Effect of Ni 4Ti 3 precipitation on martensitic transformation in Ti–Ni

Abstract

Precipitation of Ni 4Ti 3 plays a critical role in determining the martensitic transformation path and temperature in Ni–Ti shape memory alloys. In this study, the equilibrium shape of a coherent Ni 4Ti 3 precipitate and the concentration and stress fields around it are determined quantitatively using the phase field method. Most recent experimental data on lattice parameters, elastic constants, precipitate–matrix orientation relationship and thermodynamic database are used as model inputs. The effects of the concentration and stress fields on subsequent martensitic transformations are analyzed through interaction energy between a nucleating martensitic particle and the existing microstructure. Results indicate that R-phase formation prior to B19′ phase could be attributed to both direct elastic interaction and stress-induced spatial variation in concentration near Ni 4Ti 3 precipitates. The preferred nucleation sites for the R-phase are close to the broad side of the lenticular-shaped Ni 4Ti 3 precipitates, where tension normal to the habit plane is highest, and Ni concentration is lowest.