Effect of compressive load on the martensitic transformation from austenite to 5M martensite in a polycrystalline Ni-Mn-Ga alloy studied by in-situ neutron diffraction

In this study, the influences of uniaxial compressive load on martensitic transformation from austenite to 5M martensite were studied in a directionally solidified Ni-Mn-Ga polycrystalline alloy with coexisting <0 0 1>A and <1 1 0>A preferred orientations parallel to the solidification direction (SD). Based on the neutron diffraction, the direct evidence on the variant redistribution induced by the thermal-mechanical treatment was presented and the selection of preferential variants was found to be strongly dependent on the austenite orientation. For the austenite with a <0 0 1>A preferred orientation, the compressive loading direction (LD) along the SD can promote the formation of preferred variants with {0 2 0}5M⊥SD (LD). On the other hand, for the austenite with a <1 1 0>A preferred orientation, the variants with {1 2 5}5M/{1¯ 2 5}5M⊥SD (LD) are more favorable after the thermal-mechanical treatment. Such variant selection is originated from the accommodation between the anisotropic lattice distortion in martensitic transformation and the external constraint. The present investigations may offer some fundamental information on variant selection subject to external stress field and the necessary guidelines for microstructure optimization of polycrystalline Ni-Mn-Ga alloys through external field training.