ABSTRACT
Any change in the microstructure of a material involves the movement of grain boundaries, but may also involve changes other than simple grain growth. Three of the classical ways of changing the microstructure to develop a new granular array are solidification, transformation in allotropic materials, and primary re-crystallisation of deformed materials. In all of these processes, the driving force derives from energy changes that are not related to grain boundary energy. The formation of a granular array by solidification and by allotropic transformation is brought about by changes in the chemical free energy due to the phase changes. Because of this, the grain structure is initially dependent upon the nucleation and growth rates associated with the degree of under-cooling, and therefore with factors such as the cooling rate or isothermal transformation temperature. Nucleation and growth transformations of this kind involve changes in chemical free energy, surface energy (of the growing nucleus), and strain energy, (associated with volume changes during transformations). The chemical free energy provides the driving force, whilst the surface energy and strain energy terms are seen as forces opposing the change in structure. Primary re-crystallisation also shows this same characteristic, but in this process, the driving force is associated with the elimination of dislocations and their associated strain energies.
