A KAIST research team led by Professor Gi-Dong Sim in the Department of Mechanical Engineering has proposed a new strategy to strengthen aluminum alloys. Contrary to the conventional perspective that interstitial solid-solution strengthening is difficult to achieve in aluminum, the team experimentally demonstrated that interstitial carbon in sputter-deposited aluminum–carbon thin films can form a Cottrell atmosphere around dislocations for strength enhancement.

Aluminum alloys are widely used in lightweight structural applications because of their low density and excellent formability. However, their relatively low strength remains a key limitation in developing next-generation structural components that require higher load-bearing capability and improved safety. Although various strengthening approaches, including precipitation hardening, grain refinement, and carbon-based reinforcement, have been extensively explored, these strategies are often hampered by ductility reduction or processing limitations, highlighting the need for a more fundamental strengthening mechanism.

In this context, Professor Gi-Dong Sim’s team in the Department of Mechanical Engineering at KAIST has suggested a new strengthening strategy based on interstitial solute–dislocation interactions. The team initially directed their attention towards understanding the yield point phenomenon observed in sputter-deposited aluminum–carbon (Al-C) thin films. While such behavior is well known in Fe-based materials such as carbon steels, it has rarely been reported in aluminum because of the low solubility of carbon and the tendency for brittle carbide formation. In an earlier study, the group showed that sputtering can produce metastable Al-C thin films with uniformly distributed carbon in the aluminum matrix. In the present work, repeated unloading–aging–reloading tests, stress relaxation experiments, and extensive microstructural analyses revealed that the strengthening originates from Cottrell atmosphere formation (Figure 1a). Metals plastically deform as microscopic defects in their crystal structure, called dislocations, move through the material. A Cottrell atmosphere forms as interstitial solute atoms accumulate around dislocations, effectively locking them in place and suppresses their motion, thereby significantly enhancing the strength of the Al thin films (Figure 1b). This study is particularly meaningful because it provides direct experimental evidence of diffusion-controlled solute–dislocation interactions in aluminum.

The study further demonstrated that this strengthening concept is not limited to carbon addition alone. Similar yield point behavior and strength enhancement were also observed in boron-containing Al–B thin films and in carbon-doped 6000-series aluminum alloy thin films. Notably, the developed alloy-carbon thin films achieved a yield strength of approximately 715 MPa, about six times higher than that of pure aluminum thin films (Figure 2).

In a follow-up study, the group also showed that annealed aluminum alloy–carbon thin films can simultaneously benefit from conventional precipitation strengthening and Cottrell atmosphere strengthening induced by interstitial carbon (Figure 3). As a result, the annealed thin films achieved tensile strengths of up to approximately 900 MPa. These findings suggest that interstitial solute–dislocation interactions may offer a new design pathway beyond conventional strengthening strategies for aluminum alloys. 

The research team also anticipates that this concept may be extended beyond thin films to bulk-scale aluminum alloys produced by additive manufacturing (AM). As AM involves localized melting and rapid solidification during fabrication, it may enable non-equilibrium microstructures similar to those observed in the thin-film systems, thereby opening new opportunities for the development of high-strength, lightweight aluminum structural materials.

This work was conducted by Ph.D. candidate Zion Lee and Dr. Hojang Kim as co-first authors, with Professor Gi-Dong Sim serving as the corresponding author in the Department of Mechanical Engineering at KAIST. This research was published online in Advanced Functional Materials. A related follow-up study was subsequently published in the materials science journal Materials Today Nano on April 20, 2026.

Advanced Functional Materials: 

https://advanced.onlinelibrary.wiley.com/doi/full/10.1002/adfm.76205

Materials Today Nano:

https://www.sciencedirect.com/science/article/pii/S258884202600074X