Effect of minor niobium addition on microstructure of a nickel-base directionally solidified superalloy

Scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe microanalysis (EPMA) and differential scanning calorimetry (DSC) were used to study the effect of minor niobium addition on as-cast microstructure of a nickel-base directionally solidified superalloy. A proper niobium addition resulted in the formation of homogeneous γ′ particles almost with the same size throughout the whole dendrite. The ternary eutectic reaction L → (M23C6 + γ + γ′) occurred with increasing niobium content, but no eutectic was found when niobium content was high enough because the NbC preferably precipitated and the eutectic reaction was suppressed.