Microstructure and mechanical properties of Mg-Gd-Y-Zn-Zr alloy prepared by rheo-diecasting

Microstructure and mechanical properties of thin-walled Mg-11Gd-3Y-1Zn-0.2Zr (GWZ1131K, wt.%) alloy castings prepared by rheo-diecasting were characterized and analyzed in as-cast and solution-treated states, and compared with those of conventional diecasting. Rheo-diecast and conventional diecast alloys consist of externally solidified grains (ESGs) with very different morphologies and secondary solidified grains (SSGs) delineated by a network of eutectic phases ((Mg,Zn)(3)(Gd,Y)). After solution treatment of rheo-diecast alloy, the initial spherical or rose-shaped ESGs evolved into conventional grains with visible grain boundaries. Meanwhile, the brittle eutectic phases ((Mg,Zn)(3)(Gd,Y)) were transformed into H-14-type long-period stacking ordered (LPSO) structures and partially dissolved into alpha-Mg matrix, thus forming massive blocky X phases and a few lamellar LPSO structures. The room-temperature tensile mechanical properties of rheo-diecast alloy were unexpectedly decreased compared to the conventional diecast alloy, which may be resulted from the large average grain size and the aggregated distribution of eutectic phases. Due to the dissolution of eutectic phases and the contribution of LPSO structures and solution strengthening, the solution-treated rheo-diecast alloy offers a significant improvement of elongation (EL: 6.1 +/- 0.8%) and a slight increase in ultimate tensile strength (UTS: 250 +/- 10 MPa).