Thermally Decomposed Binary Fe–Cr Alloys: Toward a Quantitative Relationship Between Strength and Structure

Abstract

Binary Fe–Cr alloys are model alloys for ferritic steels proposed as structural materials for future fusion reactors. They are used to investigate the fundamental mechanisms of their degradation induced by heat and irradiation. Fe–Cr presents a miscibility gap, which induces Cr-rich (α′) regions in an Fe-rich (α) matrix. As this causes embrittlement, it is crucial to understand this phase decomposition and its role, starting with the heat impact. Fe–Cr alloys with 5−40 wt% Cr were annealed at 500 °C for up to 1008 h. The microstructure was probed by chemical mapping using scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (EDS) and atom probe tomography, and hardness was assessed by Vickers testing. Increasing Cr content increases hardness, and beyond 15 wt% Cr it further increases upon annealing. At 20 wt% Cr, nanoscale globular α′ precipitates appear, while at 40 wt% Cr an α′-percolating structure develops. In both cases, the α′ core composition reaches slightly more than 80 at% Cr, and hardness doubles. A unified relationship is found between the alloy strength and the α′ structure and it is shown that this type of hardening is a general mechanism for mature systems, independent of the nominal alloy composition.