Nanoscience and Technology

Biography

Biography: Osman Adiguzel

Abstract

Shape memory alloys take place in a class of smart materials by exhibiting a peculiar property called shape memory effect. This property is characterized by the recoverability of two certain shapes of material at different temperatures. These materials are often called smart materials with the functionality and capacity of responding to changes in the environment. These materials are used as shape memory devices in many interdisciplinary fields such as medicine, bioengineering, metallurgy, building industry and many engineering fields. Shape memory effect is initiated by cooling and deformation, and performed thermally by heating, and this behavior is called thermoelasticity.  This phenomenon is based on lattice reactions, called martensitic transformation, and this transformation is characterized by changes in the crystal structure of the material. This is plastic deformation; strain energy is stored after releasing and released on heating by recovering the original shape of material. These alloys are mainly used as deformation absorbent materials in control of civil structures subjected to seismic events, due to the absorbance of strain energy during any disaster or earthquake. These alloys exhibit another property, called superelasticity performed by stressing and releasing the material in parent phase region. Loading and unloading paths are different in stress strain diagram, and cycling loop refers to the energy dissipation. Thermal induced martensitic transformation occurs on cooling along with lattice twinning with cooperative movements of atoms by means of lattice invariant shear, which occurs in two opposite directions, <110 > -type directions on the {110} - type planes of austenite matrix. Ordered parent phase structures turn into twinned martensite structures with thermal induced transformation, and the twinned structures turn into the detwinned structures by means of stress induced martensitic transformation by stressing the material in the martensitic condition.  

Copper based alloys exhibit this property in metastable β-phase region, which has bcc-based structures at high temperature parent phase field.  Lattice invariant shear and twinning is not uniform in copper based ternary alloys and gives rise to the formation of complex layered structures, depending on the stacking sequences on the close-packed planes of the ordered parent phase lattice.

In the present contribution, x-ray diffraction and transmission electron microscopy (TEM) studies were carried out on two copper based CuAlMn and CuZnAl alloys. X-ray diffraction profiles and electron diffraction patterns reveal that both alloys 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 duration at room temperature. Especially, some of the successive peak pairs providing a special relation between Miller indices come close each other. This result refers to the rearrangement of atoms in diffusive manner.

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