| Summary: | Structural and phase transformations in the microstructure and new metastable baro- and deformation-induced phases of the Ni<sub>50</sub>Mn<sub>28.5</sub>Ga<sub>21.5</sub> alloy, typical of the unique class of ferromagnetic shape memory Heusler alloys, have been systematically studied for the first time. Phase X-ray diffraction analysis, transmission and scanning electron microscopy, and temperature measurements of electrical resistivity and magnetic characteristics in strong magnetic fields were used. It was found that in the course of increasing the pressure from 3 to 12 GPa, the metastable long-period structure of martensite modulated according to the 10<i>M</i>-type experienced transformation into a final non-modulated 2<i>M</i> structure. It is proved that severe shear deformation by high pressure torsion (HPT) entails grainsize refinement to a nanocrystalline and partially amorphized state in the polycrystalline structure of the martensitic alloy. In this case, an HPT shear of five revolutions under pressure of 3 GPa provided total atomic disordering and a stepwise structural-phase transformation (SPT) according to the scheme 10<i>M</i> → 2<i>M</i> → <i>B</i>2 + <i>A</i>2, whereas under pressure of 5 GPa the SPT took place according to the scheme 10<i>M</i> → 2<i>M</i> → <i>B</i>2 → <i>A</i>1. It is shown that low-temperature annealing at a temperature of 573 K caused the amorphous phase to undergo devitrification, and annealing at 623–773 K entailed recrystallization with the restoration of the <i>L</i>2<sub>1</sub> superstructure in the final ultrafine-grained state. The size effect of suppression of the martensitic transformation in an austenitic alloy with a critical grain size of less than 100 nm at cooling to 120 K was determined. It was established that after annealing at 773 K, a narrow-hysteresis thermoelastic martensitic transformation was restored in a plastic ultrafine-grained alloy with the formation of 10<i>M</i> and 14<i>M</i> martensite at temperatures close to those characteristic of the cast prototype of the alloy.
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