Effect of aging on mechanical behavior of single crystal Cu–Al–Ni shape memory alloys

The mechanical response of β (parent)-phase aged Cu–Al–Ni single crystal shape memory alloys (SMAs) was investigated at temperatures above and below the transition regime. Two alloys, one in austenitic (alloy A) and the other in martensitic (alloy M) condition in the as-grown state at room temperature, were examined. Both the tensile and the compression responses and their variation with aging were evaluated. With aging, successive martensitic transitions (β′1→γ′1) in both the alloys and precipitate formation (γ2) in the alloy A occur. Compression loading of the aged samples shows large mechanical hysteresis and stabilization of γ′1 martensite which results in permanent strain after unloading. In addition, a noticeable increase in critical stress for transformation in the aged samples vis-à-vis the as-quenched sample is seen. In tension, low temperature aged (473 K) samples show an increase in the critical stress for transformation in alloy M. Differences in the stress–strain hysteresis characteristics of alloys tested at various temperatures indicate the presence of different kinds of martensites (β′1   and   γ′1) in the two alloys.