Thermoelastic and Pseudoelastic Characterization of Shape Memory Alloys

Abstract

Shape-memory effect is based on martensitic transformation, which is a solid state phase transformation and govern the remarkable changes in internal crystalline structure of materials. Shape memory alloys can exhibit another property called pseudoelasticity (PE), which is performed in only mechanical manner in the parent austenite phase region. Shape Memory Effect (SME) is performed thermally in a temperature interval and this behaviour is called thermoelasticity. Pseudoelasticity is performed by stressing the material in the parent phase region, and shape recovery is performed simultaneously upon releasing the applied stress. PE is the result of stress-induced martensitic transformation. Martensitic transformations occur with cooperative movement of atoms by means of lattice invariant shear which occurs in two opposite directions on close packed planes of parent austenite phase, and possible 24 martensite variants occur. By this way the twinned martensite occurs on cooling, and the twinned structure turn into the detwinned martensite by deformation in the low temperature product phase condition. Copper based alloys exhibit this property in metastable beta-phase region, which has bccbased structures at high temperature parent phase field, and these structures martensiticaly turn into layered complex structures with lattice twinning on cooling.