Stress-induced martensitic transformations in a Cu–Al–Ni shape memory alloy studied by in situ transmission electron microscopy

Structural observations by transmission electron microscopy were performed and analyzed on in situ   strained foils of a Cu–Al–Ni shape memory alloy. The experiments were carried out at room temperature. Two stress-induced transformation processes are described in this paper. (i) Transition between two variants of γ′1γ′1 (2H) martensite. Both variants coexisted at low stresses. With increasing stress, one variant was replaced by the other one and appeared again during unloading. (ii) Transformation between β1 (D03) austenite and β′1β′1 (18R) martensite. Nucleation and behaviour of the β′1β′1 needles under stress were followed.