Stability of ageing-induced multiple-stage transformation behaviour in a Ti-50.15%Ni alloy

Abstract

A fully annealed Ti-50.15%Ni alloy was found to develop a multiple-stage transformation behaviour after ageing at 623 K. The transformation behaviour corresponded to the R-phase transition and two separate martensitic transformations on cooling and two reverse transformations of the two marteasites on heating. This study investigated the effects of overheating and thermal cycling on the stability of the multiple-stage transformation behaviour. It was found that overheating to above 723 K for a short time restored the single-stage transformation behaviour by eliminating the second martensitic transformation whilst merging the R-phase transition into the first martensitic transformation. Thermal cycling, on the other hand, was found to cause a degradation of the multiple- stage transformation behaviour by eliminating the first martensitic transformation.