Effects of Gd on hot tearing susceptibility of as-cast Mg96.94-Zn1-Y(2−x)-Gdx-Zr0.06 alloys reinforced with LPSO phase

Abstract

In the present work, the effects of Gd on hot tearing susceptibility (HTS) of as-cast Mg96.94-Zn1-Y(2−x)-Gdx-Zr0.06 (x = 0, 0.5, 1, 1.5, 2 at%) alloys reinforced with long-period stacking ordered (LPSO) phase was studied. The HTS of the alloys was predicted and confirmed by differential thermal analysis system and hot tearing test system, respectively. Based on the cooling curves, shrinkage stress curves and microstructure evolution observed by SEM, TEM and EBSD, the hot tearing mechanism of the alloys was explored. The results showed that the HTS of Mg96.94-Zn1-Y(2−x)-Gdx-Zr0.06 alloys significantly decreased with the increasing of Gd content. When Gd content was 0.5 and then increased to 1 at%, the decrease in HTS of the alloys was attributed to the combined effects of grain refinement and increased secondary phase amount. When Gd content increased to 1.5 and then to 2 at%, the grain size of the alloys increased instead, and the HTS continued to decrease. This was attributed to the increase of pinning effect of LPSO phase and the increase of residual liquid feeding capacity caused by the increase of precipitation amount of the secondary phase. The combined effects of the two made up for the negative effects of grain coarsening. In Mg96.94-Zn1-Gd2-Zr0.06 alloy, the amount of LPSO phase was the most, and the pinning effect on grain boundaries was the strongest. The intergranular bridges composed of LPSO phase hindered the initiation and propagation of hot tearing, thus, the alloy had the lowest HTS.