Nanoscale Characterization of Crystallographic Phase Transformations in Shape Memory Alloys

Author(s): Adiguzel Osman

Shape memory effect is a peculiar property exhibited a certain alloy systems with special chemical compositions in the β-phase fields. Successive crystallographic structural transformations govern shape memory effect. These are thermal and stress induced martensitic transformations, and occurs by thermally and mechanically on cooling and stressing. These transformations are diffusionless structural phase transformations; thermal induced transformations occur as martensite variants with lattice twinning in crystallographic scale on cooling below martensite finish temperature, and ordered parent phase structures turn into twinned martensitic structure. Twinned structures turn into detwinned martensite by means of stress induced transformation by stressing material. Shape memory alloys are deformed plastically in the low temperature martensitic condition with which and plastic strain is stored and recover original shape on heating, and cycles between original and deformed shapes on heating and cooling, respectively. Thermal induced martensitic transformation is lattice-distorting phase transformation and occurs   with the cooperative movement of atoms on {110}-type planes of austenite matrix by means of shear-like mechanism. The lattice invariant shears occurs, in two opposite directions, <110 > -type directions on the {110}-type basal plane. This kind of shear can be called as {110}<110> - type mode, 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 means of stress induced martensitic by deforming the material in the low temperature product phase condition. The parent phase structures turn into the detwinned structure by means of stress induced martensitic transformation by deformation in the pseudoelasticity.