Alloying, the technique of mixing metals with various elements, has been a fundamental aspect of materials science and metallurgy, enabling the creation of materials with customized characteristics. Conversely, dealloying has primarily been viewed as a destructive process, leading to the deterioration of materials over time by selectively eliminating components, thus compromising their structural integrity. Recently, researchers from the Max Planck Institute for Sustainable Materials (MPI-SusMat) have combined these two seemingly opposing processes into an inventive concept known as harmonic synthesis. Their research, published in the journal Science Advances, illustrates how alloying and dealloying can work together to produce lightweight, nanostructured porous martensitic alloys in a manner that is energy-efficient and carbon dioxide-free.
The arrangement of atoms within the crystal lattice defines the microstructure of metallic alloys, where their locations and chemical makeup significantly influence the material’s properties. Traditionally, dealloying leads to the loss of atoms from this lattice structure, causing it to degrade. However, the MPI-SusMat team posed a groundbreaking inquiry: Could the process of dealloying be utilized to form advantageous microstructures?
“Our goal was to apply the dealloying method to eliminate oxygen from the lattice structure, manipulating porosity by creating and accumulating oxygen vacancies,” said Dr. Shaolou Wei, a Humboldt research fellow at MPI-SusMat and the lead author of the study. “This approach opens up new avenues for crafting lightweight, high-strength materials.” Central to their method is reactive vapor-phase dealloying—a technique that removes oxygen atoms from the lattice through a reactive gas environment. In this technique, the reactive atmosphere “draws” the oxygen, selectively extracting it from the primary lattice. The atmosphere incorporates ammonia, which serves both as a reducing agent (because of its hydrogen content) and as a nitrogen donor, which fills the empty spaces in the lattice, thereby improving material properties. “The dual function of ammonia—removing oxygen while adding nitrogen—is a major innovation of our method, as it assigns specific roles to all atoms involved in both reactions,” explains Professor Dierk Raabe, managing director of MPI-SusMat and the study’s corresponding author.
Four essential metallurgical processes in one step
The team’s significant advancement lies in merging four key metallurgical processes into a single reactor step:
- Oxide dealloying: Eliminating oxygen from the lattice to create excessive porosity while concurrently reducing metal ores with hydrogen.
- Substitutional alloying: Facilitating solid-state intermixing of metallic elements during or after the complete removal of oxygen.
- Interstitial alloying: Infusing nitrogen from the vapor phase into the host lattice of the resultant metals.
- Phase transformation: Initiating thermally-driven martensitic transformation, the optimal method for nanostructuring.
This synthesis approach not only streamlines the production of alloys but also promotes sustainability by employing oxides as raw materials and reactive gases like ammonia or even waste emissions from industrial activities. By using hydrogen as a reductive agent and energy carrier instead of carbon, the entire dealloying-alloying process remains free of carbon dioxide emissions, with water being the only byproduct. Thermodynamic modeling indicates that this technique is viable for metals such as iron, nickel, cobalt, and copper.
Sustainable lightweight design through microstructure engineering
The end result is nano-structured porous martensitic alloys, which are both lighter and stronger owing to precise control over their microstructure from the millimeter level down to the atomic scale. Traditionally, achieving such porosity required extensive time and energy-consuming processes. In contrast, the new method expedites porosity formation while simultaneously integrating interstitial elements like nitrogen that strengthen and enhance the material’s functionality.
Future applications may extend to lightweight structural components and functional devices, such as iron-nitride-based hard magnetic alloys that could outperform rare-earth magnets. Looking forward, the researchers plan to broaden their approach by utilizing impure industrial oxides and alternative reactive gases, which could transform alloy production by reducing dependence on rare-earth materials and high-purity inputs, thus supporting global sustainability efforts.
With this ground-breaking dealloying-alloying technique, the MPI-SusMat team has shown that reimagining traditional methods can result in significant advancements in materials science. Combining sustainability with innovative microstructure engineering, they are paving the way for a new era in alloy design.
This research was supported by a fellowship to 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 for the team.
