The shape memory mechanism in 18R martensitic alloys

Abstract

This investigation describes the deformation behavior and shape memory mechanism in internally faulted 18R martensites in the CuZnGa and CuAlZn systems. The 18R martensites, like other martensites (i.e. 9R and 2H), form as self-accommodating plate groups consisting of four habit plane variants symmetrically disposed with respect to a {110} plane of the parent phase. There are six of these groups, in general, in a fully transformed specimen, initially a single crystal of the parent phase. The deformation behavior within a given plate group was found to be identical to that previously found for the 9R martensite in the CuZn system, and involves {100} and {110} twinning, in addition to variant-variant boundary movement, all of which operate to produce a single martensite orientation from an initial plate group region consisting of four orientations. In addition, various interactions between plate groups have been determined, the end result of which is that 24 initial martensite orientations (six plate groups) are converted to a single orientation, which corresponds to the particular martensite variant with shape strain most favorable with respect to the applied stress. The behavior in which 24 orientations are converted into a single crystal of martensite has revealed new martensite deformation processes. Once a plate group has been converted into a single crystal region by deformation, various processes within or around the plate group can follow. These include basal plane twinning, {110} twinning (non-basal), and the ‘invasion’ of the single crystal region into adjoining plate groups, all of which serve to convert the entire specimen to the single crystal orientation of the most favored variant. The eventually obtained single crystal of martensite, involving the entire specimen, reverses, when heated, to a single crystal of the parent phase in a unique manner, explaining the observed shape recovery, which is, in effect, the transformation of a single crystal of martensite to single crystal of the parent phase.