Crystallographic features of phase transformations in solids

The paper reviews the current knowledge and understanding of the crystallographic features of phase transformations in solid materials – metals, ceramics and alloys. It covers both of the broad classes of phase transformations in crystalline solids – martensitic or ‘displacive’ and ‘diffusional’ or ‘reconstructive’. The factors that govern the crystallographic features of these two classes of transformations are compared and contrasted. This provides an appropriate basis for examining the ‘diffusional–displacive’ transformations that appear to exhibit the characteristics of both classes. After a brief summary of the considerable body of experimental data available on the crystallographic characteristics of these various types of phase transformation, the different models/theories advanced to account for these observations are discussed. The main emphasis is on those models/theories that are capable of predicting, rather than just rationalising or explaining, these crystallographic features. The review purposely adopts a unifying approach and attempts to reconcile the controversy that has on occasions existed between the ‘displacive’ group and the ‘diffusional’ group – particularly in respect of the ‘diffusional–displacive’ transformation. Developing a comprehensive understanding of the crystallographic features of all classes of phase transformations is obviously the ultimate goal. The review concludes by assessing how close we are to this final achievement, identifies the gaps in current knowledge and suggests future work.