What is it about?
Metallurgical production traditionally involves three steps: extracting metals from ores, mixing them into alloys by liquid processing and thermomechanical processing to achieve the desired microstructures1,2. This sequential approach, practised since the Bronze Age, reaches its limit today because of the urgent demand for a sustainable economy2,3,4,5: almost 10% of all greenhouse gas emissions are because of the use of fossil reductants and high-temperature metallurgical processing. Here we present a H2-based redox synthesis and compaction approach that reforms traditional alloy-making by merging metal extraction, alloying and thermomechanical processing into one single solid-state operation. We propose a thermodynamically informed guideline and a general kinetic conception to dissolve the classical boundaries between extractive and physical metallurgy, unlocking tremendous sustainable bulk alloy design opportunities. We exemplify this approach for the case of Fe–Ni invar bulk alloys6,7, one of the most appealing ferrous materials but the dirtiest to produce: invar shows uniquely low thermal expansion6,8,9, enabling key applications spanning from precision instruments to cryogenic components10,11,12,13. Yet, it is notoriously eco-unfriendly, with Ni causing more than 10 times higher CO2 emission than Fe per kilogram production2,14, qualifying this alloy class as a perfect demonstrator case. Our sustainable method turns oxides directly into green alloys in bulk forms, with application-worthy properties, all obtained at temperatures far below the bulk melting point, while maintaining a zero CO2 footprint.
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Why is it important?
Using H2-based redox reactions, our ‘one step oxides to bulk alloy’ operation (Fig. 1a) is aimed to reform the millennia-old multi-step alloy-making process (Fig. 1a, top) in three aspects: first, eliminating CO2 emission during fossil reductant-based metal extraction; second, reducing the energy cost of liquid processing15,16 that scales with melting temperatures; and third, exploiting the diffusion processes involved directly for compaction. The a priori feasibility of our sustainable alloy synthesis route is governed by the thermodynamic nature of the traditionally separated process steps that we merge here: metal extraction from oxides, atomic-scale mixing amongst the alloying elements and bulk material compaction by diffusion. (Fig. 1a, bottom).
Perspectives
This is a redox-inspired sustainable alloy design concept fulfilling one-step synthesis of bulk alloys directly from oxides. Following the thermodynamic guideline and the integrated kinetic conception, we applied this approach to the fabrication of bulk Fe–Ni invar alloys with microstructure–bulk property combinations that are ready to be deployed in real-world applications. The as-synthesized alloy not only exhibits a near-zero thermal expansion property aligning well with the invar alloys fabricated using the traditional multi-step metal extraction, liquid alloying and thermomechanical processing routes but is also accessible to wide microstructure tunability. The universality of our approach, however, goes beyond the specific scope of Fe–Ni binary invar alloy synthesis: the same concept can be extended (1) to various dilute oxide-bonded transition metals and (2) to even highly contaminated oxidized feedstocks of diverse origins. This approach also dissolves some of the classical boundaries between extractive and physical metallurgy, inspiring direct conversion from oxides to application-worthy products in one single solid-state operation.
Professor Dierk Raabe
Max-Planck-Gesellschaft zur Forderung der Wissenschaften
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This page is a summary of: One step from oxides to sustainable bulk alloys, Nature, September 2024, Springer Science + Business Media,
DOI: 10.1038/s41586-024-07932-w.
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