Microstructure Evolution of K424 Superalloy during Near-equilibrium and Sub-rapid Solidification Processes

Abstract:

Adopting differential scanning calorimetry and copper-mould spray-casting methods, the near-equilibrium and sub-rapid solidification of K424 superalloy were fabricated with a wide range of cooling rate between 0.17 K/s and 100 K/s. Using OM, SEM and EDS, the evolutions of primary γ phase, precipitated γ′ phase, MC carbides and eutectic microstructure were described as a function of cooling rate. It is shown that γ phase is refined with increasing cooling rate, where the secondary dendrite spacing decreases from 84 μm for 0.17 K/s to that of 5 μm for 100 K/s. Moreover, the average size of precipitated γ′ phase decreases because of the shortened time for transformation. Precipitation process occurs once again as for heating the formed supersaturated solid solution during sub-rapid solidification. Both the refinement of γ phase and solute trapping decrease the extent of solute segregation, which leads to the reduction of the sizes of γ+γ′ eutectic and carbides.

Key words:Nickel-base superalloy; Cooling rate; Grain refinement; Phase structure