β-Phase thermal degradation in Zr-added Cu–Zn–Al shape memory alloy

Abstract

Small amounts of Zr, as a grain refiner, were added to the β Cu–Zn–Al shape memory alloy. The resulting microstructure was polycrystalline, with Zr-rich precipitates preferentially located in the grain boundary. An alloy of similar composition, without addition of Zr, was studied for comparison. The martensitic transformation and the stability of β-phase were studied by differential scanning calorimetry and optical microscopy. The activation energy of phase decomposition, within the β-phase metastability range, was determined by the Kissinger–Akahira–Sunose method, and values of 121 kJ mol−1 for Cu–Zn–Al(Zr) and 110 kJ mol−1 for Cu–Zn–Al were obtained. Furthermore, it was observed that the Zr-rich particles, responsible for controlling the grain size, were unstable during the heat treatments at temperatures within the β-phase stability range. The dissolution of precipitates leads to a gradual decrease in the volume fraction of the sample that transforms to martensite. Finally, after prolonged aging the martensitic transformation disappeared.