Deformation behavior of an HCP-BCC dual-phase magnesium alloy at cryogenic temperatures

Gong, W.   ; Gholizadeh, R.*; Harjo, S.   ; Kawasaki, Takuro   ; Aizawa, Kazuya  ; Tsuji, Nobuhiro*

The addition of lithium (Li) to magnesium (Mg) alloys enables the formation of a body-centered cubic (BCC) phase. These Mg alloys with a BCC phase exhibit superior ductility at room temperature, addressing a common limitation of Mg alloys: their typically low ductility. However, these alloys generally show poor work-hardening capability at room temperature, resulting in low ultimate tensile strength and limited uniform elongation. This lack of work-hardening is likely due to the ease of dislocation recovery in the BCC phase. It is well known that lowering the deformation temperature can suppress dislocation recovery, potentially enhancing work hardening. Nonetheless, studies on the cryogenic deformation behavior of dual-phase Mg-Li alloys remain limited. In this study, we investigated the deformation behavior of a dual-phase Mg-Li alloy at cryogenic temperatures using in-situ neutron diffraction. The stress-strain curves showed that the yield stress of the alloy increased consistently as the deformation temperature decreased. Moreover, work hardening was significantly enhanced at cryogenic temperatures, resulting in simultaneous improvements in ultimate tensile strength and uniform elongation. At 295 K, the dislocation densities in both the BCC and HCP phases remained low and nearly constant during straining. In contrast, at 200 K, 77 K, and 20 K, the dislocation densities in both phases increased steadily with strain, reaching higher values as temperature decreased. Strain rate jump tests revealed a strain rate sensitivity of approximately 0.03 at 295 K, while a negative strain rate sensitivity was observed below 200 K, likely due to the dynamic strain aging effect.