This paper summarises the characteristics of diffusional and displacive transformations, with emphasis on the transformation mechanism, which is distinguished from the processes controlling the rate of interface motion.
The implications of the shape deformation accompanying displacive transformation are discussed. It is sometimes argued that an invariant-plane strain relief with a large shear component arises even for reconstructive transformations, simply due to the existence of a sessile semi-coherent or coherent interface which is displaced by the movement of "disordered" ledges. The ledges themselves are considered to have an incoherent structure. It is argued that this proposal is fundamentally incorrect.
A second point of controversy concerns factors determining the morphology of precipitates, especially in steels. The shape deformation accompanying the formation of Widmanstattedn ferrite and bainite in steels has a large shear component. The strain energy due to this causes the plates to adopt a thin-plate morphology. The contrary view that the morphology is determined by the minimisation of interface energy alone fails to explain why other ferritic phases, such as allotrimorphic ferrite, do not assume a plate shape.
Scripta Metallurgica,Vol. 21, 1987, pp. 1605-1609
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