CRYSTALLOGRAPHIC SIMILARITIES IN SHAPE MEMORY MARTENSITES

The formation of martensite in shape memory alloys having 2H, 3R, 9R and 18R periodic stacking order structures occurs by the localized formation of four self-accommodating variants in a plate group. The 24 variants of martensite consist of six plate groups, each of which consists of four habit plane variants which are near to and symmetrically clustered about {001} poles of the parent phase. Optical and electron microscopy and computer calculations using the phenomenological crystallographic theory have shown that common features of the mutual orientation of the martensite lattices in a plate group and their orientation with respect to the parent phase exist, even though these martensites are both internally twinned and internally faulted. The characteristic four-variant habit plane grouping in a self-accommodating diamond-shaped morphology with respect to a {011} plane proves to be identical for all alloys studied, and therefore a common type of behavior during the shape memory deformation and recovery process may be expected. © 1979.