Memory characteristics and lattice reactions governing reversibility in shape memory alloys

Title : Memory characteristics and lattice reactions governing 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 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 is plastic deformation, due to the soft character of the material in low temperature condition, with which strain energy is stored and releases on heating by recovering the original shape. This phenomenon is governed by the lattice reactions, twinning and detwinning in atomic level. Lattice twinning occurs on cooling with cooperative movements of atoms in 110 type directions on the110 type close packed planes of austenite matrix and ordered parent phase structures turn into the twinned structures, by means of thermal induced martensitic transformation. The twinned structures turn into the detwinned structures with deformation in the martensitic condition, by means of stress induced martensitic transformation. However, detwinning structures turn into ordered parent phase structure on heating by means of reverse austenitic transformation. Atomic movements are confined into the nearest atom distances, and martensitic transformations have diffusionless character. Superelasticity 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. It is important that stressing and releasing paths are different in the stress-strain diagram, and hysteresis loop refers to the energy dissipation. Superelasticity is also result of the detwinning reaction and ordered parent phase structures turn into detwinned structure with stressing, by means of stress induced martensitic transformation. However, crystal structure of the materials cycles between detwinned martensite and ordered parent phase structures on stressing and releasing. 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 shears. Copper- based alloys exhibit this property in metastable β-phase region, which has bcc-based structures at parent phase field. Lattice twinning 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 3R, 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.

Biography:

Dr. Adiguzel graduated from Department of Physics, Ankara University, Turkey in 1974 and received PhD- degree from Dicle University, Diyarbakir-Turkey. He has studied at Surrey University, Guildford, UK, as a post-doctoral research scientist in 1986-1987, and studied were focused on shape memory effect in shape memory alloys. He worked as research assistant, in 1975-80, at Dicle University and shifted to Firat University, Elazig, Turkey in 1980. He became professor in 1996, and he has been retired on November 28, 2019, due to the age limit of 67, following academic life of 45 years. He supervised 5 PhD- theses and 3 M. Sc- theses and published over 80 papers in international and national journals; He joined over 120 conferences and symposia in international and national level as participant, invited speaker or keynote speaker with contributions of oral or poster. He served the program chair or conference chair/co-chair in some of these activities. In particular, he joined in last six years (2014 - 2019) over 60 conferences as Keynote Speaker and Conference Co-Chair organized by different companies. Also, he joined over 230 online conferences in the same way in pandemic period of 2020-2024. Dr. Adiguzel served his directorate of Graduate School of Natural and Applied Sciences, Firat University, in 1999-2004. He received a certificate awarded to him and his experimental group in recognition of significant contribution of 2 patterns to the Powder Diffraction File – Release 2000. The ICDD (International Centre for Diffraction Data) also appreciates cooperation of his group and interest in Powder Diffraction File.