Near-atomic investigation on the elemental redistribution during co-precipitation of nano-sized kappa phase and B2 phase in an Al-alloyed lightweight steel

Abstract

In the present study, correlative transmission Kikuchi diffraction transmission electron microscopy (TKD-TEM) measurements, atom probe tomography (APT), and density functional theory (DFT) calculations are used to reveal the elemental redistribution during co-precipitation of nanosized kappa and B2 phases in an FCC matrix of an Al alloyed Fe-10Al-7Mn-6Ni-1C (wt.%) steel. Upon ageing at 800 C for 15 min, two co-nanoprecipitation modes are observed: B2 forming together with kappa and B2 forming separately from kappa in the FCC matrix. APT reveals that the B2 precipitate next to kappa (referred to as B2I) is close to an FeAl type phase, while the isolated B2 precipitate (referred to as B2II) is close to a NiAl type phase. The kappa precipitates maintain a nearly constant Al content of approximately 18.4 at.% regardless of their precipitation position. DFT confirms that kappa may accommodate limited Ni substitution at Fe sites without losing structural stability, and that Fe Ni atomic exchange between kappa and B2 is thermodynamically favorable at 800 C. This exchange drives the B2 phase to evolve from a NiAl type towards an FeAl type, improving the stability of both phases during co-precipitation. These results provide understanding of kappa B2 interactions and offer insights for designing nanosized intermetallic strengthened microstructures in Al alloyed lightweight steels.