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

TL;DR

This study explores hydrogen-based direct reduction of multicomponent oxide precursors, revealing how initial phase and morphology influence reduction behavior and microstructure, advancing sustainable alloy manufacturing.

Contribution

It provides new insights into how precursor phase composition and morphology affect hydrogen reduction pathways and microstructure in multicomponent alloy production.

Findings

Both powder and pre-sintered oxides achieve ~80% reduction at 700°C.

Pre-sintered samples show delayed reduction onset (~525°C).

Presence of a BCC phase only in pre-sintered reduced samples.

Abstract

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 Co3O4-Fe2O3-Mn2O3-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 {\deg}C/min reveals that the reduction onset temperature for the compacted powder was ~175 {\deg}C, whereas it was significantly delayed to ~525 {\deg}C for the pre-sintered sample. Nevertheless, both sample types attained a similar reduction degree (~80%) after isothermal holding for 1 h at 700 {\deg}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.