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Title: Shape reversibility and the role of twinning reactions in martensitic transformations in shape memory alloys

Abstract

Shape memory alloys take place in a class of advanced smart materials by exhibiting a peculiar property called shape memory effect. This phenomenon is initiated with thermomechanical processes on cooling and deformation and performed thermally on heating and cooling, with which shape of the materials cycle between original and deformed shapes in reversible way. Therefore, this behavior can be called Thermoelasticity. This phenomenon is governed by crystallographic transformations, thermal and stress induced martensitic transformations. Thermal induced martensitic transformation occurs on cooling with cooperative movements of atoms in <110 > -type directions on the {110} – type close packed planes of austenite matrix, along with lattice twinning and ordered parent phase structures turn into the twinned martensite structures. The twinned structures turn into the detwinned structures by means of stress induced martensitic transformation with deformation in the low temperature condition. Lattice twinning and detwinning reactions play important role in martensitic transformations, and they are driven by internal and external forces, by means of inhomogeneous lattice invariant shear. These alloys exhibit another property called superelasticity, which is performed with stressing and releasing the material in elasticity limit at a constant temperature in parent phase region, and shape recovery is performed simultaneously upon releasing, by exhibiting elastic material behavior. Superelasticity is also result of stress induced martensitic transformation and ordered parent phase structures turn into detwinned martensite structure with stressing the material in parent phase region. Copper- based alloys exhibit this property in metastable β-phase region.  Lattice twinning and lattice invariant shear is not uniform in these alloys and gives rise to the formation of complex layered structures. The layered structures can be described by different unit cells as 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 two 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.

Audience take away from my presentation:

Shape memory effect is a multidisciplinary subject and in principle, I introduce the basic terms and definition in my presentation, I introduce the experimental result performed on the alloy samples.