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 alpha-Mg matrix, Mg24Y5, Mg41Nd5 and beta(1) phases. After solution treatment at 525 degrees C for 6 h, the eutectics dissolved into alpha-Mg matrix, leading to a huge improvement of plasticity. Alloys peak-aged at 200 degrees C and 225 degrees C exhibit high strength due to fine dense beta '' and beta' precipitates. Aged at 250 degrees C for 10 h, the precipitates in the alloy are coarse sparse coupled beta(1)/beta' 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 degrees 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. (C) 2016 Elsevier B.V. All rights reserved.