Orthorhombic intermediate phase originating from {110} nanotwinning in Ni50.0Mn28.7Ga21.3 modulated martensite
Abstrakt
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 \gamma=90.34^{\circ} at room temperature (297\,K) changed to an intermediate (10M') phase with a=b=0.5945\ nm, c=0.5616\,nm, and \gamma=90.25^{\circ} at a few tenths of kelvin below the transformation to austenite at T_{\mathrm{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\neq b.
The transformation temperature coincided with the martensitic transformation temperature T_{\mathrm{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 \approx20\,nm mean twin width originates from the complex hierarchical branching on the austenite nucleus-martensite interface.