Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
5436168 | Acta Materialia | 2017 | 10 Pages |
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=90.34â at room temperature (297 K) changed to an intermediate (10Mâ²) phase with a=b=0.5945 nm, c=0.5616 nm, and γ=90.25â at a few tenths of kelvin below the transformation to austenite at TA=327 K. This previously unreported 10Mâ² 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 aâ 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 â 20 nm mean twin width originates from the complex hierarchical branching on the austenite nucleus-martensite interface.
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