Atomic disordering and BCC $\to$ FCC transformation in the Heusler compound Ni$_{54}$Mn$_{20}$Fe$_{1}$Ga$_{25}$ subject to high-pressure torsional megaplastic deformation

The Heusler compound with the $L$2$_1$ Ni$_{54}$Mn$_{20}$Fe$_{1}$Ga$_{25}$ structure subject to high-pressure torsional megaplastic deformation was first systematically studied by in situ methods of X-ray phase analysis and transmission and scanning electron microscopy. It was established that the torsional shear deformation at room temperature reduces the polycrystalline structure of the compound to a nanocrystalline and partially amorphous state. It was revealed that, as the pressure increases from 3 to 5 GPa and the degree of deformation increases from 2 to 5 revolutions, total atomic disordering and sequential structural-phase transformation according to the scheme $B$2(BCC) $\to$ $A$2(BCC) $\to$ $A$1(FCC) occur. It is shown that annealing at a temperature of 570 K and below causes devitrification of the amorphous phase and annealing at a temperature of 620 K and above causes the restoration of the $L$2$_1$ structure. The dimensional effect of suppressing the thermoelastic martensitic transformation is revealed in the nanostructured austenitic $L$2$_1$ alloy with grains smaller than 80 nm when cooled to 120 K. The capability of the thermoelastic martensitic transformation and of the shape memory is restored in a submicrocrystalline ultrafine-grained alloy after its recrystallization annealing at temperatures above 600 K.