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

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.