Question

Please compare the martensitic transformation in a binary TiNi based shape memory alloy and in a binary Fe-C alloy. Explain your answer with the necessary details.

Answer 1

TiNI Shape Memory alloys

These alloys have the ability to retain their shapes. That is when SMA deformed during application and when it is heated it come back to its original undeformed shape, usually % deformation should be upto 8%.

SMA are polymorphic, as it has 02 crystal structure and shape recovery involves transformation into these crystal structure.

One phase (Austenite) which exists at elevated temperature and have a Body centered cubic structure, upon cooling it spontaneously transform to martensite structure which is similar to Iron-Carbon System and in the same way it forms without diffusion, degree of transformation depends on Temperature also it is heavily twiinned.

TiNi SMA used in martensite state and it is deformed by migration or separation of twin boundaries.

SMA for shape regain are heated to elevated temperature hold for an hour and then cooled, rate of cooling determine final shape same Fe-C system.

Fe-C System

Here Martensite is formed from Austenite phase by rapid cooling to relatively low temperature and have a needle like structure. Martensite is a Non-equillibrium phase (i.e process of formation is irreversible)

Carbon diffusion does not take place during martensite formation.

Here Face centered cubic Austensite is tranformed to Body centered tetrahedron martensite structure.