User:Braydenbekker/Superalloy

History and Development of Co-based superalloys
Historically, Co-based superalloys have depended on carbide precipitation and solid solution strengthening for mechanical properties. While these strengthening mechanisms are inferior to gamma prime (γ') precipitation strengthening, cobalt has a higher melting point than currently ubiquitous nickel-based superalloys and has superior hot corrosion resistance and thermal fatigue. As a result, carbide-strengthened Co-based superalloys are used in lower stress, higher temperature applications such as stationary vanes in gas turbines.

However, recent research has shown that cobalt can exhibit the γ' phase. Actually, the first reported existence of γ' occurred in a 1971 PhD dissertation, but was never published. The γ/γ' microstructure was rediscovered and first published in 2006 by Sato et al. That γ' phase was Co3(Al, W). It was furthermore found that Mo, Ti, Nb, V, and Ta partition to the γ' phase, while Fe, Mn, and Cr partition to the matrix γ.

The next family of Co-based superalloys was discovered in 2015 by Makineni et al. This family has a similar γ/γ' microstructure, but is tungsten-free and has a γ' phase of Co3(Al,Mo,Nb). Since tungsten is a very heavy element, the elimination of tungsten makes Co-based alloys increasingly viable in turbines for aircraft, where low density is especially important.

A recently discovered family of superalloys was computationally predicted in a high throughput study by Nyshadham et al. in 2017, and demonstrated in the lab by Reyes Tirado et al. in 2018. This γ' phase is again tungsten free and has the composition Co3(Nb,V) and Co3(Ta,V). Reyes Tirado et al. showed the predicted γ' phase to be metastable, decomposing into C36-Co3(Ta,V) and DO19-Co3(Nb,V) after aging at 900°C. In 2019 Reyes Tirado et al. Presented experimental evidence for Al and Ti as an alloying element to stabilize the γ' phase in the metastable Co3(Ta,V).