Introduction of shape memory alloy: The term shape memory alloy itself says that materials remember their original shape. They can be polymer and ceramic as well. They are use in many engineering applications. These types of materials are advance materials. Shape memory effect can achieve by phase transformation in metals and alloys and this is control by temperature and stress. Shape memory alloy are change their phase by solid state phase transformation. Some common SMAs such as Co-Ni-Ga, Co-Ni-Al, Cu-Al-Be-X(X:Zr, B, Cr, Gd), Au-Cd 46.5/50 at.% Cd, Ag-Cd 44/49 at.% Cd, Cu-Al-Ni-Hf, Cu-Sn approx. 15 at.% Sn, Cu-Zn-X (X = Si, Al, Sn), Fe-Mn-Si, Ni-Ti-Hf, Ni-Ti approx. 55–60 wt.% Ni, Fe-Pt approx. 25 at.% Pt etc.
“Go through below link and read useful article related to polymers”
History of shape memory alloy:
First time scientists take step towards discovery of shape memory effect in 1930. In 1932, Arne Ölander discovered the superelasticity pseudoelastic behavior of the Au-Cd alloy. A nickel-titanium alloy also known as nitinol which is first developed in 1962-1963 by the United States Naval Ordnance Laboratory. Another type of shape memory alloy also exist which are change their shape under the strong magnetic fields such type materials known as ferromagnetic shape-memory alloy (FSMA).
Characteristics of shape memory alloy:
It has two main characteristics such as:
- Shape memory effect
- Superelasticity effect
- Shape memory effect: Shape memory effect describes the effect of restoring the original shape of a plastically deformed sample by heating it. It is occur without diffusion on martensitic transformation. Martensitic phase transformation occurs as the result of stress or temperature change. This phenomenon results from a crystalline phase change known as Thermoelastic martensitic transformation.
Shape memory effect behaviour: It is two types…..
- One way shape memory effect
- Two way shape memory effect
- One way shape memory effect: The reverse transformation induced by heating recovers the inelastic strain; since martensite variants have been reoriented by stress, the reversion to austenite produces a large transformation strain having the same amplitude but the opposite direction with the inelastic strain and the SMA returns to its original shape. This is known as one way shape memory effect. One Way Shape Memory Effect is an intrinsic property of SMAs. Below figure explain its behaviour:
2. Two way shape memory effect: TWSME refers to the reversible and spontaneous shape change of materials with thermal cycling, in other words, this property permitted to SMA a spontaneous shape change on both heating and cooling. Shape memory effect during both heating and cooling is said to have two-way shape memory.
- Superelasticity effect: It is also known as Pseudoelasticity. It is an elastic (reversible) response to an applied stress. It is occur without temperature. “Superelasticity” implies that the atomic bonds between atoms stretch to an extreme length without incurring plastic deformation. Temperature of shape memory alloy is maintained above transition temperature. Load is increased until austenite transforms to martensitic. When load is decreased then martensitic back to austenite. SMA return to original shape as temperature is still above transition temperature.
Properties of SMA: The most widely used shape memory material is an alloy of nickel and titanium called Nitinol. These materials called as engineering materials. This particular alloy has excellent electrical and mechanical properties, long fatigue life, and high corrosion resistance. Shape memory alloys have low yield strength than conventional steel but some compositions have a higher yield strength than plastic or aluminum. The yield stress for NiTi can reach 500 MPa. The maximum recoverable strain these materials can hold without permanent damage is up to 8% for some alloys. If we compare with conventional steel, it has maximum strain 0.5%.
Applications of Shape memory alloy:
The most practical application for those SMAs that have ability to recover a significant amount of strain (superelasticity) or those can apply large force when reverting back to their original shape. SMAs use widely in several fields as described below:
Automotive: The demand for safer, more comfortable and enhanced performance vehicles have tremendously increased the number of sensors, actuators and microcontrollers installed in modern vehicles, which will increase the weight and volume of the vehicle, and are not preferable in vehicle design. Shape memory alloys proving a pioneer in this area. It is use in body frame, engine, some battery parts etc.
Spacecraft or aircraft: SMAs are being explored as vibration dampers for launch vehicles and commercial jet engines. Boeing, General Electric Aircraft Engines, Goodrich Corporation, NASA, Texas A&M University and All Nippon Airways developed the Variable Geometry Chevron using a NiTi SMA. To reduce engine noise, some designers install chevrons on engine to mix the flow of exhaust gases and reduces engine noise.
Civil structure: SMAs wire reinforced in concrete. These wires can sense cracks and contract to heal micro-sized cracks.
Telecommunication: SMAs highly used in smart phones as autofocus (AF) actuator. Some companies are researching on optical image stabilization module driven by SMAs wire.
Medical: SMAs used as fixation devices for osteotomies in orthopaedic surgery, as the actuator in surgical tools.
“Also you can read below retailed articles”
Advantages and disadvantages of SMAs:
- You should know about India’s first private rocket launch: VIKRAM-S
- About Friction stir welding.
- How chemical elements effect on the steel properties?
- How 20MnCr5 steel is different from 16MnCr5 steel?
- A lamp can generate light by the Sea water: “Waterlight”