Effects of Nb content on the solidification characteristics and hot deformation behavior of Alloy 625 Plus

Abstract

Through thermodynamic calculations and microstructural characterization, the effect of niobium (Nb) content on the solidification characteristics of Alloy 625 Plus was systematically investigated. Subsequently, the effect of Nb content on hot deformation behavior was examined through hot compression experiments. The results indicated that increasing the Nb content lowers the liquidus temperature of the alloy by 51°C, producing a denser solidification microstructure. The secondary dendrite arm spacing (SDAS) of the alloy decreases from 39.09 to 22.61 µm. Increasing the Nb content alleviates element segregation but increases interdendritic precipitates, increasing their area fraction from 0.15% to 5.82%. These precipitates are primarily composed of large Laves, δ, η, and γ″ phases, and trace amounts of NbC. The shapes of these precipitates change from small chunks to large elongated forms. No significant change in the type or amount of inclusions within the alloy is detected. The inclusions are predominantly individual Al₂O₃ and TiN, as well as Al₂O₃/TiN composite inclusions. Samples with varying Nb contents underwent hot compression deformation at a true strain of 0.69, a strain rate of 0.5 s⁻¹, and a deformation temperature of 1150°C. Increasing the Nb content also elevates the peak stress observed in the flow curves. However, alloys with higher Nb content exhibit more pronounced recrystallization softening effects. The Laves phase precipitates do not completely redissolve during hot deformation and are stretched to elongated shapes. The high-strain energy storage increases the recrystallization fraction from 32.4% to 95.5%, significantly enhancing the degree of recrystallization and producing a more uniform deformation microstructure. This effect is primarily attributed to the addition of Nb, which refines the initial grains of the alloy, enhances the solid solution strengthening of the matrix, and improves the induction of particle-stimulated nucleation.

Keywords

element segregation / precipitates / inclusion / hot deformation / recrystallization

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