Title: Shape Reversibility and Thermo-responsive Transformations in Shape Memory Alloys

Biography:

Osman Adiguzel is a professor at Firat University, Department of Physics, Elazig, Turkey.

Abstract

Shape memory effect is a peculiar property exhibited in certain alloy systems called shape memory alloys. This phenomenon is initiated by cooling and deformation treatments and performed thermally on heating and cooling after these treatments, and shape of materials recover the original and deformed shapes. These alloys are deformed plastically in the low temperature condition, with which strain energy is stored. The stored energy is released upon heating by means of reverse austenitic transformation by recovering the permanent original shape on heating, and cycles between original and deformed shapes on heating and cooling. Shape memory effect is governed by thermoresponsive structural transformations, advanced thermal induced martensitic and reverse austenitic transformations. Thermal induced transformations occur as martensite variants along with lattice twinning in atomic scale on cooling 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. Thermal induced martensitic transformation is lattice-distorting phase transformation and occurs   with the cooperative movement of atoms in <110 > -type directions on {110}-type planes of austenite matrix by means of shear-like mechanism. These alloys exhibit another property called superelasticity, which is performed mechanically by stressing and releasing at the parent phase region. Superelasticity exhibit classical elastic material behavior, materials recover original shape after releasing the applied stress.

Superelasticity is performed in non-linear way; stressing and releasing paths are different in the stress-strain diagram, and hysteresis loop refers to energy dissipation.

Superelasticity is also result of the stress induced martensitic transformation, and the ordered parent phase structures turn into the detwinned structure by means of stress induced martensitic transformation by deformation. Stressing and releasing paths are different at Stress-Strain diagram, and hysteresis loop referrers to the energy dissipation.

Copper based alloys exhibit this property in metastable beta-phase region. Lattice invariant shear and lattice twinning is not uniform in copper-based alloys and cause the formation of complex layered structures, like 6R, 9R and18R depending on the stacking sequences on the close-packed planes of the parent phase structure.

In the present contribution, x-ray diffraction and transmission electron microscopy (TEM) 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 displacive 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. In particular, some of the successive peak pairs providing a special relation between Miller indices come close each other. This result refers to a new reaction in diffusive manner.