Assessing the potential of sparsely nucleated recrystallized grains to lead grain boundary engineering during extending annealing in Alloy 600H

Abstract

Alloy 600H is subjected to a lower extent of deformation (7.5% and 10%) followed by annealing at a higher temperature (1273 K and 1373 K) for different time duration (5 min to 600 min) to explore whether sparsely nucleated recrystallized grains can promote grain boundary engineering (GBE). Since strain-induced boundary migration (SIBM) may also occur in the aforementioned deformation-annealing conditions, this study is expected to explicate the relative role of recrystallization vs. SIBM on GBE. It is observed that the sparsely nucleated grains formed during annealing (60 min at 1273 K) of the specimen subjected to critical strain (i.e., 10%) for recrystallization does not lead to a GBE microstructure following extended annealing treatment even up to 600 min. This is due to the dominant nucleation of new strain-free grains during the extended annealing treatment, rather than the growth of initially nucleated recrystallized grains. As a result, the product microstructure post extended annealing treatment yields lower fraction of Σ3n boundaries and lesser numbers of grains per twin-related domains (TRDs) with concomitant increase in random high angle boundaries (HAGBs) connectivity, as corroborated by its higher fractal dimension (Df) and lower J2/1-J3 value. In contrast, significant SIBM has happened in the specimens subjected to 7.5% deformation (i.e., lower than the critical strain for recrystallization) when annealed at 1373 K. During such SIBM, the existing TRDs in the as-received specimen have grown allowing the occurrence of multiple twinning behind the migrating grain boundaries. The evolution of a higher fraction of Σ3n boundaries, promoted via SIBM, and complete suppression of recrystallization in these specimens lead to the significant disruption in random HAGBs connectivity as substantiated by their lower Df and a higher J2/1-J3 value.