Grain Boundary Rotations near Crack Tips in Deformed Nanomaterials

A special mechanism/mode of plastic deformation occurring through stress-driven rotations of low-angle grain boundaries (GBs) near crack tips in nanocrystalline and ultrafinegrained (UFG) materials is theoretically described. It is demonstrated that such rotations of GBs represent energetically favorable processes in wide ranges of parameters characterizing precracked specimens with nanocrystalline and UFG structures. Also, these rotations of GBs in part release high stresses concentrated near crack tips and thus hamper crack growth. Nanoscale deformation processes occurring in nanowires, micropillars, nanocrystalline and UFG materials represents the subject of intensive research efforts in materials science and physics of ]P]bcadRcdaTbbTTTVNo O]]P]RahbcP[X]T and UFG bulk solids, specif ic (inherent to nanostructures) mechanisms/modes of plastic flow effectively operate due to the combined nanoscale and grain-boundary effectsN o O In nanowires and micropillars having either single crystalline or amorphous structures, the combined nanoscale and free-surface effects strongly influence plastic deformation processes and thereby strength and ductility [1,4-7,9,13]. Recently, a particular attention has been devoted to nanowires and micropillars with nanocrystalline and UFG structures Pbb[XSbfWTaTcWTcWaTTTUTRcbocWTnanoscale, grain-boundary and free-surface effectsoR TaP tively operate, which can cause the actions of unusual modes of plastic deformation [8,10,17,18]. For instance, the experiment [8] revealed GB transforPcX]b UP]TfchTo acPcX]boX] nanocrystalline Ni nanopillars under a mechanical load. In Letter [18], GB rotations were theoretically described as stress-driven processes representing a new deformation mechanism in solids. As it has been demonstrated in Letter [18], stress-driven rotations of GBs effectively carry plastic flow and transform GB defect configurations in nanocrystalline nanowires, nanopillars and thin films as well as within subsurface areas of bulk nanocrystalline solids due to the combined actions of the nanoscale, grain-boundary and free-surface effects. Plastic deformation carried by rotations of GBs is accompanied by crystal lattice rotations in the areas swept by rotating GBs [18] and thereby can be treated as a partial case of rotational deformation (defined as plastic deformation accompanied by crystal lattice acPcX]bN O [fX]VcWTTgTaXT]cP[SPcPN o 24], crystal lattice rotations intensively occur in vicinities of cracks growing in nanostructured (nanocrystalline or UFG) materials. In the context discussed, it is highly interesting to understand, if stress-driven GB rotations can occur and thereby R]caXQdcTccWTTgTaXT]cP[hQbTaeTSN o O crystal lattice rotations in vicinities of cracks grow-