Osman Adiguzel

Title: Shape memory phenomena and deformation temperature dependent for reversibility in shape memory alloys

Abstract

Shape memory alloys take place in a class of advanced smart materials by exhibiting dual memory characteristics, Shape Memory Effect and Superelasticity. Shape Memory Effect is initiated with thermomechanical processes on cooling and deformation and performed thermally on heating and cooling, with which shape of the materials cycles 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 the cooperative movement of atoms in <110 > -type directions on {110}-type close packed planes of austenite matrix along with the lattice twinning and ordered parent phase structures turn into twinned martensitic structure. Twinned structures turn into detwinned martensite 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 the internal and external forces, by means of inhomogeneous lattice invariant shears.  Superelasticity is performed with mechanically 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 also result of stress induced martensitic transformation, and the ordered parent phase structures turn into the detwinned martensite structures by stressing in parent phase region. It is important that these alloys are deformed in the low temperature condition for Thermoelasticity, and they are stressed in the parent phase region for Superelasticity. Deformation at different temperature exhibits different characteristics beyond Thermoelasticity and Superelasticity. Copper based alloys exhibit this property in metastable β-phase fields. Lattice twinning and lattice invariant shear is not uniform in these alloys and cause to the formation of complex layered structures.

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.

What the audience take away from presentation:

Shape memory alloys are multifunctional materials and shape memory effect is multidisciplinary subject. Also, every scientist is not familiar with every subject of the science. Therefore, I will introduce the basic terms and definitions related the topics of my Talk at the beginning, and I will introduce the experimental result on the specimens.