Hydrogen-based direct reduction of multicomponent oxides: Insights from powder and pre-sintered precursors toward sustainable alloy design

The co-reduction of metal oxide mixtures using hydrogen as a reductant in conjunction with compaction and sintering of the evolving metallic blends offers a promising alternative toward sustainable alloy production through a single, integrated, and synergistic process. Herein, we provide fundamental insights into hydrogen-based direct reduction (HyDR) of distinct oxide precursors that differ by phase composition and morphology. Specifically, we investigate the co-reduction of multicomponent metal oxides targeting a 25Co-25Fe-25Mn-25Ni (at.%) alloy, by using either a compacted powder (mechanically mixed oxides) comprising Co 3 O 4 -Fe 2 O 3 -Mn 2 O 3 -NiO or a pre-sintered compound (chemically mixed oxides) comprising a Co,Ni-rich halite and a Fe,Mn-rich spinel. Thermogravimetric analysis (TGA) at a heating rate of 10°C/min reveals that the reduction onset temperature for the compacted powder was ∼175°C, whereas it was significantly delayed to ∼525°C for the pre-sintered sample. Nevertheless, both sample types attained a similar reduction degree (∼80%) after isothermal holding for 1 h at 700°C. Phase analysis and microstructural characterization of reduced samples confirmed the presence of metallic Co, Fe, and Ni alongside MnO. A minor fraction of Fe remains unreduced, stabilized in the (Fe,Mn)O halite phase, in accord with thermodynamic calculations. Furthermore, ∼1 wt.% of BCC phase was found only in the reduced pre-sintered sample, owing to the different reduction pathways. The kinetics and thermodynamics effects were decoupled by performing HyDR experiments on pulverized pre-sintered samples. These findings demonstrate that initial precursor states influence both the reduction behavior and the microstructural evolution, providing critical insights for the sustainable production of multicomponent alloys.