Premartensitic and martensitic transitions and crystallography in copper based shape memory alloys

Abstract

Copper-based beta-phase alloys constitute a class of materials which exhibit shape memory effect within a certain range of composition. This effect in beta-phases of ternary alloys based on noble metal copper is evaluated by the structural changes in microscopic scale with a displacive transformation called martensitic transition which is first-order transformation, due to the metastable character and change their internal crystalline structures with changing temperature. Beta phases of copper based alloys have the disordered bcc structure, (β)-phase, at high temperatures and undergo two types of ordered reactions called premartensitic transitions with cooling. The first transition is a firstnearest neighbour (nn) ordering reaction and results in a B2-type superlattice. However, bcc to DO3 transition induces the next-nearest neighbour (nnn) ordering reaction. The martensitic transition occurs from the B2(CsCl) or DO3(Fe3Al) type ordered structures to the layered structures on further cooling. Martensitic transformations in these alloys occur occurs in a few steps. . In this kind of transformation, internal stress is effective and one of the steps is type shear mode occuring on close-packed planes of matrix phase called basal plane of martensite. Product martensite phase has the long period layered structures which consist of an array of close-packed planes with complicated stacking sequences called as 3R, 9R or 18R structures depending on the stacking sequences. It is determined that the basal plane of 9R (or 18R) martensites originates from one of the {110}planes of the parent, and a homogenous shear occurs on this plane in two opposite directions during the transformation. Also, the basal plane is subjected to the hexagonal distortion with martensite formation on which atom sizes have important effect. In case the atoms occupying the lattice sites have the same size, the hexagon becomes regular hexagon otherwise Otherwise deviations occur from the hexagonal arrangement. The knowledge of the associated distortion allows us to obtain information on the ordering degree in the martensite. In the present contribution, x-ray diffraction and transmission electron microscopy (TEM) studies were carried out on two copper based ternary alloys aged for long term, nearly 20 years.