Lattice Reactions and Nanoscale Aspects of Reversibility in Shape Memory Alloys
Abstract
Osman Adiguzel
Shape memory effect is a peculiar property exhibited by a series of alloy system, called shape memory alloys, and this property is characterized by the recoverability of two certain shapes of material at different temperatures. These materials are often called smart materials due to the functionality and capacity of responding to changes in temperature. These alloys are used as shape memory devices in many fields, such as medicine, bioengineering, metallurgy, building industry and many engineering fields. Shape memory effect is result of successive thermal and stress induced phase transformations characterized by changes in the crystal structure of the material. These transformations are governed by lattice reactions in crystallographic and nanoscale level in the material. Thermal induced martensitic transformation is a first-order lattice-distorting phase transformation and thermally occurs with cooperative movements of atoms by means of lattice invariant shear, on cooling from the parent phase region. Thermal induced martensite occurs as twinned martensite by lattice twinning, and the twinned structures turn into the detwinned structures by means of stress-induced martensitic transformation, by stressing material in the martensitic condition. Shape memory alloys become noticeable as smart materials in mechanical applications in many fields of industry. These alloys have dual characteristics and exhibit another property called superelasticity which is performed by stressing material at a constant temperature in the parent phase region just over the austenite finish temperature. Superelastic materials are deformed in the parent phase region and, shape recovery is carried out instantly and simultaneously upon releasing the applied stress. This property exhibits classical elastic material behavior, but stress-strain behavior is different; the stressing and releasing paths are different and hysteresis loop refers to the energy dissipation
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