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The Previous Keynote Speakers
Prof. Dr. Osman ADIGUZEL
Faculty of Science, Department of Physics, Firat University, USA
Speech Title: Thermal and Mechanical Reactions in Memory Behavior of Shape Memory Alloys
Shape memory effect is a peculiar property exhibited by certain alloy systems in the β-phase fields, called shape memory alloys. These alloys exhibit another property called superelasticity. These effects are governed by thermal and mechanical reactions, thermal and stress-induced martensitic transformations in atomic scale. Shape memory effect is initiated by cooling and plastic deformation and performed thermally on heating and cooling; therefore this behavior is called thermoelasticity. Superelasticity is performed mechanically stressing and releasing in the parent phase region. Thermal induced martensitic transformation occurs along with lattice twinning on cooling and ordered parent phase structures turn into twinned martensite structures. Twinned structures turn into detwinned martensite structures by means of stress induced transformation by deformation in martensitic condition. Strain energy is stored in the material with deformation and released upon heating, by recovering the original shape in bulk level, and cycles between original and deformed shapes on heating and cooling, respectively. Superelasticity is also a result of stress induced martensitic transformation and performed in only mechanical manner in the parent austenite phase region. The materials are stressed in the elastic limit just over Austenite finish temperature, and shape recovery is performed simultaneously upon releasing the applied stress. The ordered parent phase structures turn into the detwinned structures by means of stress induced martensitic transformation. Superelasticity is performed in non-linear way, unlike normal elastic materials; stressing and releasing paths are different in stress-strain diagram, and hysteresis loop refers to the energy dissipation. Deformation at different temperatures in intermediate region between Martensite start and Austenite finish temperatures exhibits different behavior beyond shape memory effect and superelasticity, and the materials partially recover original shape. Copper based alloys exhibit this property in metastable β-phase region, which has bcc-based structures in parent phase region. Lattice invariant shears are not uniform in these alloys, and the ordered parent phase structures turn into the non-conventional complex layered structures. The long-period 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. The unit cell and periodicity is completed through 18 layers in direction z, in case of 18R martensite, and unit cells are not periodic in short range in direction z In the present contribution, x-ray diffraction and transmission electron microscopy studies were carried out on two copper based CuZnAl and CuAlMn alloys. X-ray diffraction profiles and electron diffraction patterns exhibit super lattice reflections inherited from parent phase due to the diffusion less character of martensitic transformation. X-ray diffractograms taken in a long-time interval show that diffraction angles and intensities of diffraction peaks change with the aging time at room temperature. This result refers to a new transformation in diffusive manner.