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

Abstract

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) g110, where g110 is the reciprocal lattice vector, to incommensurate with q up to pseudo-commensurate q = (3/7) g110. Upon heating, reverse changes are observed with a thermal hysteresis of ≈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.