Properties of materials are sensitively influenced by the microstructure inherited from their synthesis and processing. In response to high stress, temperature and composition gradients microstructures evolve in a complex way that involves nucleation of new phases, interface migration and mass redistribution that lead to complex morphological evolution on the mesoscale. Understanding this evolution and the ways it influences properties can be key to optimizing materials for targeted applications. Due to their importance related to grain structure evolution and properties of polycrystals, significant effort has been devoted to calculation of free energies and mobilities of isolated grain boundaries using atomistic simulations. It is commonly assumed that interface properties are continuous functions of temperature, pressure and chemical composition. On the other hand there is accumulating evidence suggesting that interfaces are capable of first order structural transitions, in which case the properties like segregation, excess volume, mobility, cohesive strength and sliding resistance may change discontinuously. This talk will review the results of recent atomistic computer simulations, investigating the nature of structural phase transitions in metallic grain boundaries, induced by changes in temperature and composition. We start by reviewing changes in the structure of elemental boundaries that are observed when these interfaces are exposed to very high homologous temperatures, and the nature of the qualitatively different types of structural disordering that can arise. The transitions involve changes in atomic density in the grain-boundary plane, which was discovered only when new simulation methodologies were developed that permit such variations. We show that interfaces can absorb large number of point defects through a first order phase transition, which may play important role in recovery of materials from radiation damage. Our simulations demonstrated strong effect of the transitions on self and impurity diffusion as well as grain boundary migration and shear strength.
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