Shape memory effect and diffusionless phase transformation in shape memory alloys

Title : Shape memory effect and diffusionless phase transformation in shape memory alloys

Abstract:

Shape memory alloys take place in a class of adaptive structural materials called smart materials by exhibiting a peculiar property called shape memory effect and superelasticity with the recoverability of two shapes at different conditions.  Shape memory effect is initiated with thermomechanical treatments 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. Deformation in the low temperature condition is plastic deformation, with which strain energy is stored in the materials and released on heating by recovering the original shape. This phenomenon is governed by the thermal and mechanical transformations, thermal and stress induced martensitic transformations. Thermal induced martensitic transformations occur on cooling with cooperative movement of atoms in <110 > -type directions on a {110} - type plane of austenite matrix, along with lattice twinning reaction and ordered parent phase structures turn into the twinned martensite structures. The twinned structures turn into detwinned martensite structures with deformation by means of stress induced martensitic transformations. Superelasticity is performed in only mechanical manner with stressing the material and releasing in the parent austenite phase region, and shape recovery occurs instantly upon releasing, by exhibiting elastic material behavior. Atomic movements are confined to the nearest atom distances, atomic neighborhoods do not change, 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, shape recovery occurs instantly upon releasing, by exhibiting elastic material behavior.  Superelasticity is performed in non-linear way, and loading, 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 the detwinned martensite structures with stressing. Lattice twinning and detwinning reactions play important role at the transformations.

Copper based alloys exhibit this property in metastable beta-phase region. Lattice twinning is not uniform in these alloys and cause to the formation of unusual 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. The unit cell and periodicity are completed through 18 layers in direction z, in case of 18R martensite, and unit cells are not periodic in short range in direction z.

In the present contribution, x-ray diffraction and transmission electron microscopy (TEM) studies were carried out on copper based CuZnAl and CuAlMn alloys. X-ray diffraction profiles and electron diffraction patterns exhibit super lattice reflection. 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.

Audience Take Away Notes: 

• Shape memory alloys are functional materials, shape memory effect is multidisciplinary subject, audiences are from different disciplines. Therefore, usually I introduce the basic terms and definition related to this phenomenon at the beginning of my Talk and introduce the experimental results obtained in shape memory alloy samples. So, audiences can gain elementary knowledge on shape memory phenomena

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.