Microstructure Evolution Of K424 Superalloy During Near-Equilibrium And Sub-Rapid Solidification Processes

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