The effect of grain size and rolling reduction on microstructure evolution and annealing hardening response of a Mg-3Gd alloy

Mg-3Gd (wt.%) samples with different initial grain sizes were prepared to evaluate the grain size effect on microstructural evolution during cold rolling and subsequent annealing hardening response. The deformation behavior and mechanical response of the as-rolled and annealed samples were systematically investigated by a combination of electron microscopy and microhardness characterization. The results show that the twinning activities were highly suppressed in the fine-grained samples during rolling. Upon increasing the rolling reduction to 40%, ultra-fine grain structures with a volume fraction of ∼28% were formed due to the activation of multiple slip systems. Conversely, twinning dominated the early stages of deformation in the coarse-grained samples. After a 10% rolling reduction, numerous twins with a volume fraction of ∼23% were formed. Further increasing the rolling reduction to 40%, high-density dislocations were activated and twin structures with a volume fraction of ∼36% were formed. The annealing hardening response of deformed samples was effectively enhanced compared to that of the non-deformed samples, which was attributed to the enhanced Gd segregation along grain boundaries, twin boundaries and dislocation cores. Moreover, the grain size and rolling reduction were found to affect the microstructure evolution during annealing, resulting in a notable difference in the annealing hardening response of Mg-3Gd alloy between samples of different grain sizes deformed to different strains. These findings highlight the crucial importance of microstructural and processing parameters in the design of high-strength, cost-effective Mg alloys.