Microevolution of grain boundary character distribution in Hastelloy C-276 during the annealing process

Abstract

Grain boundary character distribution and disconnectivity of random boundaries were observed “in-situ” in the Hastelloy C-276 alloy sample through thermomechanical processing. The role of Σ3 boundaries in microstructural evolution of grain boundary character distribution was analyzed by electron back-scattered diffraction (EBSD) and the mechanisms of grain boundary engineering (GBE) during annealing were elucidated. The results showed that the fraction of Σ3n boundaries (including Σ3 boundaries) consistently increased in number with annealing time; however, the Σ3 boundaries had different effects on the disconnectivity of random boundaries because of their variants, i.e., coherent and incoherent Σ3 boundaries. At the initial annealing stage, new incoherent Σ3 boundaries were generated, according to the “Σ3 regeneration mechanism” model; Σ9 and Σ27 boundaries appeared and increased in number with increasing Σ3 boundary interactions. These incoherent Σ3 boundaries could disrupt the connectivity of the random grain boundaries and form large clusters. However, at the annealing stage, some incoherent Σ3 boundaries were annihilated, and new coherent ones emerged. As a result, the random grain boundary network evolved by obtaining significant connectivity with larger clusters, attributed to the reduction of the incoherent Σ3 boundaries. This implies that the incoherent Σ3 boundaries play an important role in the optimization of grain boundary engineering.