Al/Cr compositional effects on nanomechanical creep behavior in lightweight refractory high-entropy alloy coatings

Improving the surface mechanical performance of titanium alloy components without sacrificing lightweight design is a key requirement for protective coating development. Here, laser-cladded MoNbTiV-based lightweight refractory high-entropy alloy coatings on TC4 substrates were studied to clarify how Al/Cr alloying regulates phase constitution, interfacial structure, local nanomechanical response, and indentation creep behavior. TEM observations show that AlMoNbTiV forms a single BCC solid solution, whereas AlCrMoNbTiV develops a BCC + C15 Laves dual-phase microstructure with dense misfit dislocations near the phase interfaces. Nanoindentation mapping demonstrates that Al/Cr alloying increases nanohardness from 7.69 to 8.94 GPa and elastic modulus from 146.51 to 179.90 GPa, Among the investigated coatings, AlCrMoNbTiV exhibits the highest hardness–modulus combination, representing the most favorable local mechanical response. The coatings also display clear strain-rate-dependent indentation behavior; possible depth-dependent hardness effects are discussed cautiously because the tests were not designed as a dedicated quantitative indentation-size-effect study. The total strain recovery ratio decreases with Al addition but increases with Cr addition. Indentation creep analysis gives effective activation volumes comparable to those reported for BCC high-entropy alloys, suggesting thermally activated dislocation-mediated deformation. These results provide a composition–microstructure–property basis for designing lightweight protective coatings on titanium alloys.