Implications of carbon content for the processing, stability, and mechanical properties of cast and wrought Ni-based superalloy Nimonic 105

As the temperature and pressure requirements in land-based turbines for power generation increase, the shift towards more advanced alloys, e.g., Ni-based superalloys, requires improved understanding of their long-term stability and mechanical properties. Additionally, the larger size of the components also presents fabrication and cost-reduction problems. In this study, we investigated Nimonic 105 as a potential rotor material at two carbon content levels – the commonly used maximum of the alloy specification and a lower carbon content variant. Reducing the carbon content presented fabrication challenges, resulting in surface crack formation during hot working. Additionally, a lower M6C carbide fraction was present after fabrication/solutioning, leading to a significantly larger grain size. After the standard aging treatment, similar γ′ populations formed in the two variants, however the M6C carbides in the high-carbon variant already started transforming to intergranular M23C6. Although we did not observe a noticeable effect on the tensile properties of the two variants, the lower carbon content alloy possessed a superior creep property – mainly due to the larger grain size as the microstructural stability was inferior. Upon prolonged thermal exposure, the M6C carbides in the low-carbon variant further decomposed to intergranular and intragranular σ and μ precipitates, as opposed to mainly M23C6 as in the high-carbon variant.