Effect of W/Mo ratio on the microstructure of Ni-Mo-W-Cr-Nb based superalloys after thermal exposure

The structural materials used in molten salt reactors (MSRs) require enough high-temperature strength, good chemical compatibility with fuels and irradiation damage resistance. The Nb addition and W alloying are two effective optimization paths, which have been proven to respectively improve the embrittlement problems and high-temperature strength of alloys. It is worth exploring whether one full-featured alloy can be developed for the high-temperature MSRs by combining the benefits of Nb addition and W alloying. In this work, four Ni-Mo-W-Cr-Nb based superalloys with different W/Mo ratios were designed by substituting W for Mo in the standard Nb-modified alloy. The microstructural evolution and mechanical properties of these alloys were evaluated systematically after thermal exposure at 750 degrees C. As the W/Mo ratio increased, more undissolved primary MC carbides remained, while fewer secondary carbides formed in the matrix and at the grain boundaries after thermal exposure. Micro-hardness and tensile tests showed that the sub-stitution of Mo by W can provide an obvious strengthening effect for alloys in the solid-solution treated state. After thermal exposure, all alloys exhibited precipitation-induced strengthening, but the strength-ening degree decreased with the W/Mo ratio due to the fewer secondary MC carbides. Based on experi-mental observations and thermodynamic calculations, it was found that W atoms cannot easily occupy the lattice sites of MC carbides as Mo atoms do, and the absence of secondary carbides in the high-W alloys is due to the undissolved primary MC carbides and the low Mo concentration.(c) 2023 Elsevier B.V. All rights reserved.

Automated summary

The development of structural materials for molten salt reactors (MSRs) is crucial for their performance. This study investigates the effects of Nb addition and W alloying on the microstructural evolution and mechanical properties of Ni-Mo-W-Cr-Nb based superalloys. Results show that the substitution of Mo by W enhances the alloys' strength, but reduces the precipitation-induced strengthening due to fewer secondary carbides. The absence of secondary carbides in high-W alloys is attributed to the presence of undissolved primary MC carbides and low Mo concentration.