Influence of Nd on Hot Tearing Susceptibility and Mechanism of Mg-Zn-Y-Zr Alloys

The influence of Nd on the hot tearing susceptibility(HTS) of Mg-4.5Zn-1Y-xNd-0.5Zr (x = 0, 1, 2, 3 wt.%) alloy was systematically studied by using Clyne–Davies model, the Newton baseline method based on the thermal analysis and the “T” hot tearing die with temperature and force testing system. The HTS of the alloy decreased first and then increased with Nd content increase, reaching the minimum value at the Nd content of 2 wt.%. Based on the results of thermal analysis, the solidification path of multi-component alloy was studied, and the evolution of phase precipitation and microstructure was discussed by SEM, EBD and TEM. It was found that the precipitation of W-phase, which has a relatively high precipitation temperature and a certain coherent relationship with the parent phase, and reduced the intergranular hot tearing tendency. The results show that with the increase in Nd content to 2 wt.%, the decrease in HTS was due to α-Mg grain refinement, and the entry of Nd atoms into phase I and phase W, so that the ratio of phase I with low melting point to phase W decreases. When the content reaches 3 wt.%, the reason of HTS decrease was α-Mg grain turn to coarsening and the appearance of another T-phase with much lower melting point than W-phase, i.e., ternary (MgZn)₉₂Nd₈ phase. It was considered that the smooth feeding channels caused by fine and uniform α-Mg grains and the high grain boundary bridging strength from the higher precipitation temperature of W-phase with a certain crystallographic relationship to the matrix leads to the lowest HTS of the Mg-4.5Zn-1Y-2Nd-0.5Zr alloy.