Effects of solid solution treatment and cooling on morphology of LPSO phase and precipitation hardening behavior of Mg-Dy-Ni alloy

Effects of solid solution treatment and cooling on the morphology of long period stacking order (LPSO) phase and precipitation hardening behavior of Mg-2Dy-0.5Ni (molar fraction, %) alloy were investigated. Microstructures of the as-cast alloy mainly consisted of α-Mg phase, bamboo-like Mg12DyNi phase with LPSO structure distributed between dendrites and small amounts of cubic Dy phases. During solid solution treatment at 565 °C for 12 h and subsequent different cooling conditions, dot-shaped, block, fine lamellar and rod-shaped LPSO phases precipitate in Mg matrix, respectively. For continuous cooling conditions (furnace and air cooling), the fine lamellar LPSO phase generally forms in grain interior and its volume fraction increases and block LPSO phase coarsens with increasing cooling time. For discontinuous cooling conditions (air cooling after furnace cooling to 415 and 265 °C), the dot-shaped LPSO grows into the rod-shaped phase, which results in an decrease of cooling hardening behavior of alloy.