Sustainable production of Fe-Ni alloys by hydrogen-based direct reduction of oxides

Abstract

Hydrogen-based direct reduction (HyDR) offers a fossil-free route for metalmaking, but its simultaneous application to iron- and nickel-oxides, combining reduction, alloying, and densification in a single solid-state step, has scarcely been studied. Crucially, no work has yet demonstrated HyDR for producing bulk Fe-Ni alloys with controlled microstructures. Furthermore, grain refinement of Fe-Ni alloys via austenite reversion without prior deformation and the attendant challenge of stabilising an austenite–ferrite duplex structure at room temperature remains largely unexplored. This study applies the ‘one-step oxide-to-bulk-alloy’ concept to blended Fe₂O₃-NiO pellets, aiming for a fully reduced, dense Fe-Ni alloy with an ultrafine-grained austenite-ferrite duplex microstructure.
Thermogravimetric analysis confirmed near-theoretical mass loss (0.2936) and complete reduction (R = 1 ± 0.002) when the heating rate was limited to 5 °C min⁻¹. Sintering produced densities up to 7.3 g cm⁻³ with just 1.1 % porosity in Fe- 10wt.% Ni samples. Intercritical annealing of Fe-10 wt.% Ni at 560 °C maximised the driving force for the FCC phase, yet sluggish Ni diffusion limited the retained FCC to 5 vol.%. In contrast, Fe- 17 wt.% Ni annealed at 490 ℃, achieved 15 vol.% FCC. The optimised TF_5/20_17Ni_490 alloy exhibited < 3% porosity, 0.93 ± 0.08 μm ultrafine-grains and a stable, two-phase microstructure comprising of a 15% Ni-rich FCC phase (25.63 wt.% Ni) and a 85 vol% Ni-lean BCC phase (14.16 wt.% Ni). This demonstrates HyDR’s efficacy in producing bulk Fe-Ni alloys with controlled ultrafine-grained two-phase microstructures.