CO2-Free Nano-Alloys Redefine Sustainable Metals

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Alloying, the art of blending metals with other elements, has long been a cornerstone of materials science and metallurgy, creating materials with tailored properties. In contrast, dealloying has been known primarily as a corrosive process that degrades materials over time by selectively removing elements, weakening their structure. Now, researchers at the Max Planck Institute for Sustainable Materials (MPI-SusMat) have turned these two seemingly counteracting processes into an innovative harmonic synthesis concept. Their study, published in the journal Science Advances, showcases how dealloying and alloying can be harmonized to create lightweight, nanostructured porous martensitic alloys CO2-free and energy saving.

The microstructure of metallic alloys is defined by the arrangement of atoms within a lattice, with their positions and chemical composition being critical to material properties. Traditional dealloying naturally removes atoms from this lattice, causing degradation. But the MPI-SusMat team asked a game-changing question: What if we could harness dealloying to create beneficial microstructures?

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“We aimed to use the dealloying process to remove oxygen from the lattice structure, modulating porosity via the creation and agglomeration of oxygen vacancies,” explains Dr. Shaolou Wei, Humboldt research fellow at MPI-SusMat and first author of the publication. “This method opens new pathways for designing lightweight, high-strength materials.” At the heart of their approach is reactive vapor-phase dealloying – a technique that removes oxygen atoms from the lattice structure using a reactive gas atmosphere. In this process, the atmosphere “attracts” the oxygen, selectively extracting it from the host lattice. Hereby, the atmosphere consists of ammonia, which acts as both a reductant (via its hydrogen content) and a donor of interstitial nitrogen, filling vacant lattice spaces to enhance material properties. “This dual role of ammonia – removing oxygen and adding nitrogen – is a key innovation in our approach, since it assigns all atoms from both reaction partners specific roles” says Professor Dierk Raabe, managing director of MPI-SusMat and corresponding author of the study.

Four crucial metallurgical processes in one step

The team’s breakthrough lies in integrating four crucial metallurgical processes into a single reactor step:

1. Oxide dealloying: Removing oxygen from the lattice to create excessive porosity while simultaneously reducing the metal ores with hydrogen.
2. Substitutional alloying: Encouraging solid-state interdiffusion between metallic elements upon or after complete oxygen removal.
3. Interstitial alloying: Introducing nitrogen from the vapor phase into the host lattice of the gained metals.
4. Phase transformation: Activating thermally-induced martensitic transformation, the most viable pathway for nano structuring.

This synthesis strategy not only simplifies alloy production, but also offers a sustainable approach by utilizing oxides as starting materials and reactive gases such as ammonia or even waste emissions from industrial processes. Through the usage of hydrogen as a reductant agent and energy carrier instead of carbon, the whole dealloying-alloying process is CO2-free and the only byproduct is water. Thermodynamic modelling demonstrates the feasibility of this technique for metals like iron, nickel, cobalt, and copper.

Sustainable lightweight design through microstructure engineering

The resulting nano-structured porous martensitic alloys are lighter and stronger, thanks to precise microstructure control from the millimeter down to the atomic scale. Traditionally, achieving such porosity required time- and energy-intensive processes. In contrast, the new strategy accelerates porosity formation while allowing for the simultaneous introduction of interstitial elements like nitrogen that enhance material strength and functionality.

Future applications could range from lightweight structural components to functional devices such as iron-nitride-based hard magnetic alloys, which could surpass rare-earth magnets in performance. Looking ahead, the researchers envision expanding their approach to use impure industrial oxides and alternative reactive gases. This could revolutionize alloy production by reducing reliance on rare-earth materials and high-purity feedstocks thus aligning with global sustainability goals.

With this innovative dealloying-alloying strategy, the MPI-SusMat team has demonstrated how rethinking traditional processes can yield transformative advances in materials science. By combining sustainability with cutting-edge microstructure engineering, they are paving the way for a new era of alloy design.

Reference: Wei S, Ma Y, Raabe D. Reactive vapor-phase dealloying-alloying turns oxides into sustainable bulk nano-structured porous alloys. Sci Adv. 2024;10(51):eads2140. doi: 10.1126/sciadv.ads2140

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