Nanostructure State as Nonequilibrium Transition in Grain Boundary Defects in SPD Condition

Abstract

A promising way of generating novel physical properties of solids is possible by the modification of their microstructures. In nanostructured solids (crystalline size ca. l0 nm) the physical properties are strongly modified by the disordered structure of grain boundaries (GB). Nanocrystals are produced under severe (high pressures and heavy deformations) or nonequilibrium (growth from an amorphous state) conditions. Much attention is presently given to studies of nanocrystalline solid as nonequilibrium systems whose mechanical and physical properties are related to collective phenomena in the system of GB defects. The GB defects are thermodynamically nonequilibrium, but can be mechanically stable at ambient temperatures. The nonequilibrium nature of GBs may originate from the intrinsic properties of the dislocation structures that form these mesoscopic defects, as well as from the interactions in the system of GB defects. Since the effect of GB defects on the mechanical properties of polycrystals should be considered as a cooperative phenomenon (i.e., as the effect of the whole ensemble of GB defects), a statistical approach seems to be most relevant in accounting for both the nonequilibrium nature of a separate mesoscopic defect (a grain boundary) and the interactions of GB defects in the ensemble. In this work, we attempt to construct a statistical theory of the polycrystalline state that explicitly allows for the presence of GB defects and defect structures that develop in the ensemble of such GB defects.