Abstract: L1₂-strengthened multi-principal element alloys (MPEAs), particularly the Al-Ti bearing systems, exhibit outstanding potential as heat-resistant structural materials, but generally suffer from intermediate-temperature embrittlement (ITE), limiting their practical application. In this study, we successfully achieved a distinct brittle-to-ductile transition in the L1₂-strengthened NiCoCr-based MPEA at 700 and 800 °C, along with an improved strain-hardening capability via grain boundary engineering (GBE). Comprehensive microstructural characterization and mechanism analysis revealed that the mitigation of ITE rests upon two key factors: (1) inhibiting the discontinuous precipitates at random high-angle grain boundaries (RHAGBs), and (2) increasing the fraction of special boundaries dominated by Σ3 boundaries. The coupling between these two factors alleviates the stress concentration near RHAGBs and enhances the oxidation resistance of the alloy, thereby delaying the initiation of intergranular cracks and suppressing oxygen-accelerated GB damage. Furthermore, the dynamic recrystallization is also significantly restrained, ensuring the sustainability of the strain-hardening capacity driven by multiple deformation mechanisms, such as dislocations, stacking faults, and deformation twins. This study is a successful demonstration of applying the GBE strategy to manipulate the mechanical properties of L1₂-strengthened MPEAs, thus providing a simple and effective approach for creating high-performance MPEAs.

Key words: Multi-principal element alloys, Intermediate-temperature embrittlement, Grain boundary engineering, Precipitation strengthening, Mechanical properties