• Increasing Nb content in Alloy 625 Plus lowers the liquidus temperature by 51°C and refines the solidification microstructure, reducing secondary dendrite arm spacing from 39.09 to 22.61 µm.
• Higher Nb content alleviates element segregation but increases interdendritic precipitates (Laves, δ, η, γ″) from 0.15% to 5.82%, altering their morphology from small chunks to elongated forms.
• During hot deformation at 1150°C and 0.5 s−1, higher Nb content raises peak stress but promotes more pronounced recrystallization softening, increasing recrystallization fraction from 32.4% to 95.5%.
• The enhanced recrystallization is attributed to Nb-induced grain refinement, solid solution strengthening, and particle-stimulated nucleation, leading to a more uniform deformation microstructure.