Nanocrystalline metals are materials composed of grains (crystallites) whose sizes are of the order of 100 nanometers or less. The small grain size endows these materials with several desirable properties including very high strength and increased toughness compared to conventional metals. However, many commonly used methods to produce nanocrystalline metals lead to highly nonuniform microstructures with a wide distribution in the size and orientation of grains. Such heterogeneous microstructures lead to a large variability in their mechanical properties, which limits their practical use. This award supports fundamental research to predict and quantify uncertainty in the mechanical properties of heterogeneous nanocrystalline metals using a combined experimental and modeling approach. The ability to reliably and accurately predict the behavior of nanocrystalline metals would hasten their adoption in technological and scientific structural applications, which will directly benefit the U.S. economy and society. This research involves a multi-disciplinary approach comprising of advanced microfabrication, materials science and mechanics and will lead to broader engagement of underrepresented groups in research and positively impact engineering education.
Nanocrystalline metals with a wide or bimodal grain size distribution often exhibit the best combination of strength and ductility and hence are very attractive from an engineering perspective. However, to accurately predict the mechanical response of these materials it is necessary to consider both their intrinsic microstructural heterogeneity and the stochastic nature of nanoscale plasticity. Motivated by this, a new stochastic crystal plasticity framework informed by in situ experiments will be used to model the deformation of heterogeneous nanocrystalline metals. The model will explicitly take into account both the variation in grain sizes and orientations as well as the statistics of dislocation bursts in nanocrystalline metals. Novel in situ transmission electron microscopy experiments on nanocrystalline metal films with controlled microstructures will be used to directly obtain input parameters for the model such as the distribution of critical resolved shear stresses. The predictive capability of the model will be verified with data from independent in situ x-ray diffraction experiments.
