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April 15-16, 2019 | Frankfurt, Germany
Applied Physics & Laser, Optics and Photonics
International Conference on
Materials Science and Nanotechnology | Volume: 3
Physical basis of shape memory effect and reversibility in shape memory alloys
Osman Adiguzel
Firat University, Turkey
A
series of alloy system take place in a class of
smart materials due to 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 at different temperatures.
Shape memory materials are used as shape
memory devices in many interdisciplinary fields
such as medicine, metallurgy, building industry
and many engineering fields. Shape memory
effect is performed thermally by heating and
cooling after first cooling and stressing treatments.
Shape memory effect is result of successive
crystallographic transformations; thermal and
stress induced martensitic transformations.
Shape memory alloys exhibit another property
called superelasticity, which is performed by
stressing material at high temperature parent
phase region. This effect exhibit classical elastic
material behavior and it is performed by stressing
and releasing the material in parent phase region.
Loading and unloading paths are different in
stress strain diagram, and cycling loop reveals
energy dissipation. The strain energy is stored
after releasing, and 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.
Thermal inducedmartensitic transformation is first
order lattice-distorting phase transformations,
and thermally occurs on cooling, by which ordered
parent phase structures turn into twinned
martensitic structures. This transformation occurs
with cooperative movements of atoms by means
of lattice invariant shear. Lattice invariant shears
occur in two opposite directions, <110 > -type
directions on the {110} - type planes of austenite
matrix which is basal plane of martensite.
Thermal induced martensite occurs as twinned
martensite, 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, like 3R,
9R or 18R depending on the stacking sequences
on the close-packed planes of the ordered lattice.
Crystal structure of martensite of these alloys is
orthorhombic and basal plane is hexagonal.
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. In particular, 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.
e
:
oadiguzel@firat.edu.tr