The Role of twinning and detwinning reactions in martensitic transformations in shape memory alloys

Title : The Role of twinning and detwinning reactions in martensitic transformations in shape memory alloys

Abstract:

Shape memory alloys take place in a class of advanced smart materials with adaptive properties and   stimulus response to the external changes. These alloys exhibit a peculiar property called shape memory effect. This phenomenon is initiated on cooling and deformation processes and performed thermally on heating and cooling, with which shape of materials cycles between original and deformed shapes in reversible way in bulk level. Therefore, this behavior can be called thermal memory or thermoelasticity. This is plastic deformation, due to the soft character of the material, with which deformation energy is stored in the material and releases on heating by recovering the original shape. Thermoelasticity is governed by the crystallographic transformations, thermal and stress induced martensitic transformations. Thermal induced martensitic transformation occurs on cooling with cooperative movement of atoms in <110 > -type directions on {110} - type plane of austenite matrix, along with lattice twinning and ordered parent phase structures turn into the twinned martensite structures, and twinned structures turn into detwinned martensite structures by means of stress induced martensitic transformations with deformation in the low temperature condition. These alloys exhibit another property, called superelasticity which is performed with mechanically stressing and releasing material in elasticity limit at a constant temperature in parent phase region, and shape recovery is performed simultaneously upon releasing the applied stress, by exhibiting elastic material behavior. It is important that superelasticity is performed in non-linear way; stressing and releasing paths are different at the stress-strain diagram, and hysteresis loop refers to the energy dissipation. Superelasticity is also result of stress induced martensitic transformation and ordered parent phase structures turn into detwinned martensite structure with stressing the material in the parent phase region. Twinned and detwinned reactions play important role in martensitic transformations, and they are driven by inhomogeneous lattice invariant shear. These alloys are functional materials with these properties, and they are used in many fields from biomedical to the building industry.

Copper- based alloys exhibit this property in metastable β-phase region.  Lattice invariant shear and twinning is not uniform in these alloys and gives rise to the formation of complex layered structures, depending. 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 diffraction and electron diffraction studies were carried out on copper based CuZnAl and CuAlMn alloys. X-ray diffraction profiles and electron diffraction patterns exhibit super lattice reflections. Critical transformation temperatures of these alloys are over the room temperature. The specimens were aged at room temperature, and a series of x-ray diffractograms were taken during aging. X-ray diffractograms taken in a long-time interval show that locations and intensities of diffraction peaks change with the aging time, and this result refers to the redistribution 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 on shape memory alloys. He worked as research assistant, 1975-80, at Dicle University and shifted to Firat University, Elazig, Turkey in 1980. He became professor in 1996, and he has already been working as professor. He published over 60 papers in international and national journals; He joined over 100 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 seven years (2014 - 2020) over 80 conferences as Keynote Speaker and Conference Co-Chair organized by different companies. He supervised 5 PhD- theses and 3 M.Sc.- theses. 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.