Towards high-strength, high-ductility ferritic steels: Pathways to overcome the “475 °C embrittlement” in spinodally decomposed Fe-40Cr alloy

Abstract

Fe–Cr is a model alloy for ferritic steels used in thermal power generation systems and envisaged as primary structural material for future fusion reactors. However, upon heating and, further, under irradiation, Fe–Cr can suffer from phase decomposition leading to the formation of Fe-rich and Cr-rich regions that induce simultaneous hardening and embrittlement. In this study, we examine the origins of the degradation in mechanical properties by performing room-temperature in situ micropillar compression tests on single-crystalline Fe–40wt.%Cr alloys in solid solution, and in the spinodally decomposed state obtained after annealing at 500 °C for 1008 and 2016 h, respectively. The compressed micropillars are subjected to correlative nanoscale structural characterization using transmission electron backscattered diffraction and transmission electron microscopy. Dislocation slip occurs unequivocally on the {110}〈11¯1〉 slip system for all conditions. While the 2016 h annealed state exhibits a more evolved nanoscale phase modulation than the 1008 h annealed condition, both microstructures display approximately double the yield strength of the solid-solution state without any concurrent loss of ductility. Our findings reveal a fundamental change in plasticity mechanism across the three different microstructures. Deformation in the solid-solution state is associated with kink-mediated local plasticity occurring on multiple glide systems, which activate sequentially based upon the largest instantaneous Schmid factor. This gradually transforms into a less localized Lüders-band like plasticity associated with single-slip activation in the 1008 h annealed state, while the deformation in the 2016 h annealed state is marked by uniform strain hardening related to a homogeneous polycrystalline-like dislocation motion occurring simultaneously on multiple slip systems. Correlations between the spatial and compositional fluctuations in Cr and the associated plasticity dynamics are established. It is shown that the spatial fluctuations in Cr strongly influence dislocation strengthening and the relative mobility of the edge and screw components across all microstructural states. It is further concluded that the phase-separation effect, despite promoting strengthening, does not act as the primary cause of embrittlement, but rather plays a contrary role of enhancing ductility.