Dynamic microstructural evolution and recrystallization mechanism during hot deformation of intermetallic-hardened duplex lightweight steel

Abstract

Dynamic microstructural evolution and recrystallization mechanism during hot deformation of intermetallic-hardened duplex Fe-9Al-10.8Mn-4.5Ni-0.7C (wt.%) lightweight steel have been comprehensively examined at various deformation temperatures at a fixed strain rate of 0.001 s−1. The flow curves are predicted employing Avrami exponent obtained from the strain dependent Johnson-Mehl-Avrami-Kolmogorov relation, which is further corroborated with the dynamic microstructural response. A detailed analysis of the intermetallic precipitates and elemental partitioning in both the ferrite and austenite phases are performed. The ferrite matrix having uniformly distributed nano-sized B2 (NiAl) precipitates has a higher micro-hardness as compared to the austenite matrix, which corroborates the strain partitioning in the austenite phase during hot deformation. Two distinct restoration mechanisms are observed in this alloy viz. continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) following hot deformation. The CDRX mechanism in the ferrite and austenite phase is characterised by the progressive misorientation development of subgrains into high-angle boundary during straining. The ferrite phase is associated with CDRX mechanism at all the deformation temperatures (1223–1423 K) albeit DDRX-like mechanism, facilitated by austenite/ferrite interphase is found to be an assisting mechanism towards the higher temperatures (1323–1423 K). The austenite phase, on the other hand, exhibits DDRX mechanism during the initial stage and dominant CDRX at the later stage of deformation at lower temperature (1223–1323 K). With the increasing deformation temperature to 1423 K, the dissolution of boundary-B2 precipitates in austenite facilitates the boundary migration, thus promoting the DDRX accompanied by the twinning in this phase.