The huge variation of microstructure evolution and mechanical properties controlled by the 50℃ extrusion temperature difference

A slight difference in extrusion temperatures (450℃ and 500℃) brought about significant variation in the mechanical properties of Mg-9.5Gd-4Y-2Zn-0.5Zr (wt%) alloy during the ultra-large ratio (100:1) hot extrusion. Such variation was investigated through the comparison of microstructure evolution as well as its effect on the mechanical properties. Results showed that the sample extruded at 450℃ (T450 sample) possessed a finer average grain size of 2.3μm due to dynamic recrystallization (DRX), while increasing the extrusion temperature to 500℃ caused the obvious migration of grain boundaries in the extrusion sample (T500 sample), coarsening the grain size to 7.9μm. Importantly, such DRXed grains in the T450 sample showed random orientation, which weakened the deformation texture and consequently led to the formation of a weak [10−10] fiber texture finally. In contrast, the T500 sample exhibited preferred nucleation and growth of DRXed grains, eventually developing the strong [0001]//ED recrystallization texture. The finer DRXed grains and retained deformation microstructure in the T450 sample produced significantly higher yield strength (YS) and ultimate tensile strength (UTS) of 402 MPa and 477 MPa compared to the T500 sample (YS: 343 MPa; UTS: 433 MPa). Actually, the T450 sample should possess much higher strength originally if its weak texture did not weaken the grain boundary strengthening and dislocation strengthening effects. Additionally, the LPSO phases in the T450 sample exhibit a greater width than the T500 sample, producing smooth tearing ridges, sparse dimples, and a lower fracture elongation of 13.4 % (T500 sample: 16 %).