Effect of W/Mo ratio on the as-cast microstructure of Nb-modified Hastelloy N alloys

The effects of W/Mo ratio on the microstructures of as-cast Nb-modified Hastelloy N alloys have been investigated systematically in this study. It was found that the dendritic morphologies are independent on W/Mo ratio, but the microsegregation behaviors and the primary precipitates in the interdendritic regions are greatly affected. The electron probe microanalyses revealed that W segregates to dendrite core regions, while Mo, Cr, Nb and C to the interdendritic regions. The microsegregation of elements becomes more severe with the W/Mo ratio increasing. The primary precipitates in the interdendritic regions were identified as MC/γ eutectic phases in all alloys, and the MC carbide lamellae in the eutectic phases can be categorized into two types, namely thin (Nb, Mo)C ones in the three Mo-contained alloys and thick NbC ones in the Mo-free alloy. The sizes of MC/γ eutectic phases change with the W/Mo ratio in the same manner as the solidification temperature region, and their number densities gradually increase with the W/Mo ratio increasing. Based on the thermodynamic calculations and solidification theory, the influencing mechanism of W/Mo ratio on the microstructures of as-cast Nb-modified Hastelloy N alloys was briefly discussed. •W segregates to dendrite cores, while Mo, Cr, Nb and C to the interdendritic regions.•The primary precipitates were identified as MC/γ eutectic phase.•Size of MC/γ eutectic phase becomes larger with W/Mo ratio increasing.•MC carbide lamellae transform from (Nb, Mo)C to NbC type with W/Mo ratio increasing.