This Alloy Can Remember How It Looks!!! 😱
Introduction
First discovered by William J. Buehler with Frederick Wang in 1959 during their research at Naval Ordnance Laboratory. It is an alloy of Nickel and Titanium called Nitinol (NiTi). It is an acronym for Ni- Nickel, Ti-Titanium, Nol-Naval Ordnance Laboratory.
It has main properties like ‘Super elasticity’ and ‘shape memory’ (due
to this property it is also called a Shape memory alloy).
Let us go into details of the properties of Nitinol.
Why Nitinol is called a shape memory alloy (SMA)?
Nitinol is called a shape memory alloy because it regains its predetermined shape when it is heated above a certain temperature generally known as transformation temperature, irrespective of the shape from which it is being transformed. i.e., it actually remembers its shape!
This can also be called ‘The shape memory effect’. In simple words if a straight wire is deformed by winding it into a coil, then heating this coil to a certain temperature by placing it into a hot water bath ‘helps’ that material to remember its original shape. So, it reverts back to original straight wire form and deformation can be removed.
Equilibrium binary phase diagram of Nickel and Titanium:
As
Nitinol is an alloy of Nickel and Titanium it is worth taking a glance at binary
phase diagram of Ni-Ti.
Nitinol alloy is called Nitinol 60 or 60NiTi
because it approximately has the composition of 60 wt% Ni and 40 wt% Ti. This
is an intermetallic compound. Depending on heat treatment given to alloy, it
can be found in different phases with different mechanical and chemical
properties.
Approximately at 2-20 wt% Ni present in phases α and β (of
titanium) and an intermetallic compound Ti2Ni at higher
temperatures. By lever rule it can be said that higher the concentration of Ni
higher is the amount of intermetallic compound formed. A eutectoid reaction
occurs at 765 0C, β → α + Ti2Ni
for approximately 8 wt % of NI. If 10% of Ni by weight is there, then after
cooling down from higher temperature it will show proeutectoid Ti2Ni
phase in α matrix. A eutectic reaction L → β + Ti2Ni occurs at 942 0C
where Ni is approx. 28.4 wt %.
We
can see austenite phase can be viewed in alloy when 60NiTi is heated above 10500C.
Other phases viz. Ni3Ti, Ni4Ti3, Ni3Ti2
can
be found depending upon cooling rates provided to the alloy from 10500C.
Out of these Ni4Ti3 and Ni3Ti2 are
metastable phases. Precipitates of the above 3 phases can be found distributed in the matrix of Ni-Ti based alloys at room temperature. These phases generally are
not suitable as they affect the compatibility of 60NiTi alloy by becoming a
reason for corrosion due to their soft nature. Quenching is helpful to prevent the appearance
of phases Ni3Ti and Ni3Ti2. But Ni4Ti3
precipitation generally cannot be avoided because of the faster
nucleation of this phase.
This binary system is very sensitive to small changes in composition.
The working mechanism of shape memory alloy:
The Mechanism of Shape Memory Alloy (SMAs) is based on Solid-to-Solid Phase Transformation.
Nitinol generally shows two different types of phases or crystal structure, Martensite and Austenite.
1)Martensite: It is a phase that is stable at a lower temperature and can be plastically deformed, therefore we can have more than one martensitic form.
2)Austenite: It is a phase that is stable at a higher temperature and hard to deform.
It is the structure where one nickel atom is at the centre of the cube with titanium atoms are at the corners. This Solid-to-Solid phase transformation is athermal, therefore it depends only on temperature. Temperature and Internal stresses are the two factors that play an important role in the elasticity of shape memory alloy.
So, when the Martensitic phase is heated at transformation temperature, there is the transformation of the alloy from Martensite to Austenite phase (as this phase is stable at that temperature). The temperature at the start of the phase transformation is called as As and the end temperature is called Af
If this Austenite phase is cooled from the existing temperature, Austenite to Martensite transformation takes place at Ms temperature and ends at Mf temperature and we get a sheared structure i.e., atoms can slide over one another and the material becomes malleable.
The temperature at which alloy reverts to its original shape is called a memory transfer temperature. This temperature can be altered by making small changes in alloy composition (or even by adding tertiary elements like V, Co etc.) and by changes in heat treatments given to alloy.
During In this phase transformation the atomic arrangements changes but the position
of atoms with respect to each other do not change.
The
reversibility of this transformation makes SMAs special.
How
to set the Default Shape?
To Set the Default shape, Nitinol is heat-treated at 500 degrees Celsius into the desired shape and then quenched into water. This desired shape will remain the default shape unless this process is repeated with some other shape. The material obtained is in the Twinned Martensitic phase.
Now, if we deformed this martensitic phase in any random shape and then heat it to Memory Transfer Temperature, it will return back to its original shape which is predetermined by the Austenitic phase. When this structure is cooled, its shape does not change but microstructure resembles Twinned Martensite which can be deformed again and the process can be repeated.
Mechanical Properties of Nitinol:
|
Property |
Austenite form |
Martensite form |
|
Ultimate tensile strength |
Ranges between 754 – 960 MPa
|
|
|
Yield strength |
195-690 MPa |
70-140 MPa |
|
Elastic modulus (approx.) |
75 GPa |
28 GPa |
|
Poisson's ratio (approx.) |
0.3 |
|
Based on its mechanical properties, its different workability properties are given
Hot workability is quite good. Due to rapid work hardening, it's quite difficult to cold work. Its Machinability is also difficult.
Physical Properties:
|
Property |
Austenite form |
Martensite form |
|
Melting point |
13000C |
|
|
Physical appearance |
Silvery bright metal |
|
|
Density |
6.45 gm/cm3
|
|
|
Resistivity |
82 Ω-cm |
76 Ω-cm |
|
Thermal conductivity |
0.18 W/cm- 0C |
0.086 W/cm-
0C |
|
Heat capacity |
0.077 cal/ gm-°C |
|
|
Latent heat |
5.78 cal/ gm |
|
|
Corrosion performance |
excellent |
|
Manufacturing:
Due to high reactivity of Titanium
and requirement of greater compositional accuracy (as Transformation
temperature alters), Nitinol is extremely difficult to manufacture.
Almost all Nitinol components go
through a series of similar steps of manufacturing...
1)Nitinol is Vacuum Melted
2)Hot worked
3)Cold worked
4)Heat Treated to achieve final
properties.
Hot working of Nitinol is relatively
easy, but Cold working is difficult because the enormous elasticity of alloy
increases die contact, leading to tremendous frictional resistance and tool
wear.
Heat treating of Nitinol requires intensive knowledge of transformation temperatures.
Advantages:
i.
High
damping capacity.
ii.
High
wear resistance.
iii.
High
corrosion and chemical resistance.
iv.
High specific strength
v. Large deformation
Limitations:
1)SMAs are subjected to
a) Structural Fatigue:
It is a type of failure that occurs due to loading and
unloading cycle which results in the formation of crack and its propagation.
Eventually it results in catastrophic failure.
b) Functional Fatigue:
It is a type of failure where material do not break but it
loses its shape memory over time. The ability
of material to undergo reversible phase transformation is lost.
2)SMA actuators are actuated electrically. Here, the
actuation and deactuation period is
not same (Actuation is relatively fast). This results in asymmetric actuation.
3)Shape Memory Alloy are relatively expensive to manufacture and machine as compared to other materials such as steel and aluminium.
Applications:
a. Biological applications:
Two main properties of Nitinol, shape recovery due to temperature changes and super elasticity, have made it an excellent biomaterial.
1. The first
main application is in orthodontic field. NiTi wires are in austenite phase in our buccal cavity due to
certain temperature and hence used in
multibracket treatment. Braces of teeth also contain Nitinol.
2. In orthopaedic field Nitinol is used in orthopaedic staples or plates which
are used to correct fractures. When inserted at the place of fracture inside body,
due to body temperature it undergoes shape memory effect and helps to join two
fractured bone pieces.
3. These are also broadly used in
vascular field and neurosurgical field.
b.
They
are used as alternatives for actuators, thermostats, Helicopter blades and
Hydraulic fittings.
c. They are also developed for their use in spring tyres.
Nitinol Dental Braces Nitinol Orthodontic wire
Image source: https://www.samaterials.com/nitinol/407-nitinol-dental-braces.html
https://www.hindawi.com/journals/jm/2011/501483/
Orthopaedic staples of Nitinol
Image sources: https://medicalcomponentspecialists.com/orthopedic-staples/
https://www.medshape.com/foot-staples/
 |
| Nitinol self-expandable neurosurgical stent |
Image source: https://3d-car-shows.com/goodyear-spring-tyre/
References:
https://sites.google.com/site/me10001shapememoryalloys/nitinol
https://www.medicaldesignbriefs.com/component/content/article/mdb/features/articles/23077
https://www.chemistrylearner.com/nitinol.html
https://www.youtube.com/watch?v=EKimWj8c-MQ
https://www.hindawi.com/journals/jm/2011/501483/
https://www.researchgate.net/figure/Advantages-and-disadvantages-of-NiTi-SMA_tbl1_311202728
https://en.m.wikipedia.org/wiki/Shape-memory_alloy
Written by -
Chetaneshwari Nivargi & Praful Kadam - TEAM METAMONDAY
NOTE :-
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