Effect of ageing on the transformation behaviour of Ti–49.5 at.% Ni

It is known that ageing treatment causes complex multiple stage transformation behaviour in Ni-rich Ni–Ti alloys. This has been attributed to the formation of coherent Ti3Ni4 precipitates. There have been different views in the literature on the actual roles of the precipitates on the transformation behaviour, including the creation of microscopic variations in local composition and stress field and spatial inhomogeneous distribution of the precipitates. This study investigated the response to ageing treatment of a Ti–49.5 at.% Ni alloy, which does not develop Ti3Ni4 precipitates. It was found that multiple-stage transformation behaviour was also induced by ageing treatment, involving three stages in the forward transformation on cooling and two stages in the reverse transformations on heating. This is attributed to the formation of coherent Ti-rich precipitates in the matrix.