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 ′ 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 ′ 1 (18R) martensite. Nucleation and behaviour of the β ′ 1 needles under stress were followed.