34^th World Nano Conference

Biography

Biography: Osman Adiguzel

Abstract

Metals and alloy systems have different phases at different conditions, and these phases are described in phase diagrams as alloy composition-temperature or composition-pressure space. A series of alloy systems exhibit a peculiar property called shape memory effect in β-phase region. These alloys are called shape memory alloys and they are very sensitive to external conditions this phenomenon is initiated with thermomechanical processes on cooling and deformation, and performed thermally on heating and cooling, with which shape of the materials cycle between original and deformed shapes in reversible ways. Therefore, this behavior can be called thermoelasticity. This is plastic deformation, due to the soft character of the material in low temperature condition, with which strain energy is stored in the materials and release upon heating, by recovering original shape.

Shape memory effect is governed by thermomechanical phase transformations in crystallographic level, thermal and stress induced martensitic transformations.  Thermal induced martensitic 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 reaction, and ordered parent phase structures turn into twinned martensite structures. The twinned structures turn into detwinned martensite structures by means of stress induced transformation with mechanically stressing the material in the martensitic condition. These reactions are driven by lattice invariant shear, and lattice twinning and detwinning reactions play important role at the martensitic transformations. These alloys exhibit another property called superelasticity, which is performed with mechanically stressing and releasing the material in elasticity limit at a constant temperature in parent phase region and shape recovery occurs instantly upon releasing, by exhibiting elastic material behavior.

Stress-strain profile exhibits nonlinear behavior at stress-strain diagram, Stressing and releasing paths are different and hysteresis loops refers to energy dissipation. This phenomenon is also result of stress induced martensitic transformation and ordered parent phase structures turn into detwinned martensite structure with stressing.

Copper based alloys exhibit this property in metastable β-phase region. Lattice twinning and lattice invariant shear are not uniform in these memory alloys and gives rise to the formation of layered structures, like 3R, 9R or 18R depending on the stacking sequences on the {110} - type close-packed planes of the parent phase. Unit cell and periodicity is completed through 18 layers in 18R structures in ternary copper-based alloys. Also, parent phases of these alloys have the high symmetry, and product martensitic phases have low symmetry at low temperature.

In the present contribution, x-ray diffraction and electron diffraction studies were carried out on copper based CuZnAl and CuAlMn alloys. X-ray diffraction profile and electron diffraction patterns exhibit super lattice scattering. Critical transformation temperatures of these alloys are over the room temperature, at which alloy samples are completed in the martensitic state. These alloy samples were aged at room temperature, and a series of x-ray diffraction profiles and electron diffraction patterns were taken. X-ray diffractograms taken in a long-time interval show that scattering angles, peak intensities, and characteristics change with ageing at room temperature. This result refers to the rearrangement of atoms in diffusive manner.

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