Prof.  Osman  Adiguzel
Firat University, Elazig,  Turkey

Title: Thermomechanical Processes and Transformations for Reversibility in Shape Memory Alloys

Abstract:

Shape memory effect is a temperature dependent phenomenon exhibited by certain alloy systems called shape memory alloys. These alloys take place in the class of smart and advanced structural materials, with the response to the variation of temperature and external conditions. These alloys have dual characteristics called thermoelasticity 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 material cycles between original and deformed shapes in reversible way. Therefore, this behavior is called thermoelasticity. Strain energy is stored in the material and releases on heating by recovering the original shape. Thermoelasticity is governed by crystallographic transformations, thermal and stress induced martensitic transformations on cooling and stressing.  Thermal induced martensite occurs on cooling along with lattice twinning reaction, and ordered parent phase structures turn into twinned martensite structures by means of lattice invariant shears, and these structures turn into detwinned martensitic structures with deformation by means of stres induced martensitic transformation. Lattice twinning reaction occurs with cooperative movement of atoms in <110 > -type directions on the {110}-type plane of austenite matrix, by means of lattice invariant shear. Atom movements are confined to the nearest neighbor atom distances, and martensitic transformations are displacive transformations. Superelasticity is performed mechanically stressing the materials in elasticity limit at a constant temperature in parent phase region, and shape recovery occurs instantly upon releasing, by exhibiting elastic material behavior.  Superelasticity is governed by stress induced transformation, and ordered parent phase structure turns into the fully detwinned martensite structure with stressing. The crystal structure cycles between the parent phase structure and detwinned martensite structures on stressing and releasing.

Copper based alloys exhibit this property in metastable β-phase region.  Lattice invariant shear and twinning is not uniform in these alloys, and cause to the formation of layered structures, like 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 solution treated copper based CuZnAl and CuAlMn alloys. Electron and x-ray diffractions exhibit super lattice reflections. Specimens of these alloys were aged at room temperature, and a series of x-ray diffractions were taken at different stages of aging in a long-term interval. X-Ray diffraction profiles taken from the aged specimens in martensitic conditions reveal that crystal structures of alloys chance in diffusive manner, and this result refers to the stabilization.

Key words: Shape memory effect, martensitic transformation, thermoelasticity, superelasticity, twinning and detwinning

Biography:

Dr. Osman Adiguzel graduated from Department of Physics, Ankara University, Turkey in 1974 and received PhD- degree from Dicle University, Diyarbakir-Turkey. He studied at Surrey University, Guildford, UK, as a post doctoral research scientist in 1986-1987, and his studies focused on shape memory alloys. He worked as research assistant, 1975-80, at Dicle University and shifted to Firat University in 1980. He became professor in 1996, and he has been retired due to the age limit of 67, following academic life of 45 years.
He published over 80 papers in international and national journals; He joined over 120 conferences and symposia in international and national level as Plenary Speaker, Keynote Speaker, Invited speaker, speaker or Poster presenter. 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 Speaker, Keynote Speaker and Conference Co-Chair organized by different companies in different countries.
Additionally, he retired at the end of November 2019, and contributed with Keynote/Plenary Speeches over 60 Virtual/Webinar Conferences, due to the coronavirus outbreak in two year of his retirement, 2020 and 2021. Dr. Adiguzel served his directorate of Graduate School of Natural and Applied Sciences, Firat University in 1999-2004. He supervised 5 PhD- theses and 3 M. Sc theses. He is also Technical committee member of many conferences. He received a certificate which is being 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.
Scientific fields of Dr. Adiguzel: Shape memory effect and displacive phase transformations in shape memory alloys and other alloys, molecular dynamics simulations, alloy modeling, electron microscopy, electron diffraction, x-ray diffraction and crystallography.