Martensitic transformations in ‘unfamiliar’ systems

In stiff engineering materials, martensitic transformations are shear-dominant diffusionless processes, and their crystallographic features can be successfully predicted using the phenomenological theory of martensite crystallography (PTMC). Recently, an alternative approach has been developed wherein the interface structure is modelled in terms of interfacial dislocations. This model, referred to as the topological model (TM), provides insight into the transformation process and identifies a set of five criteria that must be met for a transformation to be diffusionless. In the case of stiff engineering materials, the crystallographic predictions of the PTMC and TM are very similar. However, the TM enables a broader range of transformations to be treated, and two examples of ‘unusual’ martensites are presented here. The first is a diffusionless transformation in a small elastically soft protein crystal. The second is a transformation in a prospective high-temperature engineering alloy which exhibits the characteristic crystallographic features of martensite but where concomitant diffusion occurs. In this case, four of the five criteria mentioned above that relate to conservation of substitutional atomic sites are satisfied.