SMA:-SHAPE MEMORY ALLOYSShare memory alloys (SMAs) are metals, which exhibit two very uniqueproperties, pseudo-elasticity and the shape memory effect"The ability of the metallic alloys to retain to their original shape whenheating or cooling is called as Shape Memory Alloys (SMA)."These metallic alloys exhibit plastic nature when they are cooled to very lowtemperature and they return to their original nature when they are heated. This effect isknown as Shape Memory Effect.It is also called as smart materials or intelligent materials or Active materials.There are two types of shape memory alloys,1. One way shape memory It returns to its memory only when heating2. Two way shape memory It returns to its memory on both heating and Cooling.Classification1. Piezo electric SMA materials.2. Electrostrictive SMA materials.3. Magnetostrictive SMA materials.4. Thermo elastic SMA materialsegNi-Ti, Cu Zn A1, Cu A1 Ni, Au Cd, Ni-Mn-Ga and Fe based alloys.Working Principle of SMAThe shape memory effect occurs in alloys due to change in the crystalline structureof the materials with the change in temperature and stress.The shape memory effect occurs between two temperature states known asMartensite and Austenite. The Martensite structure is a low temperature phase and isrelatively soft, It has platelet structure the Austenite is a high temperature phase and ishard it has needle like structure.Martensite is the relatively soft and easily deformed phase of shape memoryalloys which exists at lower temperatures. It has two molecular structures namely,twinned Martensite and deformed Martensite. Austenite is the stronger phase of shapememory alloys which occurs at higher temperatures, the shape of the Austenite structureis cubic.When we apply a constant load on a shape memory alloy and cool it, its shapechanges due to produced strain. During the deformation, the resistivity, thermalconductivity, Youngs modulus and yield strength are decreased by more than 40%.Twinned Martensite state alloy becomes deformed Martensite when it is loaded.The deformed Martensite becomes Austenite when it is heated, the Austenitetransformed to original twinned Martensite state when it is cooled.Characteristics of SMA1. HysteresisHysteresis of a SMA is defined as the difference between the temperatures atwhich the material is 50% transformed to austenite when heating and 50% transformedto martensite when cooling.When the temperature is decreased in a metallic material, the phase transformationtakes place from austenite to martensite. This transformation takes place not only at asingle temperature, but over a range of temperatures.The hysteresis curve for a shape memory alloy is shown below.2. Pseudo elasticityWhen a metallic material is cooled from a temperature T to a lower temperatureTC it deforms and changes its shape. On reheating the material to Temperature (T) theshape change is received so that the material returns to its original state. This effect isknown as pseudo elasticity or thermo elastic property.3. Super elasticitySuper elasticity is a property of SMA. When a material is deformed at atemperature slightly greater than its transformation temperature super elasticity propertyappears (Rubber like property).Applications of SMAs1. Eye glass frames2. Toys3. Helicopter blades:4. Coffee Valves