Ductile vs. brittle behavior of pre-cracked nanocrystalline and ultrafine-grained materials

Aspects of the ductile vs. brittle response of nanocrystalline and ultrafine-grained (UFG) materials are theoretically examined. The focus of this study is on the combined effects of grain boundaries (GBs) and blunting of cracks on the fracture toughness of nanocrystalline and UFG materials in a typical situation where crack blunting and growth processes are controlled by dislocation emission from crack tips. Within our description, lattice dislocations emitted from cracks are stopped at GBs, resulting in blunting of cracks. Both crack blunting and the stress fields of the arrested dislocations hamper further dislocation emission from cracks in nanocrystalline and UFG materials. As a result, crack blunting is suppressed, while crack growth is enhanced. The combined effects of GBs and blunting of cracks on the dislocation emission, further crack blunting and growth processes depend on grain size and material parameters. The dependence of the maximum number of dislocations, emitted by a crack, and the critical stress intensity factor on grain size (ranging from 10 to 300 nm) in Al and alpha-Fe is calculated. It is demonstrated that a decrease in grain size from 300 to 10 nm in Al and alpha-Fe decreases their critical stress intensity factors by a factor of 2-3 and thereby dramatically reduces the toughness/ductility of these materials. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.