Paper Title
SHAPE REVERSIBILITY AND DIFFUSIONLESS PHASE TRANSFORMATIONS IN SHAPE MEMORY ALLOYS

Abstract
effect, which is characterized by the recoverability of two certain shapes of material at different conditions. This phenomenon is initiated with thermomechanical processes on cooling and deformation, and performed thermally on heating and cooling, with which shape of materials cycles between original and deformed shapes in reversible way in bulk level. Therefore, this behaviour can be called thermal memory or thermoelasticity. Thermoelasticity is governed by the diffusionless phase transformations in crystallographic level, thermal and stress induced martensitic transformations. Thermal induced martensite occurs on cooling with cooperative movement of atoms in<110 > -type directions on the {110}-type plane of austenite matrix, along with lattice twinning and ordered parent phase structures turn into twinned martensite structures by means of lattice invariant shears.The twinned structures turn into detwinned martensitic structures with deformation by means of stress induced martensitic transformation. transformation. Atomic movements are confined to the nearest atom distances, neighbourhood of atoms do not chance. Therefore, martensitic transformations have diffusionless character. These alloys exhibit another property, called superelasticity, which is performed by stressing and releasing the material in elasticity limit at a constant temperature in the parent phase region, and material recovers the original shape upon releasing, by exhibiting elastic material behavior. Superelasticity is governed by stress induced martensitic transformations, and ordered parent phase structures turn into detwinned martensitic structure with stressing. Copper based alloys exhibit this property in metastable β-phase region, which has bcc-based structures. Lattice twinning is not uniform in these alloys, and the ordered parent phase structures martensitically undergo the non-conventional layered structures. These layered structures can be described by different unit cells as 3R, 9R or 18R depending on the stacking sequences on the close-packed planes of the ordered lattice. In the present contribution, x-ray and electron diffraction studies were carried out on copper based CuAlMn and CuZnAl alloys. X-ray diffraction profiles and electron diffraction patterns exhibit super lattice reflections. X-ray diffractograms taken in a long-time interval show that diffraction angles and intensities of diffraction peaks change with the aging duration at room temperature. This result refers to the rearrangement of atoms in diffusive manner. Keywords - Shape memory effect, martensitic transformation, thermoelasticity, superelasticity, lattice twinning, detwinning.