Lattice Reactions Governing Thermoelasticity and Superelasticity in Shape Memory Alloys

Abstract

Some materials take place in class of advanced smart materials with adaptive properties and stimulus response to the external changes. Shape memory alloys take place in this group, due to the shape reversibility and capacity of responding to changes in the environment. These alloys exhibit a peculiar property called shape memory effect, which is characterized by the recoverability of two certain shapes of material at different temperatures. These alloys have dual characteristics called thermoelasticity and superelasticity, from viewpoint of memory behavior, which is governed by two structural transformations, thermal and stress induced martensitic transformations performed by lattice twinning and detwinning reactions. Thermal induced transformation is governed by lattice twinning and occurs on cooling and ordered parent phase structures turn into twinned martensite structures, and these twinned structures turn into detwinned martensite structures by means of strain induced martensitic transformation, by stressing material in low temperature condition. Superelasticity is performed by stressing and releasing material at a constant temperature in parent phase region, and shape recovery is performed simultaneously upon releasing the applied stress.