Shape memory alloys are one of the types of smart materials. They are extensive in use since 1950. 

History  

The first time the shape memory effect (SME)was seen in Gold-Cadmium alloy in1951. But, it was not too useful. Later on, in 1962 SME was seen in titanium and nickel alloy(Nitinol). This time the effect was significant. Although this effect was found in other Copper and Iron base alloys CuZnAl, FeMnSi. But, the Nitinol effect was more comprehensive. 



How do SMAs work

Shape memory alloys pass through a high recoverable strain under solid to solid transformation. It is a shape-changing process at a specific temperature. SMAS remember its original shape, it returned to its preformed shape by heating. There are some metals that are one or two shape memory alloys. One shape memory alloy only remembers one shape while two shape memory alloy remembers two shapes at both high and low temperature. 






 Micostractural change 

In the shape-changing process, the microstructure of the alloy change from martensitic to austenite at high temperature, that is much strong than at the low-temperature martensitic structure is obtained.



shape memory process
                               

 Applications

  1. These are being used in bio-medical devices e.g stents, surgical tools, and dentistry braces 

  2. In space application e.g flaps control, and ribbons 

  3. In manufacturing e.g actuators, eyeglasses and robots 

   Pros and Cons of SMAs

Pros

  • Biocompatible

  • Good mechanical properties 

Cons 

  • Expensive SMAs process 

Summary   
 still much more researches are needed to get high benefits from SMAs. but, this alloy is beneficial in this era because it resolves some complications of handling in machines without heavy motors.