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Prof. Dr. Osman ADIGUZEL
Faculty of Science, Department of Physics
Speech Title: Thermoelasticity Pseudoelasticity and Crystallographic Basis of Martensitic Transformations in Shape Memory Alloys
Shape memory effect is a peculiar property exhibited a series of alloy system called shape memory alloys, which have dual characteristics called thermoelasticity and pseudoelasticity, from view point of memory behavior. Two successive structural transformations, thermal and stress induced martensitic transformations govern shape memory phenomena in crystallographic basis. Shape memory effect is performed thermally in a temperature interval on heating and cooling after first cooling and stressing processes, whereas pseudoelasticity is performed mechanically by stressing and releasing at a constant temperature in the parent austenite phase region. Thermal and stressing processes in bulk level in physical basis, and lattice twinning and detwinning processes govern shape memory effect in crystallographic basis. Thermal induced martensite occurs as multivariant twinned martensite in self-accommodating manner on cooling, and twinned martensite structures turn into detwinned martensite by means of stress induced martensitic transformation on stressing. The materials are deformed in pseudoelasticity, and shape recovery is performed simultaneously upon releasing the applied stress. Pseudoelasticity is performed in non-linear way; stressing and releasing paths are different in the stress-strain diagram, and hysteresis loop refers to energy dissipation.
The elementary processes involved in such martensitic transformations are essentially shear deformations, lattice invariant shears, and shuffling of atomic planes. Lattice invariant shears occur with cooperative movements of atoms on close packed planes of ordered parent phase lattice in displacive manner. The atomic plane shuffling and shearing can be considered as elementary processes activated during displacive martensitic transformations.
Lattice invariant shear is not uniform in copper based shape memory alloys, and cause to the formation of long-period layered complex martensitic structures with lattice twinning on cooling. Electron diffraction and x-ray diffraction studies performed on two copper based CuZnAl and CuAlMn alloys show that these alloys exhibit super lattice reflections in martensitic condition. Critical transformation temperatures of these alloys are over room temperature, and they are in fully martensitic state at room temperature. A series of x-ray diffractions were taken duration aging at room temperature. Diffraction results show that diffraction angles and peak intensities change with aging. This result refers to a new reaction in diffusive manner and leads to the martensite stabilization.