Dierk Raabe

Abstract

Multi-component alloys made from co-reduction of mixed oxides with hydrogen

Metallurgical manufacturing relies on a carbon-intensive serialization of metal extraction, liquid-phase mixing, casting, and thermomechanical processing to control microstructures and properties. This classic paradigm is responsible for nearly 10% of global greenhouse gas emissions.

We developed a sustainable solid-state strategy that dissolves the traditional boundaries between chemical extraction and physical metallurgy by collapsing oxide reduction, elemental alloying, and microstructural compaction into a single, integrated solid-state operation.

The physical feasibility of this one-step approach is governed by a thermodynamic framework evaluating the solid-state reducibility of oxide mixtures in hydrogen against the mixing enthalpies of the constituent metallic elements. We show that traditionally stable, hard-to-reduce oxides such as Cr- or Mn-oxides can be fully or partially co-reduced at significantly lowered temperatures when embedded within a matrix of less stable oxides such as Fe-, Co-, or Ni-oxides. This temperature-lowering effect is driven by mutual dissolution and low chemical activity of the solute metals within the evolving metallic solid solution phase, which shifts the local equilibrium to prevent re-oxidation.

By tracking the real-time thermokinetics and structural changes via in situ synchrotron high-energy X-ray diffraction, we demonstrate that the initial state of the precursors, whether mechanically mixed distinct oxide powders or pre-sintered, chemically blended solid-solution ceramic compounds, decisively dictates the transient phase evolution and final microstructure. Mechanically mixed powders follow sequential reduction steps governed by individual oxide stabilities, whereas pre-sintered complex oxides undergo a collective direct transformation where metal elements partition out of a stable oxide matrix (e.g., MnO). This exsolution mechanism yields unique dual-phase microstructures consisting of metallic nanoparticles supported on a nanoporous oxide skeleton, highlighting a single-step route to materials suitable for advanced catalysis and energy technologies.

We showcase the universality of this approach across multiple alloy classes, ranging from structural bulk Fe-Ni Invar alloys and functional near-equiatomic NiTi shape memory alloys synthesized via hydride-oxide reactions, to complex equiatomic CoFeMnNi multi-component alloys. Managing the competition between reduction fluxes and interdiffusion-driven densification through optimized heating rates and hydrogen fugacity allows for wide microstructure tunability. This approach yields fully consolidated bulk alloys with fine-grained microstructures and properties that match or outperform conventionally processed liquid-metallurgy counterparts, outlining a clean manufacturing paradigm for advanced structural and functional materials.

References

(1) Wei, S., Ma, Y. & Raabe, D. One step from oxides to sustainable bulk alloys. Nature 633, 816–822 (2024).

(2) Chen, X., Bienvenu, B., Yang, T. et al. Nature Synthesis 5, 1340–1348 (2026)

(3) S. Shankar, B. Ratzker, A. K. da Silva, T. M. Schwarz, H. Brouwer, B. Gault, Y. Ma, D. Raabe, Materials Today 90 (2025) 43-51.

(4) S. Shankar, B. Ratzker, Y. Ma, D. Raabe, Acta Materialia 301 (2025) 121528.

(5) S. Shankar, B. Ratzker, C. Pistidda, D. Raabe, Y. Ma, ACS Sustainable Chem. Eng. (2026), doi:10.1021/acssuschemeng.5c12301.

(6) L. Hitzemann, S. L. Wei, S. Zaefferer, D. Raabe, Acta Materialia 314 (2026) 122337.