High strength, ductility, and impact toughness at room temperature in hot-extruded FeAl alloys

Abstract

Abstract Ingots and powders of FeAl alloys (Fe-36Al-0.5Mo-0.05Zr-0.13C (at%), base composition) were hot extruded at 900–1100 °C to produce fine or ultrafine grained materials. Ingot material ( I M ) FeAl alloys with (100 or 210appm) and without added boron extruded at 900 °C had fully recrystallized, fine-grained (35–50 μm) microstructures and 8–10% tensile ductility in air. Boron microalloying also changed the fracture mode from intergranular to a more ductile transgranular quasicleavage mode. Charpy impact toughness tests measured an absorbed energy of 25 J in the boron-free I M FeAl, and a much higher impact energy of 63 J in the boron-doped I M FeAl alloy. Intragranular microstructures of all these I M FeAl alloys consisted of dislocation networks with some dislocation loops and no fine precipitates. Powder material ( P M ) FeAl alloys were directly consolidated by hot extrusion at temperatures of 950, 1000, and 1100 °C. This processing produced complex microstructures of deformed remnant oxide ‘envelopes’ (950, 1000 °C) or stringers (1100 °C) structures related to the prior particle interfaces, and ultrafine recrystallized grain structures within those particles. P M FeAl alloys extruded at 950 °C and 1000 °C had an ultrafine 2–4 μm grain size, 12.5–15% elongation in air (21–29% in oxygen), and ductile-dimple fracture within the prior powder particles. Despite their ductility, these P M FeAl alloys were also much stronger than I M FeAl alloys (yield strength (YS) of 600–670 MPa compared to 410–435 Mpa, respectively). These P M FeAl alloys also had high Charpy impact energies of 85–105 J. By contrast, P M FeAl extruded at 1100 °C had slightly coarser grain size (9–13 μm), had 9% elongation in air (11 % in oxygen), had a more brittle mixture of intergranular fracture and transgranular cleavage, was weaker (YS of 500–520 MPa), and had lower impact energy (25J). The ultrafine-grained P M FeAl alloys also have subtle differences in intragranular precipitate (ZrC) and dislocation (loops) components of the microstructure which may play a role in their excellent mechanical properties.