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This paper presents experimental data on the microstructure and martensite transformation temperatures of Ti49.8Ni50.2 (at%) after abc pressing (multi-axial forging) to different true strains e from 1.84 to 9.55 at 573 K. The data show that increasing the true strain results in grain–subgrain refinement on different scales at a time. With e = 9.55 at 573 K, the average grain–subgrain size measured approximately 130 nm. Decreasing the abc pressing temperature from 723 to 573 K caused a decrease in all martensite transformation temperatures, a change in the lattice parameters, R phase formation, and angular shifts of diffraction peaks and their broadening. The largest change in the microstructure of Ti49.8Ni50.2 was provided by abc pressing to e = 1.84. Increasing the true strain to e = 9.55 resulted in a much smaller effect, suggesting that the alloy obtained a high density of structural defects even at e = 1.84. Two possible mechanisms of grain–subgrain refinement are discussed.

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In the work, we studied the regularities and mechanisms of microstructure formation in binary TiNi alloys with 50.2 and 50.8 at.% Ni under warm abc pressing with a stepped decrease in strain temperature (873, 673, 623, and 573 К) and isothermal (723 К) multipass caliber rolling. In the TiNi alloy with 50.2 at.% Ni at all abc pressing stages, microstructures inhomogeneous in grain size were formed due to faster dynamic recrystallization and hence faster formation of finer grains and subgrains in strain localization bands compared to microvolumes bounded by these bands. After final abs pressing at 573 К with a total true strain е = 7.7, a microstructure composed of submicroand nanocrystalline grains and subgrains was found. In the TiNi alloy with 50.8 at. % Ni subjected to warm rolling, three stages of grain structure evolution were revealed. At the first stage with low strains, the average grain size ‹d› increased due to collective dynamic recrystallization. At the second and third rolling stage, the average grain size ‹d› decreased steeply due to discontinuous and continuous dynamic recrystallization. On rolling at е = 2.0, a microstructure composed of microand submicrocrystalline grains was formed. An algorithm was proposed for estimating the critical strain for the onset of dynamic recrystallization from dependences of grain sizes on true strain. The sequences and temperatures of martensite transformations from a cubic В2 phase to rhombohedral R and monoclinic В19′ martensite phases were studied depending on the strain accumulated in abc pressing and rolling.

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