Is diffusion creep the cause for the inverse Hall-Petch effect in nanocrystalline materials?

It has previously been demonstrated by means of molecular-dynamics (MD) simulation that for the very smallest grain sizes (typically below 20-30 nm), nanocrystalline f.c.c. metals deform via grain-boundary diffusion creep, provided the applied stress is low enough to avoid microcracking and dislocation nucleation from the grain boundaries. Experimentally, however, the nature of the deformation process in this “inverse Hall-Petch” regime (in which the yield stress decreases with decreasing grain size) remains controversial. Here we illustrate by MD simulation that in the absence of grain growth a nanocrystalline model b.c.c. metal, Mo, and a model metal oxide, UO2, also deform via diffusion creep. However, in the case of Mo both grain-boundary and lattice diffusion are observed to contribute to the creep rate; i.e., the deformation mechanism involves a combination of Coble and Nabarro-Herring creep. While our results on Mo and UO2 are still preliminary, they lend further support to the observation of diffusion creep previously documented in f.c.c. metals and in covalently bonded Si.