Effect of heat treatment on microstructure, mechanical properties and fracture behaviors of sand-cast Mg-4Y-3Nd-1Gd-0.2Zn-0.5Zr alloy

Microstructure, mechanical properties and fracture behaviors of sand-cast Mg-4Y-3Nd-1Gd-0.2Zn-0.5Zr (wt%) alloy in different thermal conditions were investigated. The as-cast alloy was comprised of α-Mg matrix, Mg24Y5, Mg41Nd5 and β1 phases. After solution treatment at 525 °C for 6 h, the eutectics dissolved into α-Mg matrix, leading to a huge improvement of plasticity. Alloys peak-aged at 200 °C and 225 °C exhibit high strength due to fine dense β″ and β′ precipitates. Aged at 250 °C for 10 h, the precipitates in the alloy are coarse sparse coupled β1/β′ phases. A desirable combination of strength and ductility was obtained with 198 MPa in yield strength, 276 MPa in ultimate tensile strength and 7.6% in elongation when over-aged at 250 °C for 10 h. Alloys under different thermal conditions show different fracture behaviors, which are closely linked to the structures of both inner grains and grain boundaries.