A novel strategy creating serrated grain boundaries to improve ductility in a Fe–Cr–Al alloy

Abstract

Grain boundaries serration has long been regarded as a way to improve mechanical properties at high temperatures in alloys. However, the traditional method of creating serrated grain boundaries (SGBs), solution treatment followed by slow cooling, could induce a significant coarsening behavior of the second phase, thereby adversely affecting the properties. In this work, a new strategy is proposed to obtain numerous irregular SGBs without producing coarser Laves precipitates (<400 nm) in Fe–Cr–Al alloys, in which these SGBs are formed through recrystallization annealing of a warm-rolled microstructure featuring kink bands (KBs). These SGBs can be divided into two types: low amplitude SGBs (LA-SGBs, in the range of 1∼10 μm), and high amplitude SGBs (HA-SGBs, over 20 μm). Through SEM and quasi in-situ EBSD observation, and combined with the Taylor factor model, the formation mechanism of irregular SGBs is revealed in detail. The pinning effect of Laves phase precipitated during annealing on the grain boundaries migration induces the formation of LA-SGBs. On the other hand, the discrepancy in deformation stored energy between the matrix and the KBs promotes the special nucleation and growth of new grains, thus resulting in the formation of HA-SGBs. Moreover, tensile tests are conducted at 298 K and 873 K. SGBs can significantly enhance ductility while maintaining strength, exhibiting an increase of 52.1% and 41.1% respectively. Compared to straight grain boundaries, at 298 K, SGBs can reduce the accumulation of the GNDs in the vicinity of themselves through the unique strain-hardening behavior mediated by high-density dislocation walls (HDDWs), alleviate the stress concentration, make the plastic strain distributions more uniform, and thus retard the crack initiation. As the deformation temperature rises to 873 K, the improved ductility is mainly ascribed to the deflection and passivation effects of SGBs on crack propagation during straining from necking to fracture while their effects on the strain-hardening stage are weakened.