Disclinations, Amorphization and Cracks at Grain Boundaries in Nanocrystalline Materials

Abstract

The processes developing in the late stages of plastic deformation in conventional (i.e., non-nanocrystalline in their initial state) crystalline materials are characterized by defect structures which have passed over some intermediate steps in their evolution [56, 263–266, 381, 389, 430–440]. Their principal difference from the defect structures inherent in the initial stages of plastic deformation is the mainly disclinational (rotational) character of the defect configurations (misorientation bands, fragment boundaries, high-angle grain boundaries of deformation origin, etc.) observed in experiments, which may be considered as carriers of rotational plasticity. At this late stage of plastic deformation in metallic materials, there exist two general paths for further evolution of the defect structures. The first path is the generation of microcracks in the regions of highest stress concentration, leading to fracture of the material. The second alternative is a transition to a nanocrystalline and/or amorphous structure of the metal [441, 442]. Obviously, the second path for structural evolution allows the deformed sample to achieve larger steps of plastic deformation than the first and this explains why it is very promising for metal-forming technologies. Nowadays, this approach is widely used in various techniques for fabricating NCMs and amorphous alloys (e.g., ball milling and mechanical alloying of powders [14], equal-channel angular pressing [28,443], etc.). In the 1980s, probably the first amorphous—nanocrystalline composites [36,37] were obtained under the combined action of high pressure and intensive shear.