Origin of microstructure instability in nanocrystalline materials

Grain growth process in the absence of drag forces in polycrystals with bi-modal grain size distribution is investigated by numerical simulations. It appears to be the abnormal growth that evolves without induction period and terminates without completion, which is in qualitative agreement with the experimental data on nanomaterials. This agreement leads to the supposition that in bulk nanomaterials, the same specific kind of abnormal growth takes place. Its necessary conditions are the ab initio presence of excess large grains and the absence of drag forces. Owing to the latter, abnormal growth terminates without completion, which results in dual microstructure.