K. A. Zheronkina

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’.