Orthorhombic intermediate phase originating from {110} nanotwinning in Ni50.0Mn28.7Ga21.3 modulated martensite

Abstract

The temperature evolution of the structure of Ni50.0Mn28.7Ga21.3 single crystals exhibiting magnetic shape memory effect was investigated near the martensitic transformation by X-ray diffraction. Upon heating, five-layered modulated monoclinic martensite (10M) with a=0.5974 nm, b=0.5949 nm, c=0.5589 nm, and γ=90.34∘ at room temperature (297 K) changed to an intermediate (10M′) phase with a=b=0.5945 nm, c=0.5616 nm, and γ=90.25∘ at a few tenths of kelvin below the transformation to austenite at TA=327 K. This previously unreported 10M′ phase between 10M martensite and austenite can be described as orthorhombic using axes diagonal to the original a and b axes derived from the cubic L21 cell. Upon cooling, the phase remained stable in a broader temperature interval down to temperature T=318 K at which it transformed back to monoclinic 10M structure with a≠b. The transformation temperature coincided with the martensitic transformation temperature TM=318 K. The SEM observations and theoretical X-ray diffraction calculations indicate that the intermediate phase is actually the nanotwinned original 10M phase. This {110} nanotwinning with ≈ 20 nm mean twin width originates from the complex hierarchical branching on the austenite nucleus–martensite interface.