Progression of Late Stage Abnormal Grain Growth of Electroformed Nanocrystalline Ni Without the Addition of Grain Refiner

Abstract

Nanocrystalline (nc) metals are very attractive research materials due to the many beneficial properties the nc structure offers. However, the thermal stability of the nc structure is often of concern due to its high energy state. One curious phenomenon, often reported in nc Ni, is the occurrence of a late stage abnormal grain growth (AGG) which results in the appearance of abnormal-sized grains with faceted grain boundaries. Previously, it was reported that such an AGG stage was brought on by a critical S concentration at certain boundaries. On the contrary, a late stage AGG was observed in electroformed nc Ni without intentionally added additives, including those containing S. The objective of this study is to further elucidate on the initiation and growth of these faceted boundaries. The progression of this AGG stage in electroformed Ni has been systematically investigated by utilizing optical and electron microscopy. Clearly, the late stage AGG did not require a critical S concentration to occur in this case. Moreover, these faceted boundaries were found to be highly stable once its planar nature is fully developed suggesting that the previous focus on the rapid migration of these faceted boundaries may be misplaced.