Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Teaching & Learning Packages Superelasticity and Shape Memory Alloys Martensitic Phase Transformations - A Simple Example
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Martensitic Phase Transformations - A Simple Example

Martensitic transformations are diffusionless shear transformations. The transformation is most commonly driven by mechanical deformation or by a change in temperature. A martensitic transformation is an example of a displacive transition, in which there is cooperative motion of a relatively large number of atoms, each being displaced by only a small distance (a fraction of an interatomic spacing) relative to its neighbours. Click here to see the difference between displacive and diffusional transformations.

A simple example of a martensitic phase transformation is provided by the transition from cubic close-packed (ccp) to hexagonal close-packed (hcp).  This occurs by the systematic sliding of close-packed planes, ie {111} planes in ccp, over one another.  As it happens, the same kind of sliding (in the same direction and by the same distance) can also lead to formation of a twin (ie the same ccp crystal structure, but in a different orientation).  A twin is formed when every (111) plane undergoes this displacement, relative to the plane below it, whereas the hcp structure is created when every second (111) plane does this.