05-06-2012, 01:48 PM
SHAPE MEMORY ALLOYS
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INTRODUCTION
Shape memory alloys are the alloys which exhibit shape memory effect
If such alloys are plastically deformed at one temperature, they will completely recover their shape on being raised to a higher temperature
INTRODUCTION
In recovering their shape alloys can produce a displacement or force (or both in some cases) as a function of temperature
We can make metals change shape, change position, expand, compress with heat as the only activator
Because of these properties shape memory alloys are widely used in mechanical, electro-mechanical and medical applications
DEFINITION
Shape Memory Alloys are a unique class of metal alloys that can recover apparent permanent strains when they are heated above a certain temperature
STABLE PHASES OF SMA
Shape Memory Alloys have two stable phases
-Austenite
-Martensite
AUSTENITE
It is the high-temperature phase
It is an interstitial solid solution of carbon in gamma iron with FCC lattice
It is a non magnetic phase
It exists at temperatures above 723C
It is soft and ductile
MARTENSITE
It is the low-temperature phase
It is produced by the decomposition of austenite when cooled rapidly
It is an interstitial super saturated solution of carbon in α-iron with body centered tetragonal lattice
It possess less toughness
COOLING WITHOUT APPLYING LOAD
Upon cooling without applying external load, the material transforms from austenite into twinned martensite
No observable macroscopic change occurs during the process
When heating twinned martensite, a reversible martensitic transformation takes place and as a result the material transforms to austenite
APPLYING LOAD
If mechanical load is applied to the material in the state of twinned martensite, it will detwin the martensite
Upon releasing the load, material remains deformed
APPLYING LOAD
When heating detwinned martensite above austenite finish temperature, a reverse phase transformation will take place i.e., martensite to austenite
Thus the material retains its original shape
AUSTENITE TO DETWINNED MARTENSITE
It is also possible to induce a martensitic transformation which would lead directly to detwinned martensite
If the load is applied in austenitic phase and the material is cooled, the phase transformation will result in detwinned martensite
large strains will be observed
PSEUDOELASTIC EFFECT
It is also possible to induce a phase transformation by applying a pure mechanical load
The result of this load application is fully detwinned martensite and very large strains are observed
The martensite formed in this process is known as stress-induced martensite
PSEUDOELASTIC EFFECT
Stress-induced martensite is only stable under the application of stress
On unloading, the reduction in stress and surrounding elastic forces generated during transformation cause the martensite to come back to the original parent phase
SHAPE MEMORY EFFECTS
Shape memory alloys have different shape memory effects
Two common shape memory effects are
-One way shape memory effect
-Two way shape memory effect
ONE WAY SHAPE MEMORY EFFECT
When a shape memory alloy is in its cold state, the metal can be bent or stretched and will hold these shapes until heated above the transition temperature
Upon heating, the shape changes to its original
When the metal cools again it will remain in the hot shape, until deformed again
TWO WAY SHAPE MEMORY EFFECT
It is a shape memory effect in which the material remembers both hot and cold shapes
Under normal circumstances, a shape memory alloy remembers its high temperature shape, but upon heating to recover the high temperature shape, immediately forgets the low temperature shape
TWO WAY SHAPE MEMORY EFFECT
However, it can be trained to remember to leave some reminders of the deformed low temperature condition in high temperature phases
TYPES OF SHAPE MEMORY ALLOYS
Main types of shape memory alloys are
- Nickel-titanium [NiTi]
- Copper-zinc-aluminium-nickel
- Copper-aluminium-nickel
APPLICATIONS
Shape memory alloys are used as actuators with heat as the only actuating element
Pseudo elastic alloys are used for making eyeglass frames so that they can be bent or stretched to any shape without breaking
They will come back to their original position after releasing the applied stress
Shape memory alloys are also used in medical and dental fields
They are also used in electro-mechanical field
ADVANTAGES
High level of recoverable plastic strain
Good mechanical properties
Diverse field of application
DISADVANTAGES
High cost
Poor fatigue properties
CONCLUSION
This seminar has presented basic facts about shape memory alloys. Shape memory alloys will be exploited in many more fields in the future. Research groups are trying to improve the properties of these alloys.
