Enhancing shape memory effect in Ni-Mn-Ga alloys through homogenization annealing: Microstructure refinement and twin boundary engineering

The anisotropy of polycrystalline Ni-Mn-Ga shape memory alloys can be significantly enhanced by directional solidification, which tailors the microstructure and crystal orientation. In this study, a homogenization annealing treatment at 800 °C for 96 h was applied to directionally solidified (DS) Ni₅₇Mn₂₅Ga₁₈ alloys consisting of non-modulated (NM) martensite and γ phases. Compared with the DS samples, the content of γ phase in the annealed alloys increased from 10.0% to 50.1%, while the interphase spacing decreased. Owing to enhanced atomic order and increased internal stress, both the forward and reverse transformation temperatures decrease after heat treatment. Stress-induced grain boundary migration causes partial martensitic variants to reorient along the solidification direction, and the (112)-oriented martensite texture is strengthened. Nanotwins form in-situ within the martensitic lamellae, exhibiting a reduced critical detwinning stress. The alloys achieve complete strain recovery at a pre-strain of 10%. The post-heat-treated alloys enter the yield stage earlier and display a more pronounced plateau region in the stress–strain curve. The nanotwin boundaries are modified during heat treatment, forming weaving conjugated boundaries (CBs). These unique boundary structures not only enhance work-hardening capacity by impeding dislocation motion and decomposing applied stress into shear components, but also facilitate complete recovery of plastic deformation through reversible detwinning/reorientation of nanotwins constrained by the CBs. Consequently, the post-heat-treated DS alloys exhibit a shape memory strain of 7.56%, which is twice that of the DS alloys.