The objective of this research is to employ molecular dynamics simulations to shed light on the proper forms for the constitutive description of stress-state dependent inelastic deformation modes for some nanocrystalline metals, including grain boundary shuffling/sliding, grain boundary dislocation nucleation, absorption and desorption, and free volume exchange of grain boundaries with triple junctions. The research approach will emphasize methods to simulate and quantitatively characterize distinct contributions of competing deformation mechanisms of grain boundary sliding, grain boundary dislocation nucleation, absorption and desorption, and grain core dislocation activity in three-dimensional nanocrystalline ensembles. To accomplish the above goals, the research integrates concepts and methods that bridge between materials science/physics and the engineering sciences, in particular mechanics of materials.
The broader outcomes of this project will be used to gain fundamental new understanding of behavior of nanocrystalline materials, which can be used to inform improved continuum models of nanocrystalline materials with competing modes of deformation; to motivate proper, atomistically-consistent forms of yield surfaces in which to embed continuum models, including tension-compression asymmetry and general stress state effects; and to estimate the spectra of activation energy and volumes for both bulk- and grain boundary-mediated inelastic deformation modes, useful for modeling kinetics at long time scales. The research will provide advanced training for both graduate and undergraduate students. Moreover, outreach activities will be conducted to motivate high-school students to pursue careers in engineering research and education.
Nanocrystalline metals and their alloys with grain size in the range of 5 to 20 nanometers offer significant improvements in several mechanical properties over their conventional coarse-grained counterparts. However, grain boundaries play a major role in the inelastic deformation of nanocrystalline metals, leading to important effects such as tension-compression strength asymmetry and competition of grain boundary sliding and dislocation activity even at moderate temperatures. Furthermore, it is very difficult to conduct controlled experiments on nanocrystals under combined stress states, yet there are fundamental questions regarding the extent to which the underlying (atomic-scale) physics of inelastic deformation in NC metals is affected by stress components other than the shear stress. In general, an understanding of the role of these other stress components is also crucial from an application standpoint, since complex, multiaxial loading conditions are quite commonly encountered in practical structural applications of these materials. A generalized framework has been developed, based on molecular dynamics simulations and the notion of avalanches based on dissipation due to defects, and used to probe the onset and evolution of plasticity in simulated nanocrystals for a variety of complex loading paths and combined stress states. This framework was employed to systematically explore the role of hydrostatic pressure on the shear deformation of copper nanostructures (including nanocrystals as well as bicrystal interfaces), and to quantify the effect of pressure on the stress-strain behavior and the evolution and interaction of deformation unit processes at the nanoscale. For the latter, several recently-developed data analytics tools were applied to resolve correlations between continuum-based measures of localized deformation, internal stresses, and free volume, obtained explicitly from atomic trajectories and interatomic forces. Finally, we assessed the effects of grain size and imposed pressure on the activation parameters for the nonlinear, inelastic behavior of nanocrsystalline materials. In terms of intellectual merit, the present work has made several important new contributions to the understanding of interfaces in deformation of nanocrystalline fcc metals. We confirmed theoretical assertions regarding the kinetics of interface-mediated deformation, and found reasonable agreement between the deformation kinetics observed in our simulations and those reported in recent experiments. Additionally, we made novel and non-intuitive observations, such as the association of high internal stresses with jammed configurations of atoms in interfaces, and instances of shear-weakening under compressive pressure in some copper interfaces due to the annihilation of free volume. The present research provides broad impact in developing the essential "machinery" (methods, algorithms, metrics) to help bridge length scales between atomistic and continuum descriptions of material deformation behavior. The present work demonstrates methods via which inelastic deformation can be interpreted both in terms of atomic rearrangement cascades in the bulk, as well as the evolution of continuum-like fields of stress, strain, rotation, and free volume at the nanoscale. This information is not only useful as input to various mesoscale continuum modeling efforts, but also provides insight into the atomistic origins of complex deformation behavior such as that exhibited by nanostructured materials.