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Lallit Anand, Dept. of Mechanical Engineering, MIT
"Crystal-plasticity and grain-boundary slip and separation: application to the deformation and fracture response of nanocrystalline metals"
It is well known that in polycrystalline metals, a substantial increase in strength and hardness can be obtained by reducing the grain size to the nanometer scale. These at-tributes have generated considerable interest in the use of nanocrystalline metallic materials (grain sizes less than 100 nm), for a wide variety of structural applications. Typically, relative to their microcrystalline counterparts, nanocrystalline metals exhibit a very high tensile strength, but at the expense of a much reduced tensile ductility. The limited ductility is of major concern. For example, while the ultimate tensile strength levels approach 1500MPa in electro-deposited nanocrystalline nickel, the ductility that can be obtained in this material is generally low and usually does not exceed 3%. Physical experiments and atomistic simulations reported in the literature, show that grain-boundary-related slip and separation phenomena begin to play an important role in the overall inelastic response of a polycrystalline material when the grain-size decreases to diameters under 100 nm, and dislocation activity within the grain interiors becomes more difficult. In order to model the effects of grain boundaries in polycrystalline materials we have coupled a crystal-plasticity model for the grain interiors with a a new elastic-plastic grain- boundary interface model which accounts for both reversible elastic, as well irreversible inelastic sliding-separation deformations at the grain boundaries prior to failure. We have used this new computational capability to study the deformation and fracture response of nanocrystalline nickel. The results from the simulations capture the macroscopic experimentally-observed tensile stress-strain curves, and the dominant microstructural fracture mechanisms in this material. The macroscopically-observed nonlinearity in the stress-strain response is mainly due to the inelastic response of the grain boundaries. The stress concentrations at the tips of the distributed grain-boundary cracks, and at grain-boundary triple junctions, cause a limited amount of plastic deformation in the high-strength grain interiors. The competition of grain-boundary deformation with that in the grain interiors determines the observed macroscopic stress-strain response, and the overall ductility. In nanocrystalline nickel, the high yield strength of the grain interiors and relatively weaker grain-boundary interfaces account for the low ductility of this material in tension.
THURSDAY, October 23, 2003