Effects of extrusion ratio and temperature on the mechanical properties and microstructure of as-extruded Mg-Gd-Y-(Nd/Zn)-Zr alloys

This study investigates the effects of extrusion ratio and temperature on the microstructure and mechanical properties of as-extruded Mg-11.5Gd-4.5Y-(1Nd/1.5Zn)-0.3Zr (wt %) alloys. After hot extrusion the studied alloys exhibit a bimodal microstructure consisting of fine dynamic recrystallized (DRXed) grains with relatively random orientations and coarse un-DRXed grains with strong basal texture. The increase of extrusion ratio promotes the DRX, increases the volume fraction of Mg₅RE phase, and refines the DRXed grains, whereas the increase of extrusion temperature decreases the volume fraction of Mg₅RE phase and coarsens the DRXed grains. The increase of extrusion temperature suppresses the DRX of Mg-11.5Gd-4.5Y-0.3Zr (GW) and Mg-11.5Gd-4.5Y-1Nd-0.3Zr (GWN) alloys, but it has a limited effect on that of Mg-11.5Gd-4.5Y-1.5Zn-0.3Zr (GWZ) alloy. The increase of extrusion ratio improves the mechanical properties of GWZ and GWN alloys, while it deteriorates the mechanical property of GW alloy. The increase of extrusion temperature leads to a decreased strength and increased ductility of the studied alloys. The change of mechanical properties is a result of the competition between the “strengthening effect” of DRXed grains and the “hardening effect” of un-DRXed grains in the changing bimodal microstructure. The Mg₅RE phase also contributes to the alloy strengthening, but the extensive Mg₅RE precipitates acting as the crack resources are detrimental to the ductility, especially for the GWN alloy. With the optimum extrusion condition (temperature of 450 °C and ratio of 20:1) the GWZ alloy exhibits the best mechanical performance, which is superior to that of its competitor 6000 series aluminum alloys.