Abstract
Nanoscale fracture processes initiated at intersections of deformation twins and grain boundaries (GBs) in nanocrystalline and ultrafine-grained materials are theoretically described. Within the suggested description scheme, nanocracks are formed at GBs at which deformation twins are stopped and thereby produce specific GB defect configurations that create high local stresses initiating nanocracks. The conditions for realization of this new fracture mode in nanocrystalline and ultrafine-grained materials with face-centred cubic and body-centred cubic crystal lattices are calculated. The free-surface effect on crack generation at deformation twins stopped by GBs in thin-film nanocrystalline specimens is theoretically described. The critical parameters for crack generation in nanocrystalline and ultrafine-grained bulk materials and thin films are revealed. It is shown that in bulk nanomaterials nanocracks are generated at twin thicknesses of several nanometres and grow very fast with increasing twin thickness. At the same time, in thin-film nanomaterials, the equilibrium nanocrack lengths significantly decrease if the distance between the twin lamella and the closest film surface does not exceed several twin thicknesses.
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