Microstructural modification and mechanical characterization for a laser-induced composite coating during thermal exposure

In this work, a composite coating prepared by laser cladding on a Ni-17Mo-7Cr based superalloy was thermally exposed at 1073 K for 240 h. A detailed electron microscopy examination was performed to characterize the microstructure of the aged coating. Results indicate that: the original Mo6Ni6C dendrites in the aged coating were coarsened while the coarsening layers were essentially the Mo-deficient and Cr-rich Mo6Ni6C, due to low diffusivity of Mo. Long-term thermal exposure promoted a great amount of nano-sized Mo6Ni6C and micro-sized Cr3C2 particles being precipitated in the interdendritic zones. In the aged coating, three types of Cr3C2 were presented: discrete Cr3C2, Cr3C2 close to the graphite and Cr3C2 approaching to the Mo6Ni6C. The transformation of Mo6Ni6C to Cr3C2 merely occurred in the newly formed Mo6Ni6C rather than in the original Mo6Ni6C dendrites, because deficiency of Mo and enrichment of Cr in the new Mo6Ni6C depressed their stability. Lastly, the mechanical behavior of the aged coating was characterized by Vickers hardness, nano-indentation and nano scratch tests with the aim to unveil the effect of microstructural evolution on mechanical properties. Our findings provide basic information to understand the aged-induced microstructural modification and its effect on mechanical properties for a laser-induced coating prepared on the Ni-17Mo-7Cr based superalloy. The results obtained cannot only apply to the investigated Ni-based superalloys, but also to other Hastelloy series superalloys.