Grain boundary precipitation behavior in Re-containing nickel-based directionally solidified superalloys with carbon and boron additions

The grain boundary precipitation behaviors in Re-containing nickel-based directionally solidified (DS) superalloys with carbon and boron additions wereinvestigated using high-resolution scanning electronic microscopy and transmission electron microscopy. The results indicate that the grain boundary becomes more tortuous as the carbon and boron contents increase. When the carbon content increases from 0 to 0.130 wt%, the average size and area fraction of the intergranular and intragranular precipitations increase. Similarly, as the boron content increases from 0 to 0.014 wt%, the average size and area fraction of the intragranular precipitations also increase; however, the average size and area fraction of the intergranular precipitations decrease. Furthermore, it quantitatively estimates that the precipitations preferentially occurred on the grain boundaries at low carbon (<0.040 wt%) and boron (<0.005 wt%) contents; the increases in the carbon (≥0.085 wt%) and boron (≥0.010 wt%) contents promote the transformation from the intergranular to the intragranular precipitation. During heat treatment, the carbides are mainly of the MC and M₂₃C₆ types, whereas no borides is identified. It is also confirmed that the M₂₃C₆ carbides contain Re, whereas no Re is detected in the MC carbides.