Abstract: This study investigates the influence of tungsten (W) on the microstructure and mechanical properties of high-entropy superalloys, focusing specifically on the formation and evolution of the hierarchical microstructure. Transmission electron microscope analysis indicates that aging at 750 °C for 5-10 h induces the formation of nanoscale γ particles (γ_p) within the γ' precipitates, thereby constructing a hierarchical microstructure. Vickers hardness testing reveals a strong correlation between microstructural evolution and hardness. The formation of the hierarchical microstructure significantly increases hardness (∼40 HV). Furthermore, increasing the W content (from 0.40 to 0.60 to 1.00 at. %) delays the butterfly-shaped splitting of the γ', resulting in the maximum achieved hardness. Atom probe tomography confirms that the supersaturation of γ-forming element Cr within the γ' drives the formation of the γ_p. The emergence of Cr-rich clusters reduces the mixing enthalpy of the γ' (∼0.60 kJ/mol), providing a thermodynamic driving force. After aging, the γ matrix (γ_m)/γ' interface width is also observed to increase (∼0.36 nm). The partitioning behavior of most elements aligned with Thermo-Calc predictions, which only W exhibited a transition from preferential partitioning to the γ matrix (in γ_m/γ') to the γ' precipitate (in γ'/γ_p). Calculations based on Vegard's law show that an increased W content reduces the lattice parameter of the γ_p (∼0.002 Å), thereby increasing the lattice misfit between γ_p and γ' (∼0.08 %).

Key words: High entropy superalloy, Atom probe tomography, Transmission electron microscopy, Hierarchical microstructure, Phase separation