CHANGES IN FEATURES OF INVARIANT PLANE-STRAIN WITH MONOCLINIC DISTORTION IN 18R-TYPE MARTENSITIC-TRANSFORMATION

In order to clarify the relationship between the monoclinic distortion and the invariant plane strain in the beta-18R martensitic transforma tion of Cu-based shape memory alloys. analytic solutions for the invar iant plane strain have been derived. This involves the coupling of the phenomenological theory of the martensitic crystallography with the m onoclinic distortion model For the M18R structure. A higher monoclinic distortion has been found to bring about reduction in the tensile and /or compressive strains along the principal axes, and thus the magnitu de of the invariant plane strain is lineally related to the monoclinic distortion. Similarly, a higher monoclinic distortion leads to a decr ease in the lattice invariant shear, which can be further reduced by t he volumetric contraction due to the transformation. The stacking faul t density of the 18R martensite has been found to diminish with increa sing monoclinic distortion, and its actual stacking fault density has been calculated to be greater than 1/3 when the volumetric contraction occurs during the martensitic transformation. (C) 1998 Acta Metallurg ica Inc. Published by Elsevier Science Ltd. All rights reserved.