Hierarchical interfacial L1₂ shell formation enables stable high-temperature mechanical performance in FCC/B2 dual-phase high-entropy alloys

L1₂-strengthened FCC/B2 dual-phase high-entropy alloys (HEAs) exhibit excellent mechanical performance across a broad temperature range, positioning them promising candidates for high-temperature structural applications. However, microstructural coarsening and associated mechanical degradation under prolonged thermal exposure remain key challenges. In this study, a representative alloy with the composition Ni_41.9Co₁₉Cr₁₀Fe₁₀Al₁₅Mo₂Ti₂B_0.1 (at. %) was subjected to long-term aging at 800 °C, revealing an unusual microstructural evolution. Beyond the expected L1₂ coarsening within the FCC phase, an interfacial L1₂ shell formed via the progressive consumption of L1₂ precipitates from both the FCC and B2 phases, ultimately encapsulating the B2 domains. This transformation produced a unique three-level hierarchical architecture: FCC matrix with intragranular L1₂ precipitates, an interfacial L1₂ shell, and a B2 core. Remarkably, despite this pronounced microstructural evolution, the alloy maintained stable strength-ductility synergy from room temperature up to 800 °C. This stability is attributed to the additional strengthening imparted by the interfacial L1₂ shell and the favorable cooperative deformation among the FCC, B2, and interfacial L1₂ phases. A quantitative strengthening model was established, revealing that the strengthening contribution of the L1₂ shell increases with increasing shell thickness and exceeds 100 MPa after 720 h of aging. These results provide valuable guidance for the design of thermally stable precipitation-strengthened dual-phase HEAs for long-term high-temperature applications.