Formation of Nanostructured Steels by Phase Transformation

A thermodynamic criterion is developed to define the finest grain size that can be achieved during phase transformation, as a function of the driving force, interfacial energies and the parent phase grain size. It is found that at large undercoolings below the equilibrium temperature, the grain size achieved in practice is far greater than predicted theoretically. This is because of the heat of transformation, which causes recalescence and reduces the effective undercooling. Whereas these conclusions apply generally, there is a special effect associated with displacive transformations, i.e. the plastic accommodation of the shape deformation, which limits the growth of grains but which has yet to be modelled quantitatively.