Martensitic transformation is the name for any transformation that takes place in a diffusionless and military manner - that is, these transformations take place through atomic movements which are less than one atomic spacing; and in these transformations atoms change their positions in a coordinated manner (unlike thermally activated diffusional, or, so-called, civilian processes). In shape memory alloys such as Ni-Ti (nitinol), it is the martensitic transformation that is responsible for the shape memory effect. In this module, we describe some characteristic features of the martensitic transformations (with specific reference to steels in which, this transformation is responsible for hardening by quenching). This module is very cursory in details and we refer the interested reader to [1] and [2] for details.

Since martensitic transformations are diffusionless, necessarily, the local composition does not change during the transformation. It is only the crystal structure that changes. For example, in Fe-C alloys, the austenite (fcc) transforms into martensite (bct); in Ni-Ti, an ordered bcc (called austenite) transforms to another ordered CsCl type structure (called martensite). Note that since martensitic transformation is diffusionless, if the austenitic phase is ordered, the martensitic phase is also ordered.

Martensites are typically found in lath and plate morphologies: see the schematic in Figure 39. The lens shape martensites are coherent with the surrounding austenite as shown in the schematic Figure 40. As is clear from this schematic, after transformation, lines drawn on a prepolished surface show tilts.