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

Abstract The temperature evolution of the structure of Ni 50.0 Mn 28.7 Ga 21.3 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 T A = 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 L2 1 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 T M = 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.