Influence of heat treatment on properties of copper-based shape-memory alloy

Abstract

An investigation was performed on a Cu-Zn-Al ternary alloy to examine the influence of heat treatment on its shape-memory effect. Four heat treatments were carried out, namely, step quenching, ice-water quenching, water quenching and glycol quenching from three different temperatures of 800, 850 and 900°C, It was observed that step-quenched samples showed the best martensitic structure for high resistance to shape-memory degradation. Ice-water quenching induced vacancy-pinning effects and hence lowered transformation temperature and degradation life compared to step quenching. However, no transformation was detected in water and glycol-quenched specimens due to the stabilization of martensite. The results showed that shape-memory effect is strongly influenced by many heat-treatment parameters, such as betatizing temperature, betatizing duration and rate of quenching. Step-quenched specimens also showed a higher number of cycles to failure in comparison to ice-quenched specimens. Owing to the dominating effects of large grain size and martensitic plates, the advantages of step quenching, however, disappeared when a high betatizing temperature of 900°C was used. Several kinds of defects were observed after fatigue testing, namely microvoids, cracks near martensitic plates, cracks at the interface of inclusions and inside inclusions.