Phase transformations in the alumina-forming austenitic stainless-steel Fe-20Cr-30Ni-2Nb-5Al during creep at 750°C 45 MPa

Heat resistant steels are widely used in the energy production industry. However, pushing their operating temperatures higher means reducing component lifetimes due to the acceleration of the time-dependent processes of deformation and oxidation. The goal of next-generation alloy design is therefore to increase the temperature capability of these materials, while simultaneously maintaining low component costs. The creep lifetime of the newly developed alumina-forming-austenitic stainless steels can be improved by controlling the precipitation and microstructural evolution during creep. The current research aims to further the knowledge on the transformation pathways of Fe-20Cr-30Ni-2Nb-5Al (at.%) during aging and creep at 750 degrees C / 45 MPa. It was found that the initial gamma-gamma'-Laves (aged) microstructure transformed to a gamma-gamma'-Laves-beta-alpha'-sigma (crept) microstructure as various phases rejected or attracted different elements. Atom probe tomography (APT) and transmission electron microscopy (TEM) were employed to evaluate the composition and structure of the matrix and various inter- and intra-granular precipitates and to understand their formation mechanisms. The formation of sigma-CrFe was related to dislocation slip on {111}, with Laves-sigma co-precipitation growing along the < 111 > directions. Meanwhile grain boundary alpha'-Cr precipitation was related to the rejection of Cr, mainly by S-NiAl. Interestingly a few nm thin but up to mm long Laves phase plates formed along annealing twin boundaries, however, a significant effect on the creep properties associated to their formation was not observed.