Study on the mutual effect of La and Gd on microstructure and mechanical properties of Mg-Al-Zn extruded alloy

Abstract

The effects of rare earth elements (La, Gd) addition on the microstructure and the mechanical properties of as-extruded Mg-Al-Zn magnesium alloy under conditions of a medium temperature (230 °C) and a slow ram speed (0.1 mm/s) have been throughly investigated. The results show that the as-extruded bar presents smooth surface from the macroscopic view due to the fact that many non-basal <a> dislocations and few pyramidal 〈c + a〉 dislocations could be activated to enhance the formability in the extrusion process. A bimodal grain structure composed of fine dynamic recrystallized (DRXed) grains with globular Mg17Al12 precipitates located at grain boundaries and non-recrystallized (non-DRXed) coarse grains with thin continuous precipitation laths inside is generated in the alloy. More dislocation pile-ups and subgrain boundaries are exhibited in the DRXed region which shows a typical texture of extruded alloys containing rare earth elements. The modified extruded alloy shows much higher strength than that of as-extruded AZ80 under either tension or compression condition owing to grain refinement strengthening, precipitation strengthening, solid-solution strengthening, dislocation strengthening and subgrain strengthening. The finer DRXed grain size and the fine precipitates in the modified extruded alloy bring about its lower yield anisotropy. In addition, large and fragile Al2Gd and Al8Mn4Gd phases mainly give rise to the decrease in ductility of the modified extruded alloy.