Topological Model of Austenite-Martensite Interfaces in Cu-Al-Ni Alloy
Cu Al Ni alloy is one of the most widely studied shape-memory alloys because of a very large reversible strain and relative easiness of single crystal preparation. The β1 austenite phase has DO3 structure with the lattice parameter of 0.5836 nm. Three types of martensite are observed in this alloy: γ′ 1 with 2H, β ′ 1 with 18R, and α′ 1 with 6R structures. The orthorhombic γ ′ 1 martensite has the lattice parameters a = 0.4382 nm, b = 0.5356 nm, and c = 0.4222 nm [1]. The experiments indicate that at interfaces between the austenite and a single variant of 2H martensite can exist on a microscopic scale [2]. Unfortunately, no invariant plane can be found for martensitic transformations in this material [3]. Consequently, the classical phenomenological theory of martensite cannot predict interface plane between the austenite and a single variant of 2H martensite. It is possible to nd only an average macroscopic habit plane between the austenite and two twin-related martensite. The at interfaces between the austenite and single variant of the martensite were discussed in our previous paper [4] where a simple shear strain of martensite was included in the phenomenological model in order to nd the invariant plane. A comparatively large set of possible habit planes was obtained in that way and it was shown that this set is not in contradiction with the experimental results. However, the simple shear deformation of martensite was included in the model in a purely mathematical way without physical justi cation. The aim of the present paper is to discuss the austenite single-variant-martensite interfaces in Cu Al Ni alloy in the frame of topological model of martensite interfaces [5 7]. This model takes into account admissible defects lying in the interface. Hence, it allows prediction of the austenite martensite habit planes only by analysis of such defects without considerations of the invariant plane.