Out-of-plane order retention below the solubility threshold in V-strengthened Cr₂TiAlC₂ and its mechanical benefits

The development of MAX materials with out-of-plane chemically ordered structures (o-MAX) represents a key frontier in advanced structural ceramics. However, their synthesis is challenging due to elemental combination constraints and molar ratio limitations. This study successfully synthesized a series of novel Cr₂(Ti_1−xV_x)AlC₂ (x = 0–0.6) quinary o-MAX materials by rationally selecting element combinations and regulating their proportions. Analysis via X-ray diffraction (XRD) Rietveld refinement and aberration-corrected transmission electron microscopy revealed that the solid solution threshold for V-induced suppression of out-of-plane chemical ordering is x = 0.5. When x ≤ 0.5, V selectively forms solid solutions with Ti at the 2a Wyckoff site, preserving out-of-plane chemical order. However, when the V proportion exceeds the solid solution limit (x > 0.5), the structure undergoes an ordered–disordered transition, accompanied by the migration of V and Ti to the 4f Wyckoff site. Furthermore, the out-of-plane chemically ordered introduction of V achieves comprehensive enhancement of mechanical properties. The optimal composition Cr₂(Ti_0.5V_0.5)AlC₂ sample exhibits significant improvements in both hardness and toughness. Based on the nanoindentation hardness (H) and elastic modulus (E), the materials’ capacity for energy dissipation (E²/H) and resistance to plastic deformation (H³/E²) are analyzed. Accordingly, its E²/H and H³/E² ratios increase by approximately 7% and 27%, respectively, highlighting the synergistic strengthening–toughening effect. This work not only expands the variety of o-MAX phases but also establishes a clear microstructure–property relationship, demonstrating that a selective site solid solution is an effective strategy for designing high-performance MAX phase ceramics.