Stress-free and stress-assisted aging of NiTi shape memory alloys

The present work investigates coherent Ni4Ti3 precipitates in a Ti-50,7at%Ni shape memory alloy after different thermo-mechanical treatments. The study is based on the quantitative TEM (transmission electron microscopy) image analysis using the ACC (Adaptive Contrast Control) programme. The attention is given to a number, size and volume fraction of individual crystallographic variants of the Ni4Ti3 precipitates. Preliminary results suggest that, after common heat treatments, the precipitation of Ni4Ti3 particles is inhomogeneous not only as far as the density of particles in the original B2 grains is concerned but also with respect to the distribution of the crystallographic particle variants. The possible influence of the heterogeneous Ni4Ti3 precipitation on the characteristics of the martensitic transformations B2-R-B19’ is discussed