Grain boundary engineering of AL6XN super-austenitic stainless steel: Distinctive effects of planar-slip dislocations and deformation twins

Abstract

To explore the influence of deformation microstructures on the gain boundary engineering (GBE), a series of thermo-mechanical processes (TMPs) were performed on an AL6XN super-austenitic stainless steel (ASS). The deformation microstructures of cold-rolled samples and the TMPed microstructures were characterized by transmission electron microscopy and electron back-scatter diffraction, respectively. The results show that the deformation microstructures, including planar slip bands and deformation twins (DTs), play a distinctive role in the GBE treatment. Planar slip bands are in favor of the GBE treatment, since the migrating boundaries of twin related domains might generate “stacking accidents” in a sequence of closely packed atomic planes via absorbing the planar-slip dislocations, and thus derive the formation of annealing twins. In contrast, DTs are detrimental to GBE for they hinder the growth of twin related domains. Therefore, the optimal prior strain (namely cold-rolling reduction) of TMP for the GBE treatment of AL6XN super-ASS is suggested to be slightly less than the threshold strain for the appearance of DTs. The GBE-quantifying parameters are greatly improved by cold-rolling with 7% reduction and subsequently annealing at 1323 K for 12 h, and the fraction of special boundaries is increased up to 74.5%, thus effectively disrupting the connectivity of random high angle grain boundary network.