Hysteretic structural changes within five-layered modulated 10M martensites of Ni-Mn-Ga(-Fe)

Modulated structure of Ni-Mn-Ga-based alloys is decisive in their magnetic shape memory (MSM) functionality. However, the precise nature of their five-layered modulated 10M martensite is still an open question. We used x-ray and neutron diffraction experiments on single crystals to investigate structural changes within 10M-modulated martensite of the Ni50Mn27Ga22Fe1 MSM alloy. The modulation vector gradually increases upon cooling from commensurate q = (2/5) g (110), where g (110) is the reciprocal lattice vector, to incommensurate with q up to pseudo-commensurate q = (3/7) g (110). Upon heating, reverse changes are observed with a thermal hysteresis of approximate to 60 K. The same hysteretic behaviour was detected in the electrical resistivity and the effective elastic modulus. Scanning electron microscopy showed that the changes are accompanied by the refinement of the a/b laminate. These observations indicate that the commensurate state is a metastable form of 10M martensite. Upon cooling, this phase evolves through nanotwinning into a more irregular and more stable incommensurate structure.