Microstructure and phases structure in nickel-based superalloy IN713C after solidification

Highlights: • Microstructure of IN713C solidified at varying cooling rates were fully characterised. • The term "eutectic cluster" including large γ′ and Borides/Ni₇Zr₂ were defined. • Composition and morphology of Carbides, Borides and Ni₇Zr₂ were revealed. • Borides and Ni₇Zr₂ precipitated either in "weaved" or neighbourhood structure. • Combined effects of γ′ size and grain orientation determined hardness values. The effects of cooling rate on microstructure and microtexture development during solidification of IN713C nickel-based superalloy have been studied with a range of cooling rate from 0.3 °C/s to 4 °C/s (air cooled). The results showed an increase in grain size (from 140 μm to 2 mm) and carbides volume fraction (1.3% to 2%) with increasing cooling rate. It was also found that primary γ′ size decreased with increasing cooling rate. Furthermore, it was demonstrated that the dendrite structure was weakened with decreasing cooling rate. The dendrite structure was nearly eliminated at slowest cooling rate applied here (0.3 °C/s). Eutectic clusters that mainly consisted of large γ′, Ni₇Zr₂, M₃B₂ type boride and/or MC-type carbide precipitates were identified in air cooled samples. These undesirable clusters presented specific morphologies and their density (volume fraction) was found to decrease with decreasing cooling rate. Moreover, predominant phase separation of γ′ is observed in the slowest cooling sample and in interdendritic areas in the fastest cooling sample. Hardness test results showed a slight decrease in hardness with decreasing cooling rate. This may result from the combined effects of detrimental effect of larger γ′ and beneficial effect of smaller grain size in slowest cooling sample. The observations demonstrated here can contribute greatly to the modification of standard investment casting parameters that applied during IN713C turbine wheels production for turbochargers.