Shape Memory Alloy

This article provides an overview of shape memory alloys (SMAs), focusing on their characteristics, properties, and applications. It discusses the martensitic transformation, shape memory effect, and trends in SMA materials applications. The article also highlights the research gap in understanding the thermochemical behavior of SMA springs under thermo-mechanical loads.

SHAPE MEMORY ALLOY
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Introduction
Memory alloys are an integral aspect in the biomedical sector owing to its characteristics of
being temperature controlled dependant. This alloy of nickel and titanium is as well called
Nitinol which provides great performance besides biocompatible. The usual make up of 50/50
percentage of atom nickel titanium is extensively applied as experiments as well as analyses
conducted on the alloy have resulted in a conclusion that titanium generate a layer of TiO2 on the
sample when it undergoes corrosion hence safeguarding the sample from undergoing more
carrion rendering it the most biocompatible SMA [1].
SMA comes in two main phases: high temperature phase and low temperature phase with the
difference being in the thermal conductivity, Young’s modulus as well as resistivity. Another
unique behavior is attainable in the high temperature phase by the manner in which the material
may be deformed up to about 10% and then brought to its initial shape, a condition defined as
super elasticity [2]. Both shape memory effect and super elasticity are a factor of the behavior of
strain and stress and have extensively been covered in different areas of applications including
cardiovascular devices, orthopedics implants, endodontic devices among numerous other sectors
and field. Numerous materials are used in the manufacture of SMA including NiTi which is the
most widely used of the materials in the industrial applications as well as NiTiCu which is often
applied in the medical field [12].
Properties of shape memory alloys
Martensitic transformation
Maternsite is formed in near-equiatomic Ni-Ti allots when cooling is done from the high
temperature phase of the body centered cubic which is defined as austenite by a shear process

type. This phase of maternsite is often heavily twinned. The transformation that produces super
memory effect and super elasticity is often diffusion less transformation of phase in solids known
as martensitic transformation [3]. The atoms undergo cooperative arrangement during the
transformation to form a crystal structure that is different but bears the same identical
composition chemically through a distortion process that is displacive.
The martensitic transformation alters the material from the initial phase which was a low
temperature phase known as austenite to a low temperature phase called maternsite show [12].
The martensitic variants are often formed in a pattern that is twinned during the transformation
that occurs where the atoms are displacement with mirror symmetry. This takes place as the
crystal struggles to attain minimal potential energy states of a specific temperature. A deformed
maternsite restores to austenite upon heating and the restrictions of the crystallography are done
in such a way that it changes back to the original orientation hence getting back to the original
shape [4].
Shape Memory effect
Shape memory effect is yet another feature of thermoplastic martensitic transformation in SMAs.
Maternsite generated through cooling is composed of a random mixture of numerous variants
even though maternsite that are stress induced are made up of just a single preferential variation
as per the stress applied [5]. Application of stress is capable of easily displacing twin boundaries.
Movement in the twin boundaries of the crystal may result in a change in the orientation of the
lattice, a process defined as detwinn. During the process of detwinn of maternsite crystal
structure, the stress remains relatively unchanged when the crystal is facing a unidirectional
loading to the point where complete detwinning occurs. Deformation first occurs to the crystals
that are favourably aligned to the direction of the load at lower levels of stress [13].

Trends in shape memory alloys materials applications
The performance of a device for passive control for stay cable in bridges that are cable stayed
manufactured using shape memory alloys was studied by BenMekki et al. The focus in the study
was on the upper elasticity feature of SMA and its application in the development of a device for
supplementation energy dissipation for stay cable. The natural structure efficiency with and
without embedded shape memory alloy wires has been estimated by Kin-tak Lau [11].
More experimental studies were conducted using pre-strain SMA wires and the findings were
that the natural frequencies slightly changed when the temperature was above the finish
temperature of austenite of the composite beams that had embedded non-prestrained wires of
SMA [15]. Nevertheless, the theoretical prediction as well as the measurements from
experiments indicated that there is an increase in the natural frequencies of beams that have
embedded prestrained SMA wires [6]. There was an increase in the ration of damping of the
composite beams of SMA as the temperature of the embedded wires increased in the presence or
absence of prestraining. It was established in the experimental findings that embedded wires of
SMA wire able to modulate the natural frequency of beams in some levels [10].
The SMA of helical springs when under the influence of axial force was evaluated by Rexza
Mirzaeifar et al. the experiment was carried out in various phases and conditions. The initial
experiment was done without taking into consideration the effects of the curvature of the helical
spring [7]. The next phase was inclusive of the effects of the curvature and an analysis of the
SMA helical spring was done with the same solution that was provided for torsion of the curved
bars of SMA. A comparison of all the results was done for a Nitinol helical spring. Some of the
findings of the experiment were offered was recommendations for enhancing the working of
SMA helical springs that are used as devices for dissipating energy [14].

Research Gap
It is noted from numerous literature survey not enough of efforts have been made in describing
the thermochemical behavior of the wire of SMA springs when under thermo-mechanical loads
and thus there is not enough data at disposal on the same. This study is geared towards gaining a
proper comprehension of the behavior of NiTi shape memory alloy springs using various
methods including shape recovery force experiment and isothermal fore deflection experiment
[9].

References
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