Osman Adiguzel
Firat University, India
Title: Crystallographic reactions in memory behavior of shape memory alloys
Biography
Biography: Osman Adiguzel
Abstract
A series of alloy systems take place in a class of adaptive structural materials called intelligent materials by giving stimulus response to changes in the external conditions. Shape memory alloys take place in this group by exhibiting a peculiar property called shape memory effect, which is characterized by the recoverability of two certain shapes at different temperatures. These alloys are cooled deformed plastically and recover the original shape after these processes, and cycle between the deformed and original shapes on cooling and heating, respectively. The strain energy is stored with plastic deformation, and released on heating, by means of reverse austenite transformation. These alloys are used as functional materials in many fields from biomedical to the aeronautical industry. They are used as deformation absorbent materials in control of civil structures subjected to seismic events. Shape memory effect is initiated by successive cooling and deformation processes and performed thermally by heating and cooling, shape of materials cycle between original and deformed shapes in bulk level, whereas the crystal structure cycles between the twinned and ordered parent phase structures, by means of forward martensitic and reverse austenitic transformations. This behaviour is called thermoelasticity. Shape memory effect is governed by two crystallographic transformations, thermal and stress induced martensitic transformations. Thermal induced transformation occurs along with lattice twinning on cooling and ordered parent phase structures turn into twinned martensite structures. Twinned martensite structures turn into detwinned martensite structures by means of stress induced transformation by deforming plastically in martensitic condition. Thermal induced martensitic transformation is lattice-distorting phase transformation and occurs as martensite variants with the cooperative movement of atoms in <110>-type directions on {110}-type planes of austenite matrix by means of shear-like mechanism. Martensitic transformations occur by two or more lattice invariant shears on {110}-type planes of austenite matrix which is basal plane or stacking plane for martensite. In the martensitic transformation, the lattice of high temperature austenite phase has greater crystallographic symmetry than that of the low-temperature product phase.