Shape recovery and crystallographic reactions in shape memory alloys

Title : Shape recovery and crystallographic reactions in shape memory alloys

Abstract:

Some materials take place in class of smart materials with adaptive properties and stimulus response to the external conditions. Shape memory alloys take place in this group by exhibiting a peculiar property called shape memory effect, which is characterized by the recoverability of two certain shapes of material at different conditions. Shape memory effect is initiated by successive cooling and deformation treatments, and activated thermally on heating and cooling, with which shape of the materials cycles between original and deformed shapes in reversible way in bulk level. Therefore, this behavior can be called Thermoelasticity. This deformation is plastic deformation, with which strain energy is stored in the materials keeping the deformed shape and released on heating by covering original shape on heating.  Two successive crystallographic reactions, thermal and stress induced martensitic transformations govern shape memory phenomena in crystallographic basis. Thermal induced transformation occurs on cooling with the cooperative movement of atoms by means of shear-like mechanism in <110>-type directions on {110} -type planes of austenite matrix, along with crystal twinning on cooling and ordered parent phase structures turn into twinned martensite structures. The twinned structures turn into the detwinned structures by stressing material in low temperature condition by means of stress induced transformation. These alloys exhibit another property called superelasticity, which is performed mechanically by 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. Superelasticity is performed in non-linear way; stressing and releasing paths are different in the stress-strain diagram, and hysteresis loop refers to energy dissipation. The elementary processes involved in such the transformations are lattice invariant shear, lattice twinning and detwinning. It is well known that crystal twinning and detwinning reactions play a considerable role in shape memory effect and superelasticity.  Martensitic transformations have diffusionless character and movements of atoms are confined to inter atomic distances. Copper based alloys exhibit this property in metastable β-phase region, with chemical composition. Lattice invariant shears and lattice twinning are not uniform in these alloys, and the ordered parent phase structures martensitically undergo the non-conventional complex layered structures on cooling. 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 is 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 copper based CuZnAl and CuAlMn alloys. X-ray diffraction profiles and electron diffraction patterns exhibit super lattice reflections inherited from parent phase due to the displacive character of martensitic transformation. 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 reveals a new transformation in diffusive manner. Some materials take place in class of smart materials with adaptive properties and stimulus response to the external conditions. Shape memory alloys take place in this group by exhibiting a peculiar property called shape memory effect, which is characterized by the recoverability of two certain shapes of material at different conditions. Shape memory effect is initiated by successive cooling and deformation treatments, and activated thermally on heating and cooling, with which shape of the materials cycles between original and deformed shapes in reversible way in bulk level. Therefore, this behavior can be called Thermoelasticity. This deformation is plastic deformation, with which strain energy is stored in the materials keeping the deformed shape and released on heating by covering original shape on heating.  Two successive crystallographic reactions, thermal and stress induced martensitic transformations govern shape memory phenomena in crystallographic basis. Thermal induced transformation occurs on cooling with the cooperative movement of atoms by means of shear-like mechanism in <110>-type directions on {110} -type planes of austenite matrix, along with crystal twinning on cooling and ordered parent phase structures turn into twinned martensite structures. The twinned structures turn into the detwinned structures by stressing material in low temperature condition by means of stress induced transformation. These alloys exhibit another property called superelasticity, which is performed mechanically by 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. Superelasticity is performed in non-linear way; stressing and releasing paths are different in the stress-strain diagram, and hysteresis loop refers to energy dissipation. The elementary processes involved in such the transformations are lattice invariant shear, lattice twinning and detwinning. It is well known that crystal twinning and detwinning reactions play a considerable role in shape memory effect and superelasticity.  Martensitic transformations have diffusionless character and movements of atoms are confined to inter atomic distances. Copper based alloys exhibit this property in metastable β-phase region, with chemical composition. Lattice invariant shears and lattice twinning are not uniform in these alloys, and the ordered parent phase structures martensitically undergo the non-conventional complex layered structures on cooling. 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 is 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 copper based CuZnAl and CuAlMn alloys. X-ray diffraction profiles and electron diffraction patterns exhibit super lattice reflections inherited from parent phase due to the displacive character of martensitic transformation. 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 reveals a new transformation in diffusive manner.

Audience take away:

• Shape memory alloys are multifunctional materials, and they are used as shape memory devices in many fields from biomedical to the building industry.
• This is a multidisciplinary conference, and I will introduce the basic terms and definitions related to shape memory phenomenon at the beginning of my talk. Therefore, the delegates will be familiar with shape memory effect.

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.