A comparative study of ausforming of shape memory alloys with A2 and B2 structures

Ausforming implies plastic deformation of austenite (beta) at temperatures T-AF > M-d, at which no stress- or strain-induced transformation can occur. It introduces a variety of extrinsic lattice defects, which in turn modify the course of transformation, the structure of martensite, and increase th e conventional strength of the alloys. The temperature range of ausforming has to be subdivided into three subranges, depending on whether the beta-ph ase is (1), disordered; (2). ordered; or (3) capable of precipitation of a second phase or massive transformation. For the Cu-base alloys the ranges 1 and 3, and for Ni-Ti 2 and 3 may apply. This causes a different hot-deform ation behavior of the two types of alloys: the formation of dislocation gro upings (2-d, 3-d-nets, and Moires) in the brass-type alloys which undergo o rdering during cooling from T-AF. In addition a particular twinning mechani sm and the formation of a rolling texture are found in ordered Ni-Ti. In Ni -Ti-alloys premartensitic R-phase formation can be caused by ausforming. Au sforming leads to lower temperatures but not to suppression of martensitic transformation cycles. Conventional strength is increased in both types of alloys. (C) 1999 Elsevier Science S.A. All rights reserved.