Effects of Ni Content on Solidification Behavior and Hot Tearing Susceptibility of LPSO-Reinforced Mg-4Y-xNi alloys

In this work, the solidification behavior and hot tearing susceptibility (HTS) of Mg-4Y-xNi (x = 0.5, 1, 2, 3, 4 wt%) alloys were investigated through the thermal analysis (TA) method and a constrained rod casting system. The solidification characteristics parameters measured by TA method were coupled with the prevailing hot tearing criteria for predicting HTS of the alloys, and then compared with the experimental results. Subsequently, the hot tearing mechanism was studied upon shrinkage stress development, solidification characteristics and microstructure evolutions. The results showed that Kou's criterion accurately and effectively predicted HTS of the alloys, that is, HTS of the alloys exhibited a Lambda-shaped trend with increasing Ni content, peaking at a Ni content of 1 wt%. HTS of the alloys depended primarily on the dendrite coherency, residual liquid and LPSO-bridging. The delay in dendrite coherency prolonged the free feeding interval, which in conjunction with the increased residual liquid fraction improved the feeding capacity of the alloys at the end of solidification and decreased HTS. The higher degree of bridging between adjacent grains of alloys with higher solid fraction at dendrite coherency implies a higher skeleton strength of the alloys. In particular, when the LPSO phase precipitates at grain boundaries to form grain boundary bridges, it effectively hinders the initiation and propagation of hot tearing. This is due to its certain orientation relationship with the alpha-Mg matrix, which will significantly increase the grain boundary strength by strongly pinning the grains on both sides.