Crystallographic aspects of shape memory effect and reversibility in shape memory alloys

Title : Crystallographic aspects of shape memory effect and reversibility in shape memory alloys

Abstract:

Some materials take place in class of advanced smart materials with adaptive properties and   stimulus response to the external changes. Shape memory alloys take place in this group, with the capacity of responding to changes in the environment by exhibiting shape reversibility. These alloys exhibit a peculiar property called shape memory effect, which is characterized by the recoverability of two certain shapes of material at different conditions. These alloys have dual characteristics called thermoelasticity and superelasticity, from viewpoint of memory behavior. Shape memory effect 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. Superelasticity is performed by stressing and releasing the material at a constant temperature in the parent phase region. Superelasticity exhibits ordinary elastic material behavior, but it is performed in non-linear way; loading and unloading paths are different at the stress-strain diagram, and hysteresis loop reveals energy dissipation. These phenomena are performed by crystallographic transformations called martensitic transformation. Thermoelasticity is governed by the thermal and stress induced martensitic transformations on cooling and stressing, and reverse austenitic transformation on heating. Superelasticity is governed by stress induced martensitic and reverse austenitic transformations by stressing and releasing materials, with which ordered parent phase structures turn into detwinned martensitic structure and ordered parent phase structures, respectively. These transformations occur with the movements of atoms in atomic scale in sub-nano level.  Thermal induced martensite occurs on cooling along with lattice twinning 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 stress induced transformation. Lattice twinning occurs in two opposite directions, <110 > -type directions on the {110}-type plane of austenite matrix in self-accommodating manner, by means of lattice invariant shear.  Superelasticity is also governed by stress induced martensitic transformation and ordered parent phase structures turn into detwinned martensite structures with stressing.  Copper based alloys exhibit this property in metastable beta-phase region, which has bcc based structures at high temperature parent phase field. Lattice invariant shear is not uniform in shape memory  alloys and cause to the formation of complex layered structures with thermal induced martensitic transformation. In the present contribution, electron diffraction and x-ray diffraction studies performed on two copper- based CuZnAl and CuAlMn alloys. Electron diffraction patterns and x-ray diffraction profiles exhibit super lattice reflections in martensitic condition. Specimens of these alloys were aged at room temperature in martensitic condition, and a series of x-ray diffractions were taken duration aging at room temperature. Reached results show that diffraction angles and peak intensities change with aging time at room temperature, and this result refers to the rearrangement of atoms in diffusive manner.

Keywords: Shape memory effect, martensitic transformation, thermoelasticity, superelasticity, lattice twinning, detwinning.

Audience Take Away Notes:

• Shape memory effect is a multidisciplinary subject, and I will introduce the basic terms and definition at the beginning of my Talk, and introduce the experimental results performed on two copper-based alloys

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.