Abstract
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.
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