Stress-induced crystallographic and microstructural evolution for hierarchically twinned martensite in single- and dual-phase Ni-Mn-Ga alloys

Highlights: • Different twinning types were observed in the micro-variants and internal nano-lamellae. • Single phase alloy showed a high strain recovery while dule-phase alloy exhibited improved strength and ductility. • Microstructural evolution during the entire deformation stages were examined for different alloys. Polycrystalline Ni_54+xMn₂₅Ga_21-x high temperature shape memory alloys were developed to examine the Ni-content dependent crystallographic features, phase transformation and deformation behavior. The alloys were characterized by a hierarchically twinned martensite structure with the co-existence of ductile γ phase in Ni₅₇ and Ni₅₈ alloys. Two types of twinning relationships existed, i.e., (112) compound twin in the internal nano-lamellae and $\left(1\overline{1}2\right)$ type-I twin in the adjacent micro-variants. The martensitic start temperature, compressive strength and ductility increased with increasing Ni content. However, the corresponding shape recovery capability was significantly deteriorated in the higher Ni-containing alloys. For the single-phase alloy, the detwinning/reorientation of internal nano-lamellae and activation of deformation twins contributed to the macroscopically recoverable strain in case of low pre-strains. In contrast, large pre-strains led to piles-up of dislocation, bending and kinking of twinning interfaces, formation of deformation bands and crossing of twin structures. These irreversible processes produced massive unrecoverable strain. Almost no detwinning and twining processes were observed in the dual-phase alloys due to the severe lattice distortion. Instead, most deformation occurred via dislocation motion in γ phase.