Shape reversibility and thermomechanical reactions in shape memory alloys

Title : Shape reversibility and thermomechanical reactions in shape memory alloys

Abstract:

Shape memory effect is a peculiar property exhibited by certain alloy systems, called shape memory alloys. These alloys have dual characteristics called thermoelasticity and superelasticity. Shape memory effect is initiated thermomechanical processes in the bulk level, on cooling and deformation and performed on heating and cooling after these processes. Therefore, this behavior is called thermoelasticity or thermal memory. Thermoelasticity is governed by governed by successive thermal and mechanical reactions, in the atomic level, thermal and stress induced martensitic transformations. Thermal induced transformation occurs on cooling with cooperative movement of atoms in <110> -type directions on the {110}-type planes of austenite matrix, along with lattice twinning and ordered parent phase structures turn into twinned martensite structures. Twinned structures turn into detwinned martensite structures by means of stress induced transformation by deformation in martensitic condition. This is plastic deformation, strain energy is stored in the material with deformation, and released upon heating, by recovering the original shape in bulk level, and cycles between original and deformed shapes on heating and cooling, respectively. Superelasticity is performed by mechanically stressing the material at a constant temperature in parent phase region, and can be called mechanical memory, too. The materials are mechanically just over Austenite finish temperature, and shape recovery is performed simultaneously upon releasing the applied stress, by exhibiting the classical elastic material behavior. Superelasticity is also a result of stress induced martensitic transformation, and the ordered parent phase structures turn into the detwinned structures by means of stress induced martensitic transformation. Superelasticity is performed in non-linear way, unlike normal elastic materials; stressing and releasing paths are different in stress-strain diagram, and hysteresis loop refers to energy dissipation. Deformation at different temperatures exhibits different behavior beyond shape memory effect and superelasticity. Copper based alloys exhibit this property in metastable β-phase region, which has bcc-based structures. Lattice twinning is not uniform in these alloys, and the ordered parent phase structures turn into the non-conventional complex layered structures.  The long-period 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 studies were carried out on two copper based CuZnAl and CuAlMn alloys. X-ray diffraction profiles and electron diffraction patterns exhibit super lattice reflections inherited from parent phase.  X-ray diffractograms taken in a long-time interval show that diffraction angles and intensities of diffraction peaks change with the aging time at room temperature. This result refers to new reactions in diffusive manner.

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

Audience take-away: 
Shape memory effect is a multidisciplinary subject from physics, chemistry to materials sciences and metallurgy. Shape memory alloys are functional advanced materials and used shape memory devices in biomedical and other applications. I will introduce the details on thermoelasticity and superelasticity.

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 been retired due to the age limit of 67; following academic life of 45 years. 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 six years (2014 - 2019) over 60 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.