Crystallography and interfacial structure of twinned shape memory martensite

Abstract
This paper reports observations of the interface and surface relief of twinned martensite in a Cu-14wt%Al -3.4wt%Ni shape memory alloy. The transformation in this alloy is from ordered cubic DO3 parent phase ( ) to 2H (orthorhombic) martensite ( ) with Type II transformation twins.

Optical and atomic force microscopy were used to study single isolated plates. These plates consisted of lamellae of martensite "parent" (A) and twin (B) volumes. The average width fraction of Twin A was 0.667, close to the theoretical value of 0.698. Although twin relief was generally evident in the random sections examined, well defined interfacial facets corresponding to the terminating twin volumes were not observed. Instead, side-plates extending beyond the habit plane were common, being associated with the smaller of the two twin volumes (Twin B). This structural feature was common to both thermal and stress-induced martensite. The twin interface plane was close to a habit plane variant, and the extension of the twin ahead of the general interface, with its own twinned substructure, is inferred to be related to the need to accommodate localised interfacial misfit. Analysis of the crystallography of the transformation indicated that two distinct solutions are possible, adding to the flexibility of the transformation in accommodating forces applied during shape memory and superelastic cycling.