Out-of-plane order retention below the solubility threshold in V-strengthened Cr 2 TiAlC 2 and its mechanical benefits

Abstract 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(Ti1−xVx)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(Ti0.5V0.5)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.