Strong-bond networks and basal-cleavage resistance govern strength–toughness balance and elevated-temperature retention in Ta-based medium-entropy 211 MAX ceramics

Ta-containing M_n+1AX_n (MAX) phases are promising high-temperature structural ceramics, but high density and concurrent optimization of strength, fracture toughness, and elevated-temperature property retention remain 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 (HP) 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 the 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.