MAX phase ceramics, ternary layered carbides and nitrides, combine the toughness of metals with the strength of ceramics, making them promising candidates for structural applications. Among this family, o-MAX phases with out-of-plane chemical ordering of two distinct transition metals offer superior properties but remain difficult to synthesize due to strict elemental compatibility and stoichiometric constraints.
A research team led by Associate Professors Cheng-Feng Du and Hong Yu from the Center of Advanced Lubrication and Seal Materials, State Key Laboratory of Solidification Processing at Northwestern Polytechnical University, has successfully synthesized a series of Cr2(Ti1-xVx)AlC2 (x = 0–0.6) quinary o-MAX materials through rational elemental selection and precise proportion control. Their findings establish the vanadium solubility threshold in this system and demonstrate a clear route to property optimization via site-selective solid solution.
The team published their work in Journal of Advanced Ceramics on August 25, 2026.
"The key challenge in o-MAX synthesis is maintaining long-range chemical order while incorporating additional elements," said Hong Yu, corresponding author and associate professor at the Center of Advanced Lubrication and Seal Materials, State Key Laboratory of Solidification Processing, Northwestern Polytechnical University. "We found that by carefully controlling V content, we can selectively occupy specific atomic sites and preserve the out-of-plane ordered structure up to x = 0.5. Beyond this threshold, the structure collapses into disorder."
Using XRD Rietveld refinement and aberration‑corrected scanning transmission electron microscopy combined with energy‑dispersive X‑ray spectroscopy, the team determined that when x ≤ 0.5, V atoms selectively form solid solutions with Ti at the 2a Wyckoff site, maintaining the ordered Al‑Cr‑(Ti,V)‑Cr‑Al stacking sequence. When V exceeds this threshold, both V and Ti migrate to the 4f site, triggering an order‑disorder transition.
"First‑principles DFT calculations revealed that this order‑disorder transition is driven by two synergistic factors: decreased formation energy and increased configurational entropy," said Cheng‑Feng Du, co‑author of the study. "The mixed‑occupancy configuration shows a formation energy of −0.430 eV∙atom−1, significantly lower than that of the ordered configuration at −0.356 eV∙atom−1, while the configurational entropy increases from 8.1×10−6 to 4.48×10−5 eV·K−1·atom−1. These two factors collectively stabilize the disordered structure at elevated temperatures."
The ordered incorporation of V achieves comprehensive mechanical enhancement. The optimal composition Cr2(Ti0.5V0.5)AlC2 exhibits a 16.4% increase in nanoindentation hardness (11.56 ± 1.21 GPa), a 35.2% enhancement in Vickers hardness (6.68 ± 0.11 GPa), and a 5.1% improvement in fracture toughness (6.39 MPa·m1/2) compared to the undoped counterpart. The H3/E2 ratio, a key indicator of resistance to plastic deformation, increased by approximately 27.0%, confirming a synergistic strengthening‑toughening effect.
"This work demonstrates that site‑selective solid solution is a viable route to high‑performance MAX ceramics," said Hong Yu. "Beyond expanding the o-MAX family, we establish a clear microstructure‑property relationship that can guide future material design."
The substitution of Ti with smaller V atoms induced continuous lattice contraction, with a‑axis and c‑axis shrinking by 0.62% and 0.47%, respectively, confirming solid solution formation. Additionally, average grain size decreased from 9.11 µm to 6.33 µm as V content increased, providing grain refinement strengthening. The E2/H ratio of the V0.5 sample increased by approximately 7.0%, reflecting enhanced energy dissipation capacity and crack initiation resistance.
This research opens new avenues for designing advanced structural ceramics with enhanced mechanical performance through controlled atomic‑scale ordering, with potential applications in high‑temperature structural components, aerospace materials, and protective coatings.
About Author
Hong Yu is an associate professor at Northwestern Polytechnical University and a principal investigator at the Center of Advanced Lubrication and Seal Materials. She received her Ph.D. from Nanyang Technological University, Singapore, in 2016, and joined NPU in 2017. Her research focuses on novel two-dimensional materials and related applications.
Cheng-Feng Du is an associate professor at Northwestern Polytechnical University and a core member of the Center of Advanced Lubrication and Seal Materials (Academician Weimin Liu's team). He received his Ph.D. from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, in 2016, and conducted postdoctoral research at Nanyang Technological University from 2016 to 2018 before joining NPU. His research focuses on the synthesis, structural regulation, and tribological properties of MAX phase ceramics.
Funding
This work was supported by the National Natural Science Foundation of China (Nos. 52275212 and 52572085), the fund of the State Key Laboratory of Solidification Processing in NPU (2025-TS-09), and the "Special Lubrication and Sealing for Aerospace" Shaanxi Provincial Science and Technology Innovation Team (2024RSCXTD-63), and the research fund from Analytical & Testing Center of Northwestern Polytechnical University (2023T012).
DOI LINK: 10.26599/JAC.2026.9221336
About Journal of Advanced Ceramics
Journal of Advanced Ceramics (JAC) is an international academic journal that presents the state-of-the-art results of theoretical and experimental studies on the processing, structure, and properties of advanced ceramics and ceramic-based composites. JAC is Fully Open Access, monthly published by Tsinghua University Press, and exclusively available via SciOpen . JAC's 2025 IF is 14, ranking in Top 1 (1/34, Q1) among all journals in "Materials Science, Ceramics" category, and its 2025 CiteScore is 24.6 (6/133) in Scopus database. ResearchGate homepage: https://www.researchgate.net/journal/Journal-of-Advanced-Ceramics-2227-8508
