Microstructure evolution and mechanical properties of a high strength Mg-11.7Gd-4.9Y-0.3Zr (wt%) alloy prepared by pre-deformation annealing, hot extrusion and ageing

A high strength Mg-11.7Gd-4.9Y-0.3Zr (wt%) alloy with a weak tension-compression yield asymmetry has been successfully developed by pre-deformation annealing, hot extrusion and ageing. The effects of pre-deformation annealing on the microstructure evolution and mechanical properties are studied. The results reveal that pre-deformation annealing generates a large number of Mg₅RE (RE: rare earth) phase particles and raises the fraction of dynamic recrystallization (DRX). The preformed Mg₅RE particles not only enhance the DRX by particle simulated nucleation (PSN), but also facilitate the continuous DRX (C-DRX) by promoting the grain subdivision during hot extrusion. Without pre-deformation annealing, the as-extruded alloy exhibits a high tensile yield strength (TYS) of 376 ± 9.6 MPa but a low elongation to failure (EL) of 4.3 ± 0.1% due to the bimodal microstructure consisting of coarse un-DRXed grains with strong basal texture and fine DRXed grains with weak basal texture. After T5 treatment, the TYS further increases to 500 ± 5.5 MPa, whereas the EL reduces to 2.7 ± 0.4%. An excellent balance of strength and ductility (TYS of 343 ± 0.2 MPa and EL of 9.3 ± 0.9%) can be realized by pre-deformation annealing for 1 h due to the raised fraction of DRX and the weakened basal texture. T5 treatment further increases the TYS to 446 ± 3.8 MPa but reduces the EL to 3.0 ± 0.2%. The studied alloy exhibits good compressive performance, resulting in a weak tension-compression yield asymmetry. The grain refinement, Mg₅RE and β′ phases, and solute-segregated SFs contribute to the alloy strengthening.