Mo-driven strengthening mechanisms in cobalt-free Cr20-xFe30Mn20Ni30Mox high-entropy alloys

High-entropy alloys (HEAs) with face-centered cubic structures are renowned for their exceptional ductility but suffer from low strength, limiting their suitability for advanced applications such as Gen-IV nuclear reactors. The use of cobalt, a common FCC stabilizer, raises concerns due to its high neutron absorption and induced-radioactivity in such environments. To address these challenges, we developed Co-free Cr_20-xFe₃₀Mn₂₀Ni₃₀Mo_x (x = 0.6, 1.2, 2.4, molar ratio) HEAs and systematically investigated the role of Mo addition in enhancing their microstructural stability and mechanical performance. Cold-rolled alloys were annealed at 550–950 °C for 0.5h and characterized using X-ray diffraction, electron backscatter diffraction, transmission electron microscopy, tensile testing, and nanoindentation. The results show that increasing Mo content delays the precipitation and dissolution of the nanoscale σ phase, enhancing thermal stability by suppressing grain growth and recrystallization. Above 750 °C, higher Mo content significantly boosts strength and hardness, albeit at reduced ductility. The Mo2.4 alloy annealed at 850 °C shows the optimal strength-ductility product. In most cases, alloys with a higher Mo content tend to have a higher dislocation density and a smaller grain size. Most of the yield strength increment for HEAs is provided by dislocation strengthening and grain boundary strengthening. However, during annealing, the effect of dislocation strengthening is significantly reduced and grain boundary strengthening becomes the dominant mechanism. These findings elucidate Mo-driven strengthening mechanisms, providing critical insights for designing robust Co-free HEAs tailored for nuclear reactor applications and beyond.