Structural modification and twinning stress reduction in a high-temperature Ni-Mn-Ga magnetic shape memory alloy

Mechanical and synchrotron diffraction experiments were performed to investigate the temperature dependent structural changes in a Ni-Mn-Ga single crystal. The initial sample exhibits a mixture of seven-layered (7M) and non-modulated (NM) martensites at room temperature. Compression along ⟨100⟩ resulted in a strain of 18%, indicating a stress-induced intermartensitic transformation from the 7M to the NM phase. The thermally induced intermartensitic transformation follows the sequence 5M→7M→NM during cooling from the austenite phase. The structural changes are quantitatively reflected in the mechanical response. A twinning stress of 3.8 MPa is measured at 90 °C, which is the lowest reported in high-temperature Ni-Mn-Ga structures.