The effect of rare earths （RE） ranging from 0.1% to 1.2%（mass fraction） on hot cracking resistant property of Mg-Al alloys was investigated. The results show that hot cracking resistant property of Mg-Al alloys re...The effect of rare earths （RE） ranging from 0.1% to 1.2%（mass fraction） on hot cracking resistant property of Mg-Al alloys was investigated. The results show that hot cracking resistant property of Mg-Al alloys remarkably declines with an increase of RE addition. The causes of the decline are as follows： First, grain coarsening of Mg-Al alloys caused by RE addition reduces the fracture strain required for hot crack initiation. Second, RE reduces the eutectic microstructure of Mg- Al alloys, and as a result, shortens the time that the feeding channel remains open, making it difficult to feed the alloy. Furthermore, RE elevates the eutectic reaction temperature, which leads to the decrease in the strength of the interdendritic liquid film at the late stage of solidification. Third, when a-Mg dendrites form continuous skeletons, the interdendritic Al11 RE3 phase tends to block the feeding channels and increases the difficulty of feeding. Last, the shrinkage ratio discrepancy between Al11RE3 phases and α-Mg matrix is prone to cause shrinkage stress and promote hot crack initiation.展开更多
Small addition of mischmetal （MM） into aluminum alloys can lead to grain refinement. However, it is still uncertain whether the same effect applies to Mg-Al alloys. This work indicated that small amount of mischmeta...Small addition of mischmetal （MM） into aluminum alloys can lead to grain refinement. However, it is still uncertain whether the same effect applies to Mg-Al alloys. This work indicated that small amount of mischmetal addition ranging from 0.1% to 1.2% （mass fraction） did not cause grain refinement in Mg-Al alloys. On the contrary, they tended to coarsen the grains. When added into Mg-Al alloys, MM reacted preferentially with Al to form Al11 MM3 phase. As Al11 MM3 phase mainly distributed within α-Mg grains than at grain boundaries, it had little effect in restricting grain growth. In addition, MM reacted with Al8（Mn, Fe）5 or ε-AlMn particles to form Al-MM-Mn compounds, thus it reduced the amount of heterogeneous nuclei in the melt and resulted in remarkable grain coarsening.展开更多
Adding yttrium to magnesium can improve the mechanical properties, especially the mechanical behavior at high temperature. The valence electron structures of Mg-Y alloy were analyzed with the empirical electron theory...Adding yttrium to magnesium can improve the mechanical properties, especially the mechanical behavior at high temperature. The valence electron structures of Mg-Y alloy were analyzed with the empirical electron theory of solids and molecules (EET). Calculation shows that yttrium is on the fifth hybrid level and magnesium is on the third one in pure metal crystals. By comparing with aluminum in Mg-Al alloy, it is shown that the reason why the solution strengthening effect of yttrium is better than that of aluminum in Mg-Al alloy is the enhanced bond energy according to EET. And it is concluded from EET analysis that intermetallics Mg_(24)Y_5 can significantly improve the properties of magnesium alloys because yttrium atoms occupy the centers of the octahedron and Mg-Y bonds efficiently strengthen the alloy matrix.展开更多
文摘The effect of rare earths （RE） ranging from 0.1% to 1.2%（mass fraction） on hot cracking resistant property of Mg-Al alloys was investigated. The results show that hot cracking resistant property of Mg-Al alloys remarkably declines with an increase of RE addition. The causes of the decline are as follows： First, grain coarsening of Mg-Al alloys caused by RE addition reduces the fracture strain required for hot crack initiation. Second, RE reduces the eutectic microstructure of Mg- Al alloys, and as a result, shortens the time that the feeding channel remains open, making it difficult to feed the alloy. Furthermore, RE elevates the eutectic reaction temperature, which leads to the decrease in the strength of the interdendritic liquid film at the late stage of solidification. Third, when a-Mg dendrites form continuous skeletons, the interdendritic Al11 RE3 phase tends to block the feeding channels and increases the difficulty of feeding. Last, the shrinkage ratio discrepancy between Al11RE3 phases and α-Mg matrix is prone to cause shrinkage stress and promote hot crack initiation.
基金Project supported by Beijing Municipal Science & Technology Research Project (Y0405004040211)
文摘Small addition of mischmetal （MM） into aluminum alloys can lead to grain refinement. However, it is still uncertain whether the same effect applies to Mg-Al alloys. This work indicated that small amount of mischmetal addition ranging from 0.1% to 1.2% （mass fraction） did not cause grain refinement in Mg-Al alloys. On the contrary, they tended to coarsen the grains. When added into Mg-Al alloys, MM reacted preferentially with Al to form Al11 MM3 phase. As Al11 MM3 phase mainly distributed within α-Mg grains than at grain boundaries, it had little effect in restricting grain growth. In addition, MM reacted with Al8（Mn, Fe）5 or ε-AlMn particles to form Al-MM-Mn compounds, thus it reduced the amount of heterogeneous nuclei in the melt and resulted in remarkable grain coarsening.
文摘Adding yttrium to magnesium can improve the mechanical properties, especially the mechanical behavior at high temperature. The valence electron structures of Mg-Y alloy were analyzed with the empirical electron theory of solids and molecules (EET). Calculation shows that yttrium is on the fifth hybrid level and magnesium is on the third one in pure metal crystals. By comparing with aluminum in Mg-Al alloy, it is shown that the reason why the solution strengthening effect of yttrium is better than that of aluminum in Mg-Al alloy is the enhanced bond energy according to EET. And it is concluded from EET analysis that intermetallics Mg_(24)Y_5 can significantly improve the properties of magnesium alloys because yttrium atoms occupy the centers of the octahedron and Mg-Y bonds efficiently strengthen the alloy matrix.
