Ti3Ni4 precipitation features in heat-treated grain / subgrain nanostructure in Ni-rich TiNi alloy

Abstract

The effect of the annealing temperature of a Ti-50.9 at.% Ni nanocrystalline alloy with an inhomogeneous grain/subgrain structure on Ti3Ni4 precipitation was investigated. The evolution of sizes, morphology, and spatial distribution of Ti3Ni4 particles after annealing at 300 °C, 400 °C and 500 °C studied. Found that Ti3Ni4 particles nucleate by a heterogeneous mechanism predominantly in the substructure and almost not observed in nanograins. With an increase in the temperature of annealing the Ti3Ni4 particles increase in size and their spatial distribution changes from location in subgrains at dislocations (at 300 °C) to precipitation at subboundaries (at 400 °C). The features of recovery and recrystallization in the grain/subgrain structure at 500 °C identified and their relationship with the processes of dissolution and coagulation of Ti3Ni4 particles revealed. The large-scale microstructural heterogeneity leading to different precipitation behavior between substructure regions and nanograins groups is considered as an additional cause of the multi-stage martensite transformation sequence B2 ↔ R ↔ B19′.