Multiscale investigation of tungsten/isostatic graphite joints brazed with phase-separated HEA: From thermodynamic-kinetic driven evolution to stress-governed fracture

Tungsten and isostatic graphite are critical plasma-facing materials for next-generation nuclear fusion reactors, where the structural integrity of their heterogeneous joints is paramount. To overcome the persistent bottleneck of excessively high brazing temperatures in conventional high-entropy alloy joining, this study develops a semi-solid brazing strategy utilizing phase-separated CoCrFeNiCu_x fillers. By exploiting the unique dual-endothermic melting behavior and a stable semi-solid window, the liquid-solid ratio was precisely modulated via Cu-content adjustment. Specifically, the CoCrFeNiCu_3.0 filler achieved exceptionally dense joints with a ∼ 100% bonding ratio at significantly reduced temperatures. The interfacial evolution is governed by distinct mechanisms: at the W interface, a continuous μ-M₇W₆ layer forms via the competitive displacement of Cu-rich liquid by the FCC phase under chemical potential gradients. Simultaneously, an in-situ M₇C₃ carbide layer develops at the graphite interface, following a diffusion-controlled parabolic growth law. Mechanical evaluations reveal that the shear strength exhibits a parabolic dependence on brazing parameters, peaking at 55.03 MPa (1180 °C/15 min). This performance enhancement is attributed to the synergistic effect of controlled reaction layer thickness and the mitigation of interfacial residual stress through the multi-phase HEA microstructure. This work provides a fundamental framework for the multiscale design of HEA-based joining in advanced nuclear applications.