Materials Science and Chemistry

Biography

Biography: Dr. Osman Adiguzel

Abstract

Metals and metallic alloys exist different crystal phases at the different temperatures and pressures with crystal structures depending on alloy compositions. Crystal structures of these alloys turn to other structures with variation of temperature and stressing by means of crystallographic transformations. A series of alloy system take place in a class of advanced smart materials with the stimulus response to external effect. Shape memory alloys take place in this class by exhibiting a peculiar property called shape memory effect. This phenomenon is characterized by the recoverability of two certain shapes of material in reversible way at different conditions. These alloys are used as shape memory devices in many fields from medicine, biomedical to the building industry. Shape memory effect is initiated on cooling and deformation and performed thermally in a temperature interval on heating and cooling, with which shape of materials cycles between original and deformed shapes in reversible way. Therefore, this behavior is called thermo elasticity.

This phenomenon is governed by structural transformations thermal, and stress induced martensitic transformations. Thermal induced martensitic transformation occurs on cooling with cooperative movements of atoms by means of lattice invariant shears in <110>-type directions on the {110}-type planes of austenite matrix and ordered parent phase structures turn into the twinned martensite structures, along with lattice twinning. Twinned structures turn into the detwinned martensite structures by means of stress induced martensitic transformation with stressing. Movements of atoms are confined to interatomic distances. Therefore, these transformations have diffusion less character.

These alloys exhibit another property, called super elasticity, which is performed with stressing and releasing in elasticity limit at a constant temperature in parent phase region, and shape recovery is performed instantly and simultaneously upon releasing the applied stress, by exhibiting elastic material behavior. Stress-strain profile is nonlinear in stress-strain diagram, also stressing and releasing paths are different, and hysteresis loops refers to energy dissipation, and these alloys can be used in building industry against seismic events with this property.

Copper based alloys exhibit this property in metastable β-phase region which has bcc-based structures. Lattice invariant shear and twinning is not uniform in these alloys and gives rise to the formation of complex layered structures. These structures can be described by different unit cells as 3R,9R or 18R depending on the stacking sequences.

In the present contribution, X-ray diffraction and Transmission Electron Microscopy (TEM) studies were carried out on copper based CuAlMn and CuZnAl alloys. X-ray diffraction profiles and electron diffraction patterns exhibit super lattice reflections. X-ray diffractograms taken in a long-time interval show that diffraction angles and peak intensities change with aging duration at room temperature. This result refers to the rearrangement of atoms in diffusive manner.