at room temperature and high temperature is also gradually increased, reaches a maximum at2.4%and5.7%, respectively, were increased by85.6%and83.9%. Impact toughness and Brinell hardness of the alloy is also gradually increased with the increase in the amount of Ca-containing master alloy reaches a maximum when the Ca content as1.5wt%, for7.42J/cm2and67.5HB, respectively, than adding not Ca-containing master alloys increased44%and29.1%, respectively.Due to adding Mg-10Sr-xCa master alloy to ZA125alloy modification, the performance of alloy has been significantly improved, so Ca can be obviously increased the modification effect of Mg-10Sr master alloy. |
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