THEME: "Innovating the Future: Transforming Ideas into Reality with 3D Printing"
Firat University, Elazig, Turkey
Title: Crystallographic Transformations Governing Dual Memory in Shape Memory Alloys
Adiguzel graduated
from Department of Physics,
Shape memory alloys take
place in a class of advanced smart materials by exhibiting dual memory
characteristics, shape memory effect and superelasticity with the
recoverability of two shapes at different conditions. Shape memory effect is initiated with
thermomechanical treatments on cooling and deformation and performed thermally
on heating and cooling, with which shape of the material cycles between
original and deformed shapes in reversible way. This phenomenon is governed by
the thermal and mechanical transformations, thermal and stress induced
martensitic transformations. Thermal induced martensitic transformations occur
on cooling with cooperative movement of atoms in <110 > -type directions
on {110} - type planes of austenite matrix, along with lattice twinning
reaction and ordered parent phase structures turn into the twinned martensite
structures. The twinned structures turn into detwinned martensite structures
with deformation by means of stress induced martensitic transformations. Superelasticity
is performed in only mechanical manner with stressing the material and
releasing in the parent austenite phase region, and shape recovery occurs
instantly upon releasing, by exhibiting elastic material behavior.
Superelasticity is performed in non-linear way, unlike normal elastic materials
behavior, loading and releasing paths are different at the stress-strain diagram,
and hysteresis loop refers to the energy dissipation. 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 the
transformations, and they are driven by inhomogeneous lattice invariant shears.
Copper based alloys exhibit this
property in metastable ?-phase region. Lattice
twinning is not uniform in these alloys and cause the formation of unusual
complex layered structures. The 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
are 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 and electron diffraction studies were carried out on copper
based CuZnAl and CuAlMn alloys. X-ray diffraction profiles and electron
diffraction patterns exhibit super lattice reflection. 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. This result refers to the rearrangement of
atoms in diffusive manner.