Stress-induced variant rearrangement in Ni-Mn-Ga single crystals with nonlayered tetragonal martensitic structure

Three Ni-Mn-Ga alloys with different chemical compositions and nonmodulated tetragonal martensitic phase (NM) were studied. In two of the alloys, the polydomain nonmodulated tetragonal phase was obtained in cubic-tetragonal martensitic phase transformation at temperatures below and above Curie point, while in the third one, NM-phase is a result of intermartensitic transformation (7M-NM). Magnetic susceptibility and differential scanning calorimetry were used to study these phase transformations. X-ray diffraction, optical polarised microscopy and mechanical testing were used to study variant rearrangement in compression. Twinning strain approximately 19% was observed for all alloys. It was found also that twinning stress is high (approximately 6-18 MPa) and the alloys did not show detectable magnetic field-induced strain. The influence of magnetic field on stress-strain curves and temperature dependencies of twinning stress are presented.