Revolutionizing Materials: From Corrosion to Eco-Friendly Lightweight Alloys

The microstructure of metallic mixtures is shaped by the arrangement of atoms within a lattice, where their positions and chemical compositions are essential to material attributes. Conventional dealloying inherently extracts atoms from this lattice, leading to degradation. However, the MPI-SusMat group posed a revolutionary question: What if we could utilize dealloying to develop advantageous microstructures?

“We intended to employ the dealloying process to eliminate oxygen from the lattice configuration, modulating porosity through the creation and clustering of oxygen vacancies,” states Dr. Shaolou Wei, a Humboldt research fellow at MPI-SusMat and the primary author of the study. “This approach opens new pathways for the design of lightweight, high-strength materials.” Central to their strategy is reactive vapor-phase dealloying — a method that extracts oxygen atoms from the lattice structure using a reactive gaseous environment. In this procedure, the environment “draws” the oxygen, selectively removing it from the host lattice. The atmosphere used is ammonia, functioning as both a reductant (due to its hydrogen content) and a source of interstitial nitrogen, which occupies vacant lattice sites to enhance material attributes. “The dual functionality of ammonia — eliminating oxygen and introducing nitrogen — is a vital innovation in our approach, as it assigns specific roles to all atoms involved in the reaction,” explains Professor Dierk Raabe, managing director of MPI-SusMat and corresponding author of the article.

Four essential metallurgical processes in one action

The team’s innovation lies in merging four vital metallurgical processes into a singular reactor step:

1. Oxide dealloying: Extracting oxygen from the lattice to generate excessive porosity while concurrently reducing the metal ores using hydrogen.
2. Substitutional alloying: Promoting solid-state interdiffusion among metallic elements during or after the complete removal of oxygen.
3. Interstitial alloying: Incorporating nitrogen from the vapor phase into the host lattice of the acquired metals.
4. Phase transformation: Triggering thermally-induced martensitic transformation, the most viable method for nano structuring.

This synthesis methodology not only streamlines alloy fabrication but also presents a sustainable solution by employing oxides as precursor materials and reactive gases like ammonia or even waste emissions from industrial activities. By utilizing hydrogen as a reducing agent and energy carrier rather than carbon, the entire dealloying-alloying process remains CO₂-free, with water being the only byproduct. Thermodynamic modeling confirms the practicality of this technique for metals such as iron, nickel, cobalt, and copper.

Sustainable lightweight design through microstructure manipulation

The resultant nano-structured porous martensitic alloys are lighter and stronger, thanks to meticulous control of the microstructure from the millimeter scale down to the atomic level. Traditionally, achieving such porosity demanded time- and energy-consuming processes. In contrast, the novel approach fast-tracks porosity creation while enabling the concurrent integration of interstitial elements like nitrogen that boost material strength and functionality.

Potential applications could range from lightweight structural elements to functional devices such as iron-nitride-based hard magnetic alloys, which might outperform rare-earth magnets. Looking forward, the researchers hope to broaden their method to utilize impure industrial oxides and alternative reactive gases. This could revolutionize alloy production by decreasing dependency on rare-earth elements and high-purity feedstocks, thereby aligning with global sustainability objectives.

Through this pioneering dealloying-alloying strategy, the MPI-SusMat ensemble has showcased how re-evaluating traditional processes can result in transformative breakthroughs in materials science. By combining sustainability with advanced microstructure engineering, they are setting the stage for a new era of alloy formulation.

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