Nanoscale characterization of martensite structures in copper based shape memory alloys

Abstract

Martensitic transformations are first order displacive transitions and occur in the materials on cooling from high temperature. Shape memory effect is an unusual property exhibited by certain alloy systems, and leads to martensitic transition. Copper-based alloys exhibit this property in beta phase field which possess simple bcc- structures, austenite structure at high-temperatures. As temperature is lowered the austenite undergoes martensitic transition following two ordering reactions, and structural changes in nanoscale govern this transition. Atomic movements are also confined to interatomic lengths in sub-mu m or angstrom scale in martensitic transformation. The formation of the layered structures in copper based alloys consists of shears and shear mechanism. Martensitic transformations occur in a few steps with the cooperative movement of atoms less than interatomic distances by means of lattice invariant shears on a {110} - type plane of austenite matrix which is basal plane or stacking plane of martensite. The lattice invariant shears occurs, in two opposite directions, < 110 > -type directions on the {110}-type plane. These shears gives rise to the formation of layered structure.