Mg-6Al-xNd(x=2,4)合金的组织结构与凝固行为

Microstructure and Solidification Behavior of Mg-6Al-xNd(x=2,4) Alloys

Nd添加到Mg-6Al合金后,析出针状的Al11Nd3和多边形状的Al2Nd,前者主要分布于枝晶界或晶界,后者主要分布于晶粒内.两相含量均随Nd含量增加而增加,其中前者含量明显高于后者.含Nd合金的凝固过程经历Al2Nd析出、α-Mg析出、Al11Nd3+α-Mg同时析出等过程.合金热裂倾向随着Nd含量的增加而升高,这是由于Al11Nd3和Al2Nd析出温度高,导致合金凝固区间收缩和共晶相对量减少,降低了剩余液相的流动性;枝晶界或晶界上的针状Al11Nd3相增加了合金凝固后期流动补缩的难度;Al11Nd3相和α-Mg凝固收缩率不同,易导致两者界面上产生较高的热应力.

The needle-like Al11Nd3 phase and irregular polygon-like Al2Nd phase are the existing forms of Nd in Mg-6Al alloy prepared by metal mould casting method.The former is located on dendritic and grain boundaries while the latter is situated within grains interior.The weight fractions of Al11Nd3 and Al2Nd phase both increase with increase in the content of Nd and the former is much more than the latter.The solidification path of Mg-6Al-xNd（x=2,4） alloys experiences several stages mainly including the precipitation of Al2Nd,the precipitation of part of α-Mg and the simultaneous precipitation of Al11Nd3 and part of α-Mg.The hot cracking susceptibility increases with increase in the content of Nd,which results from three factors as below： The precipitation of Al11Nd3 and Al2Nd leads to the shrinkage of solidification gap and decreases the liquidity of surplus liquid phase.The needle-like Al11Nd3 phase located in dendritic and grain boundaries makes it difficult for the surplus liquid phase to flow and feed.It is easy for the thermal stress to occur due to the difference in solidification shrinkage rate between Al11Nd3 and α-Mg.