Solidification characteristics and microstructures evolution of Ni-based superalloy K424 with different solidification cooling rates

The solidification behavior and microstructures evolution of cast Ni-based superalloy K424 under different cooling rates were investigated by isothermal solidification quenching experiments and differential scanning calorimetry (DSC). The microstructures and segregation characteristics of K424 were analyzed at different isothermal temperatures and cooling rates using optical microscopy, scanning electron microscopy, and energy-dispersive spectrometry (EDS). The isothermal solidification, microstructure, and distribution characteristics of elements at the final solidification stage were also investigated, and the liquidus, solidus, and the formation temperatures of the main phases were evaluated. Furthermore, the influence of cooling rate on the morphology and size of MC carbides, (γ +γ') eutectic, and precipitated γ' phase were discussed. The results show that the solidification of the K424 alloy follows the sequence:(1) L → L + γ at 1345℃:the solidification begins with the formation of primary γ, and the liquidus temperature is 1345℃; (2) L → L + γ + MC at 1308℃:as the solidification continues, Ti and Nb are enriched in the liquid interdendrite, which results in the formation of MC carbides at 1308℃; (3) L → γ + (γ + γ') at 1260℃:the precipitation of the (γ + γ') eutectic occurs at about 1260℃ because of non-equilibrium solidification. Finally, the solidification ends with a solidus temperature of 1237℃. Furthermore, it is found that the precipitation of (γ + γ') eutectic at the interdendtitic regions is closely related to the cooling rate and the severe segregation behavior of Al and Ti into the residual liquid at the final solidification stage. With the increase of cooling rate, the quantity and size of MC carbides and eutectic first increase and then decrease. Moreover, with increasing cooling rate, γ' shapes transform from irregular petal-like structure to near cuboidal and spherical patterns, and the size scale of γ' precipitates decreases from 2 μm to 60 nm.