Void initiation in fcc metals: Effect of loading orientation and nanocrystalline effects

摘要

It is shown, through molecular dynamics simulations, that the emission and outward expansion of special dislocation loops, nucleatedat the surface of nanosized voids, are responsible for the outward flux of matter, promoting their growth. Calculations performed fordifferent orientations of the tensile axis, 0 0 1, 1 1 0 and 1 1 1, reveal new features of these loops for a face-centered cubic metal, cop-per, and show that their extremities remain attached to the surface of voids. There is a significant effect of the loading orientation on thesequence in which the loops form and interact. As a consequence, the initially spherical voids develop facets. Calculations reveal thatloop emission occurs for voids with radii as low as 0.15 nm, containing two vacancies. This occurs at a von Mises stress approximatelyequal to 0.12G (where G is the shear modulus of the material), and is close to the stress at which dislocation loops nucleate homoge-neously. The velocities of the leading partial dislocations are measured and found to be subsonic (～1000 m s~(-1)).It is shown, for nano-crystalline metals that void initiation takes place at grain boundaries and that their growth proceeds by grain boundary debonding andpartial dislocation emission into the grains. The principal difference with monocrystals is that the voids do not become spherical and thattheir growth proceeds along the boundaries. Differences in stress states (hydrostatic and uniaxial strain) are discussed. The critical stressfor void nucleation and growth in the nanocrystalline metal is considerably lower than in the monocrystalline case by virtue of the avail-ability of nucleation sites at grain boundaries (von Mises stress —0.05G). This suggests a hierarchy of nucleation sites in materials, start-ing with dispersed phases, triple points and grain boundaries, and proceeding with vacancy complexes up to divacancies.