Methods for atomistic input into the initial yield and plastic flow criteria for nanocrystalline materials
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Nanocrystalline (NC) metals and alloys are known to possess superior mechanical properties, e.g., strength, hardness, and wear-resistance, as compared to conventional microcrystalline materials. NC metals are characterized by a mean grain size <100 nm; in this grain size regime, inelastic deformation can occur via a combination of interface-mediated mechanisms viz., grain boundary sliding/migration, and dislocation nucleation from grain boundary sources. Recent studies have suggested that these interface-mediated inelastic deformation mechanisms in fcc metals are influenced by non-glide stresses and interfacial free volume, unlike dislocation glide mechanisms that operate in microcrystalline fcc metals. Further, observations of tension-compression strength asymmetry in NC metals raise the possibility that yield and inelastic flow in these materials may not be adequately described by solely the deviatoric stress. Unfortunately, most literature concerning the mechanical testing of NC metals is limited to uniaxial deformation or nanoindentation techniques, and the multiaxial deformation behavior is often predicted assuming initially isotropic yield and subsequent flow normal to the yield surface. The primary objective of this thesis is to obtain a better understanding of the nature of inelasticity in NC metals by simulating multiaxial deformation at the atomistic resolution, and developing methods to interpret the results in ways that would be useful from a continuum constitutive modeling viewpoint. First, we have presented a novel, statistical mechanics-based approach to unambiguously resolve the elastic-plastic transition as an avalanche in the proliferation of mobile defects. This approach is applied to nanocrystalline Cu to explore the influence of pressure and multiaxial stress states on the inelastic deformation behavior. The results suggest that initial yield in nanocrystalline Cu under biaxial loading is only weakly anisotropic in the 5 nm grain size regime, and that plastic flow evolves in a direction normal to the von Mises yield surface. However, triaxial deformation simulations reveal a significant effect of the superimposed hydrostatic stress on yielding under shear. These results are analyzed in detail in order to assess the influence of pre-existing internal stresses and interfacial excess volume on the inelastic deformation behavior. Further, we have studied the effects of imposed hydrostatic pressure on the shear deformation behavior of Cu bicrystals containing symmetric tilt interfaces, as well as Cu nanocrystals of different grain sizes. Most interfaces exhibit an increase in shear strength with imposed compressive hydrostatic pressure. However, for some interfaces, this trend is reversed. Neither the sign nor the magnitude of the pressure-induced elevation in shear strength appears to correlate with interface structure or particular deformation mechanism(s). In Cu nanocrystals, we observe that imposed compressive pressure leads to strengthening under shear deformation, and the effect of imposed pressure on the shear strength becomes stronger with increase in grain size or temperature. Activation parameters for shear deformation have been computed for these nanocrystals, and computed values seem to agree with existing experimental and theoretical estimates. Finally, we have proposed some modifications to conventional isothermal molecular dynamics algorithms, in order to isolate dislocation nucleation events from interfacial sources, and thereby permit explicit computation of the activation parameters for such events.