• Mo addition transforms the microstructure of Al1.25CoCrFeNi3−xMox HEAs from eutectic to hypereutectic and dendritic, promoting BCC phase nucleation.
• The Al1.25CoCrFeNi2.8Mo0.2 alloy achieves an outstanding combination of tensile strength (1234.80 MPa) and fracture strain (19.33%).
• Strengthening mechanisms are identified as solid solution, grain boundary, and heterogeneous interface strengthening.
• The study provides a strategy to overcome the strength-ductility trade-off in dual-phase HEAs via BCC phase modulation.