Harnessing corrosion to create sustainable lightweight alloys

Berlin, Germany (SPX) Jan 13, 2025
Alloying, the technique of combining metals with other elements, has been a cornerstone of materials science, enabling the creation of materials with tailored properties. Traditionally, dealloying has been seen as a corrosive process that selectively removes elements, leading to material degradation. However, researchers at the Max Planck Institute for Sustainable Materials (MPI-SusMat) have reimagined this process, transforming dealloying into a tool for designing lightweight, nanostructured porous martensitic alloys that are CO2-free and energy-efficient. Their findings are detailed in the journal Science Advances.

The microstructure of metallic alloys, determined by atomic arrangement and composition, is critical to material properties. Conventional dealloying processes degrade these structures by removing atoms. In contrast, the MPI-SusMat team explored how dealloying could be used to create beneficial microstructures. "We aimed to use the dealloying process to remove oxygen from the lattice structure, modulating porosity via the creation and agglomeration of oxygen vacancies," explained Dr. Shaolou Wei, Humboldt research fellow at MPI-SusMat and the study's lead author. "This method opens new pathways for designing lightweight, high-strength materials."

Central to their approach is reactive vapor-phase dealloying, a technique that employs a reactive gas atmosphere to remove oxygen atoms from the lattice. This gas, composed of ammonia, serves dual roles: it acts as a reductant through its hydrogen content and introduces nitrogen 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," said Professor Dierk Raabe, managing director of MPI-SusMat and corresponding author of the study.

Four integrated metallurgical processes

The team's innovation lies in merging four metallurgical processes into one reactor step:

1. Oxide dealloying: Removes oxygen to create porosity and simultaneously reduces metal ores with hydrogen.

2. Substitutional alloying: Encourages interdiffusion between metallic elements after oxygen removal.

3. Interstitial alloying: Introduces nitrogen from the vapor phase into the metal lattice.

4. Phase transformation: Activates martensitic transformation to achieve nanoscale structuring.

This method simplifies alloy production and promotes sustainability by starting with oxides and using ammonia or industrial waste emissions as reactive gases. Replacing carbon with hydrogen as a reductant ensures the process is CO2-free, with water as the only byproduct. Thermodynamic modeling has confirmed the feasibility of this approach for metals such as iron, nickel, cobalt, and copper.

Advancing lightweight design

The resulting nanostructured porous martensitic alloys are both lightweight and strong, thanks to precise control of their microstructure. Unlike conventional methods that require energy-intensive processes to create porosity, this approach accelerates porosity formation while introducing nitrogen to strengthen the material.

Potential applications include lightweight structural components and functional devices such as iron-nitride-based hard magnetic alloys, which could outperform rare-earth magnets. The team also envisions adapting their method to use impure industrial oxides and alternative reactive gases, reducing reliance on rare-earth materials and high-purity feedstocks. Such advancements align with global sustainability goals and could revolutionize alloy production.

By rethinking traditional metallurgical processes, the MPI-SusMat researchers have showcased how sustainability and cutting-edge microstructure engineering can drive significant advances in materials science.

This research was supported by a fellowship for Shaolou Wei from the Alexander von Humboldt Foundation, a European Advanced Research Grant awarded to Dierk Raabe, and a Cooperation Grant from the Max Planck and Fraunhofer Societies.

Research Report:Reactive vapor-phase dealloying-alloying turns oxides into sustainable bulk nano-structured alloys