A Study on the Precipitation of Carbides in Ti-Modified Ni–Mo–Cr-based Alloy During Thermal Ageing at Elevated Temperature

Ni–Mo–Cr–Ti alloys are one of the proposed structural materials for molten salt breeder reactors. In the current work, the evolution of precipitates in the alloy at various stages of thermo-mechanical processing followed by thermal ageing at 750 °C is studied. The microstructure after thermal ageing for various durations shows the presence of ‘MC’-type carbides with different morphologies and compositions. Formation of carbide networks with bulky titanium-rich ‘MC’ carbides of irregular morphology and spherical ‘MC’ around it is observed in the early stages, while fine uniform dispersion of ‘MC’-type needle-like carbides that possess a cube-on-cube orientation relationship with the FCC matrix is observed in the latter stages. High-resolution transmission electron micrograph reveals the presence of misfit dislocations on the semi-coherent interface between the fine needle-like MC carbides and the matrix which is attributed to the lattice parameter mismatch between them.