The effect of morphology of the long-period stacking ordered phase on mechanical properties of the Mg-7Gd-3Y-1Nd-1Zn-0.5Zr (wt.%) alloy

The effect of morphology of the long-period stacking ordered (LPSO) phase on mechanical properties of the Mg-7Gd-3Y-1Nd-1Zn-0.5Zr (wt.%) alloy have been studied. The results reveal that the block-like 14H-LPSO phases in and near grain boundaries can be formed during heat treatment at high temperature, but low temperatures is conducive to form the needle-like 14H-LPSO phases within α-Mg grains. During heat treatment at fixed temperature for different time, the preferential nucleation sites of the 14H-LPSO phases are located in grain boundaries, then the14H-LPSO phases grow into the α-Mg matrix in the form of needle-like, the needle-like 14H-LPSO phases gradually decrease its length after growing a certain stage, at last the needle-like 14H-LPSO phases disappear completely. The decomposition of the block-like 14H-LPSO phases is found during furnace cooling after homogenization. After decomposition of the block-like 14H-LPSO phases, the rapid diffusion of Zn atoms into α-Mg matrix leads to form the needle-like 14H-LPSO phases within α-Mg matrix, but the retention of RE atoms in grain boundaries results in forming the Mg-RE particles. After extrusion and subsequent ageing, it is found that the alloy with the needle-like 14H-LPSO phases exhibit superior elongation than the alloy without the needle-like 14H-LPSO phase. The precipitates of peak-aged alloys are studied by TEM. The <112−0>α demonstrates that the stacking faults can be observed, the serious lattice distortion appears near the stacking fault. The [0001] zone axis shows that the β′ and a new rod phase are found, and the enrichment of RE atoms are detected in the two phase by HADDF-STEM. It seems to be that the rod phase can be subdivided into a plurality of mutually parallel structure unit.