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 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. The materials’ capacity of energy dissipation (E2/H) and resistance to plastic deformation (H3/E2) are analyzed based on the nanoindentation hardness (H) and elastic modulus (E). 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 selective site solid solution is an effective strategy for designing high-performance MAX phase ceramics.