Atomistic Study of Fracture of Nano-Crystalline Materials

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In a crystalline materials with very fine (nanometer-size) grain, percentage of the atoms which form the grain boundary is fairly large. Because of randomness and quasi-stableness of the grain boundary structure, certain unique properties appear in it. Under the purpose to make clear the dynamical structural factors controlling the strength properties of the nano-crystalline materials, large scale computer simulation was performed and the following results have been obtained.(1) Inverse Hall-Petch relation which states softening of the materials with decease of the grain size, is found. This relation is expressed as the relationship between the strength and the fraction of the crystal defect, which suggests the strength mainly determined by the dynamical rearrangement in the boundary atomic structure.(2) Not only the crystal defect but also the source of dislocation such as the Frank-Read source cannot exist stationarily in the grain. But because of width of the extended dislocation becomes comparable to size of the grain, the stacking fault able to be present through the grain and it brings about the anisotropic properties, which causes the strain and cyclic hardening.(3) In nano-wire or thin plate with nano-thickness, the intra-boundary rapture is bought about due to grain boundary slide for materials with high stacking fault energy. But the inter-grain slid becomes dominate for the case that the stacking fault energy is low, which raise the ductility due to the suppression of the secondary inter-grain slide.(4) In an amorphous materials, an intensively localized deformation is taken place such as a localized necking, deformation induced recrystalization appears and nano-size crystal grain nucleates and grows, with the assist of the local temperature rise.