Sergey M. Bitter

In this work, we study the effect of annealing at temperatures of 300–500°C on martensitic transformations, structure, and deformation response of a NiTinanocrystalline alloy with a high nickel content using mechanical load/unload experiments. It is shown that the specific features of Ti₃Ni₄ precipitation, such as a change in particle size and their distribution in the nanostructure from location on dislocations to precipitates at subboundaries, is accompanied by a shift in martensitic transformation temperatures, leading to the appearance of the R-phase and providing a change in mechanical responses from pseudoelastic to shape memory.

The paper studies the regularities of the spatial distribution of Ti₃Ni₄ particles in the inhomogeneous grain-subgrain structure of the nanocrystalline Ti-50.9 at. % Ni alloy depending on the aging temperature. It was found that the presence of different types of internal boundaries influences the inhomogeneous precipitation of coherent Ti₃Ni₄ particles in the temperature range of their intense precipitation at 400°C. Ti₃Ni₄ precipitate in the substructure around the low-angle subgrain boundaries, while the decomposition of the B2-solid solution is suppressed in nanograins with high-angle boundaries. It is proposed that the difference in the Ni concentration in the B2-austenite in the regions of the substructure and nanograins is the main reason for the implementation of the anomalous effect of the R-phase transformation B2↔R1,R2 in the martensitic transformation sequence B2↔R↔B19’.

This paper presents the results of a transmission electron microscope study of the influence of the size, morphology, and spatial distribution of coherent Ti₃Ni₄ particles on the nature of the R transformation; the morphology of the R phase; and the deformation response depending on the aging temperature in nanocrystalline Ti–50.9 at.% Ni alloy with a grain–subgrain structure. It has been established that with an increase in the aging temperature, the size and spatial distribution of Ti₃Ni₄ particles change from being located on dislocations at an aging temperature of 300 °C to precipitation at sub-boundaries at an aging temperature of 400 °C, 450 °C. Correspondingly, the morphology of the R phase changes from nanodomain-like to a lamellar self-accommodation structure. Studies have shown that the morphology of the R phase, in turn, affects the deformation response of the material. In the case of lamellar self-accommodation morphology, localized R transformation develops in a Lüders-like manner.