Nanosciences and Nanotechnology

Biography

Biography: Osman Adiguzel

Abstract

Shape memory alloys take place in advanced smart materials, by exhibiting a peculiar property called shape memory effect, which is characterized by the recoverability of two certain shapes of material at different conditions. These alloys have dual characteristics called thermoelasticity and superelasticity, from viewpoint of memory behavior. Shape memory effect is initiated with thermomechanical processes on cooling and deformation and performed thermally on heating and cooling, with which shape of materials cycles between original and deformed shapes in reversible way in bulk level. Therefore, this behavior can be called thermoelasticity. Superelasticity is performed with stressing and releasing the material in elasticity limit at a constant temperature in the parent phase region, and material recovers the original shape upon releasing. Superelasticity exhibits ordinary elastic material behavior, but it is performed in non-linear way; loading and unloading paths are different at the stress-strain diagram, and hysteresis loop refers to energy dissipation. These phenomena are result of crystallographic transformations called martensitic transformation.

Thermoelasticity is governed by the thermal and stress induced martensitic transformations. Thermal induced martensitic transformation occurs on cooling with the cooperative movements of atoms in <110 > -type directions on the {110} - type planes of austenite matrix and ordered parent phase structures turn into twinned martensite structures along with lattice twinning reactions. The twinned structures turn into detwinned structures, by means of stress induced martensitic transformation with deformation. Superelasticity is also governed by stress induced martensitic transformation and ordered parent phase structures turn into detwinned martensite structures with stressing.

On heating after these treatments, detwinned martensite structures turn into the ordered parent phase structures, by means reverse austenitic transformation. Twinned structures are result of lattive invariant shears  on {110} - type planes of austenite matrix. Atomic movements are confined to the nearest atom distance, and these transformations are diffusionless transformations. 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 martensitic transformations. These alloys are functional materials with these properties, and used in many fields, from biomedical to the building industry.

Copper based alloys exhibit this property in metastable beta-phase region, which has bcc based structures at the parent phase field. Lattice invariant shear twinningis not uniform in these alloys and cause the formation of complex layered structures, depending on the stacking sequences on the close-packed planes of the ordered lattice, like 3R, 9R and 18R depending on the stacking sequences. Periodicity and unit cell is completed through 18 layers in 18R structures.

In the present contribution; x-ray and electron diffraction studies were carried out on two solution treated copper based CuZnAl and CuAlMn alloys. Electron and x-ray diffraction exhibit super lattice reflections. Specimens of these alloys were aged at room temperature, and a series of x-ray diffractions were taken at different stages of aging at room temperature in a long-term interval. X-Ray diffraction profiles taken from the aged specimens in martensitic conditions reveal that crystal structures of alloys chance in diffusive manner, and this result refers to the stabilization.

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