DISCLINATION-DISLOCATION MODEL OF METALLIC GLASS STRUCTURES

A new model for the structures of metallic glasses is proposed which is based on a continuous three-dimensional network of mixed-character disclinations containing a subnetwork (discontinuous) of dislocation segments which terminate on disclination jogs. Given the known geometrical properties of disclinations and their function as sources, sinks, and termination sites for dislocations, the mechanism of formation of the proposed double network structure is discussed. The approximate strain energy density of the model is derived from the known linear elastic solutions for disclinations. Using the measured elastic constants of several typical metallic-alloy glasses, the calculated strain energy densities agree to within about 20% with the respective measured heats of crystallization, demonstrating the energetic feasibility of the model. The structural features of the model and its conceptual relationships to the microcrystalline and DRPS models are discussed and it is concluded that the disclination-dislocation model is not conceptually inconsistent with either and may in fact represent a useful refinement of the microcrystalline model and provide additional insight into processes involving cooperative atomic motions in DRPS models. Using the known relations between glide dislocations and disclinations combined with classical dislocation theory, an expression for the low-temperature yield stress of a disclination-dislocation glass is derived. The calculated yield stresses for several typical alloy glasses agree with the experimental values to within several percent and the experimentally observed linear dependence of yield stress on the shear modulus is correctly predicted. Some speculations as to glass formation and stability at low temperatures are discussed qualitatively from the perspective of the disclination-dislocation model. This discussion is based on the severely limited glide mobility of disclination cores which must move into coincidence and annihilate for crystallization to occur. Since the disclination cores can move conservatively via the generation or annihilation of glide disocations, it is proposed that glass formation from liquids and retention at low temperatures will be facilitated in systems in which the reference crystal has a high Peierls stress for dislocation glide.