High-temperature shape memory effect and the $B2$–$L1_0$ thermoelastic martensitic transformation in Ni–Mn intermetallics

The properties and structure of the martensitic phase of alloys with a near-stoichiometric equiatomic Ni$_{50}$Mn$_{50}$ composition, as well as martensitic transformations in them, are investigated in a wide temperature range by measuring the resistivity and thermal expansion coefficient and applying transmission electron microscopy, scanning electron microscopy, electron diffraction, and X-ray diffraction. It is found that Ni$_{50}$Mn$_{50}$ and Ni$_{49}$Mn$_{51}$ alloys experience the $B2\leftrightarrow L1_0$ highly reversible thermoelastic martensitic transformation and its related high-temperature deformation of the transformation and shape memory effect. Critical temperatures, volume ($\Delta V/V$ = $\pm$ 1.7%) and linear size effects attributed to the direct and reverse martensitic transformations, and the high-temperature dependences of the martensitic and austenite lattice parameters are determined. It is found that the morphology of tetragonal $L1_0$ martensitic represents a hierarchy of thin coherent sheets of submicrocrystallites and nanocrystallites with plane near-$\{111\}L1_0$ habit boundaries, the crystallites being pairwise twinned according to the $\{111\}L10\langle 11\bar2\rangle_{B2}\parallel\{011\}\langle 01\bar1\rangle_{B2}$ twinning shear scheme.