Virginie Dupont

Grain growth behavior at absolute zero during nanocrystalline metal indentation

The authors show using atomistic simulations that stress-driven grain growth can be obtained in the athermal limit during nanocrystalline aluminum indentation. They find that the grain growth results from rotation of nanograins and propagation of shear bands. Together, these mechanisms are shown to lead to the unstable migration of grain boundaries via process of coupled motion. An analytical model is used to explain this behavior based on the atomic-level shear stress acting on the interfaces during the shear band propagation. This study sheds light on the atomic mechanism at play during the abnormal grain coarsening observed at low temperature in nanocrystalline metals.

Grain boundary structure evolution in nanocrystalline Al by nanoindentation simulations

The nanoindentation of a columnar grain boundary (GB) network in nanocrystalline Al has been examined by atomistic simulation. The goal of this study was to gain fundamental understanding on the relationship between structure evolution at GBs and incipient plasticity for indenter tips significantly larger than the average grain size. The nanoindentation simulations were performed by quasicontinuum method at zero temperature. A GB network made of vicinal and high-angle tilt GBs was produced by generating randomly-oriented 5-nm grains at the surface of a 200 nm-thick film of Al. The major findings of this investigation are that (1) nanocrystalline GB networks profoundly impact on the nanoindentation response and cause significant softening effects at the tip/surface interface; (2) GB movement and deformation twins are found to be the predominant deformation modes in columnar Al, in association with shear band formation by GB sliding and intragranular slip, and crystal growth by grain rotation and coalescence; and (3) the cooperative processes during plastic deformation are dictated by the atomic-level redistribution of principal shear stresses in the material.

Multiscale Modeling of Contact-induced Plasticity in Nanocrystalline Metals

Predicting the integrity of metallic thin films deposited on semiconductors for microelectromechanical systems (MEMS) applications requires a precise understanding of surface effects on plasticity in materials with nano-sized grains. Experimentally, the use of nanoscale contact probes has been very successful to characterize the dependence of flow stress on mean grain size in nanocrystalline metals. From atomistic simulations, several models of plastic yielding for metal indentation have also been proposed based on the nucleation and propagation of lattice dislocations, and their interaction with grain boundaries beneath penetrating tips. However, model refinement is needed to include the characteristics of materials whose grain size is much smaller than the typical plastic zones found in contact experiments. Particularly, cooperative deformation processes mediated by grain boundaries, such as grain rotation, deformation twinning, and stress-driven grain coarsening, can simultaneously emerge for very small grain sizes (< 20 nm), thus making a predictive understanding of plastic yielding elusive. This chapter summarizes our recent progress in using multiscale modeling to gain fundamental insight into the underlying mechanisms of surface plasticity in nanocrystalline face-centered cubic metals deformed by nanoscale contact probes. Two numerical approaches to model contact-induced plasticity in nanocrystalline materials, the quasicontinuum method and parallel molecular dynamics simulation, are reviewed. Using these techniques, we discuss the role of a grain boundary network on the incipient plasticity of nanocrystalline Al films deformed by wedge-like cylindrical tips, as well as the processes of stress-driven grain growth in nanocrystalline films subjected to nanoindentation