High-Entropy Alloy Filler Tames Brittle Layers in Carbon-Carbon Brazed Joints

Joining carbon fiber reinforced carbon matrix composites to metals has long been one of the most stubborn problems in high-temperature aerospace manufacturing. These composites, known as Cf/C, can survive conditions that would melt or creep most conventional alloys, which makes them attractive for rocket nozzles, thermal protection panels, and other components that face extreme heat. The trouble is that a Cf/C component rarely works alone; it must be bolted, welded, or brazed to metallic structures, and every one of those connections becomes a potential point of failure. A team of Chinese researchers now reports a new brazing strategy based on a high-entropy alloy filler metal that suppresses the brittle interfacial phases responsible for many of these failures, while also cutting joint stress significantly. Their work was published in the journal Advanced Composites and Hybrid Materials.

The core problem lies in chemistry. Traditional active brazing fillers contain elements such as titanium or chromium that react with carbon to form carbides, which is precisely what allows the molten filler to wet the composite surface in the first place. Wetting requires some reaction at the interface. But when that reaction runs out of control, it produces a continuous layer of brittle carbide phases along the entire contact area. Such a layer acts like a built-in crack: it has almost no tolerance for the shear and tensile loads that a real joint experiences, and the mismatch in thermal expansion between ceramic-like composites and metals adds large residual stresses during cooling after brazing. The result is joints that may look perfect under the microscope yet fail at low loads in service.

Researchers led by Jinzheng Li, Pengcheng Wang, Haiyan Chen, and Wenya Li at Northwestern Polytechnical University, together with collaborators from Xi’an Space Engine Company, the Harbin Institute of Technology, and the Zhengzhou Research Institute of Mechanical Engineering, attacked the problem with an unusual choice of filler: a face-centered cubic FeCoNiAlCuTi high-entropy alloy. High-entropy alloys, or HEAs, are composed of several principal elements mixed in near-equal proportions, and their defining feature is a very high configurational entropy of mixing. In a conventional alloy, one dominant element dictates the crystal structure and behavior; in a high-entropy alloy, the chaotic atomic-level mixing itself stabilizes a single solid-solution phase and complicates the formation of ordered, brittle intermetallic compounds.

That disorder proved to be the key. When the team used the FCC FeCoNiAlCuTi alloy to braze Cf/C composites to a refractory metal interlayer, the same high mixing entropy that stabilizes the bulk filler also hampered the continuous, cooperative reactions that normally produce an unbroken brittle layer on the composite side. Instead of growing as a continuous film, the interfacial phases were fragmented and dispersed. The resulting joint did not contain a continuous brittle layer at the Cf/C interface at all, which is exactly the failure-prone feature the researchers set out to eliminate. Without that weak seam running across the joint, cracks lose the easy path they would otherwise follow.

The microstructure that emerged is strikingly hierarchical. The brazed seam develops as a multi-layered structure with three principal regions: a reaction zone where the filler meets the Cf/C composite, a band of niobium solid solution, and a eutectic zone within the seam. Crucially, the interface with the composite takes on a gully-shaped, interlocking geometry, anchored by a ductile FeCoNiAl-rich phase rather than by a brittle carbide film. This mechanical interlocking does more than improve adhesion. Because the ductile FCC phase bridges the interface, it redirects a portion of the load away from the fragile composite surface and into the tougher interior of the joint, where the FCC structure can deform plastically and absorb energy instead of fracturing.

The mechanical consequences were quantified with finite element simulations. When the team modeled the stress state of a conventionally brazed joint, the peak Mises stress, a standard measure of the combined stresses that drive yielding and fracture, reached 371 megapascals. With the hierarchical microstructure engineered by the high-entropy filler, that peak dropped to 274 megapascals, a reduction of roughly a quarter. Lower peak stress means less driving force for crack initiation at the interface and a longer fatigue life in service. It also means the joint tolerates the thermal expansion mismatch between composite and metal far more gracefully, since stress is spread through the ductile phase rather than concentrated against a brittle layer.

The experimental performance matched the modeling. Under optimized brazing conditions of 1340 degrees Celsius held for 20 minutes, the joints achieved a shear strength of 40.7 megapascals. For carbon-carbon composite joints, where brittle interfacial layers and residual stresses have historically limited reliability, that figure represents a meaningful benchmark. The brazing window itself is technologically significant: vacuum brazing at around 1340 degrees Celsius is compatible with existing industrial equipment used for joining refractory metals and high-temperature assemblies, so the process does not demand exotic infrastructure to be adopted.

The study also highlights how a refractory metal participates in the design. Niobium, used as an interlayer material in the joint, dissolves into the seam to form a banded solid-solution zone. This band serves as a compliant, ductile buffer between the composite and the eutectic regions of the filler, further moderating the transfer of stress. Refractory metals are natural partners for carbon-carbon composites in aerospace systems because they retain strength at high temperature, but they bring their own joining difficulties. By integrating the niobium band into a deliberately layered seam architecture, the researchers turned what is often a liability into an engineered component of the joint’s mechanics.

What makes the approach conceptually interesting beyond this specific material pair is the use of thermodynamics as a design tool. Rather than adding inhibitors or controlling reaction time more tightly, the team exploited the intrinsic high mixing entropy of a multi-principal-element alloy to make brittle phase formation simply less favorable. The same principle could be transferable to other difficult joining problems, for example between silicon carbide composites and superalloys, or titanium aluminides and steels, wherever continuous brittle reaction layers undermine joint strength. High-entropy fillers remain a young field, and questions about cost, oxidation behavior, and long-term performance at sustained high temperature will need answers before flight-critical adoption, but the fundamental demonstration is clear.

The work, which was supported by the National Natural Science Foundation of China and several provincial and state key laboratory programs, arrives at a moment when aerospace designers are pushing carbon-carbon composites from niche thermal-protection roles toward broader structural use. Every step in that direction multiplies the number of composite-to-metal joints required, and each of those joints is only as reliable as its weakest interfacial layer. By showing that hierarchical microstructure engineering with an FeCoNiAlCuTi high-entropy filler can eliminate the continuous brittle layer, redistribute stress into ductile phases, and deliver a shear strength of more than 40 megapascals, the Northwestern Polytechnical University-led team has provided a concrete recipe for stronger, more dependable bonds. The research was published as an open-access article, making the detailed microstructural analysis available to joining engineers worldwide who are racing to make extreme-temperature composite hardware a routine reality.

Subject of Research: High-entropy alloy brazing of carbon fiber reinforced carbon composites to suppress brittle interfacial layers

Article Title: Brittle interfacial layer suppression in Cf/C brazed joints with refractory metal via hierarchical microstructure engineering

Article References: Li, J., Wang, P., Chi, J., Shao, H., Pan, Z., Wang, J., Long, W., Song, X., Chen, H., & Li, W. (2026). Brittle interfacial layer suppression in Cf/C brazed joints with refractory metal via hierarchical microstructure engineering. Advanced Composites and Hybrid Materials. https://doi.org/10.1007/s42114-026-02101-1

Image Credits: AI Generated

DOI: 10.1007/s42114-026-02101-1

Keywords: carbon-carbon composites, high-entropy alloy, brazing, vacuum brazing, interfacial reactions, refractory metals, niobium interlayer, residual stress, shear strength, microstructure engineering, aerospace materials, finite element simulation