Ta-containing MAX phases are promising high-temperature structural ceramics, but high density and concurrent optimization of strength, fracture toughness, and elevated-temperature property retention remains challenging. Entropy engineering can tailor M-site chemistry; however, it remains unclear how M-site combinations control the strength–toughness balance and elevated-temperature retention. Here, (TaNbV)₂AlC, (TaTiNb)₂AlC, (TaTiV)₂AlC, and (TaTiNbV)₂AlC were synthesized by vacuum hot pressing and investigated through phase and microstructural characterization, density functional theory (DFT)-based crystal orbital Hamilton population (COHP) analysis, Deep Potential molecular dynamics (DP-MD), and mechanics-based bridging models. Characterization confirmed a layered M₂AlC-type framework and showed no obvious M-site elemental segregation within the examined regions. Compared with Ta₂AlC, all medium-entropy ceramics showed reduced density and improved room-temperature flexural strength and fracture toughness, with (TaNbV)₂AlC achieving 578 MPa and 8.5 MPa·m^1/2. COHP revealed stronger M–C and M–Al bonding in Ta/Nb-centered environments than in Ti- and V-centered environments, explaining the flexural-strength grouping. However, fracture toughness required path-dependent basal-cleavage descriptors beyond average bond strength. A validated DP-MD model demonstrated structural retention up to 1500 K under inert (oxygen-free) conditions and within the simulation time scale, intrinsic tensile softening, and composition-dependent cleavage-work retention. Qualitative-to-semiquantitative bridging projections indicate that (TaNbV)₂AlC provides the highest absolute strength and toughness, whereas (TaTiNbV)₂AlC has the best elevated-temperature retention. These results reveal that Ta/Nb-rich bonding and basal-cleavage resistance act as complementary, composition-dependent mechanisms governing the strength–toughness balance and its elevated-temperature retention in this Ta-based 211 MAX series, providing a mechanistic basis for designing entropy-engineered MAX ceramics with room-temperature performance and elevated-temperature property retention as distinct targets.

MAX-phase ceramics combine metallic and ceramic characteristics, while their two-dimensional (2D) derivatives, MXenes, have shown great potential as reinforcements for high-temperature structural applications. Leveraging the structural similarity between MXenes and their MAX-phase precursors, Ti₃C₂T_x was incorporated into two 211-type MAX ceramics, Cr₂AlC and Ta₂AlC, to investigate its effects on mechanical properties and strengthening mechanisms. The addition of MXene improved both flexural strength and fracture toughness. The optimal enhancement was observed at 2 wt% for Cr₂AlC (22% strength increase) and 4 wt% for Ta₂AlC (33% strength increase). Microstructural analysis revealed partial solid solution and TiC_y formation in Cr₂AlC, while Ta₂AlC exhibited complete solid solution behavior. ensity functional theory (DFT) calculations confirmed that Ti ion diffusion into Ta₂AlC was energetically more favorable due to weaker Ta–Al bonding and larger interlayer spacing. A multi-mechanism Δσ model was used to decouple the strengthening contributions from solid solution, grain refinement, dislocation density, and load transfer. In Cr₂AlC, grain refinement and second-phase strengthening dominated, whereas in Ta₂AlC, solid solution and grain refinement prevailed. Theoretical predictions matched well with experimental data after incorporating a correction term into the shear-lag model. These findings provide insights into MXene-induced strengthening in layered ceramics and offer guidance for designing high-performance, damage-tolerant MAX-phase materials.