The evolution of the microstructure and system of Ti$_{3}$Ni$_{4}$ particles upon heat treatments of a Ti–50.9 at.% Ni nanocrystalline alloy

The structure of a Ti–(50.9 at.% Ni) nanocrystalline alloy is studied by transmission electron microscopy after annealing at 300–500$^\circ$C. It is found that B2-TiNi solid solution decomposition according to a heterogeneous mechanism with formation of Ti$_{3}$Ni$_{4}$ particles develops in the subgrain structure and is suppressed in nanograins. The regularities of recovery, polygonization, and recrystallization in the grain/subgrain structure of a nanocrystalline TiNi alloy are established and their interconnection with processes of dissolution and coagulation of Ti$_{3}$Ni$_{4}$ particles is identified.