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

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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 Ni 50 Mn 27 Ga 22 Fe 1 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 ≈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.
Funding Information
  • Grantová Agentura České Republiky (19-09882S 20-12624S)
  • Strategic Research Council (SRC) of Finland (313349)
  • Ministerstvo Školství, Mládeže a Tělovýchovy (LTT20014 MATFUN CZ.02.1.01/0.0/0.0/15_003/0000487 SOLID21 CZ.02.1.01/0.0/0.0/16_019/0000760)
  • České Vysoké Učení Technické v Praze (SGS19/190/OHK4/3T/14)

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