Role of carbon in modifying solidification and microstructure of a Ni-based superalloy with high Al and Ti contents

Abstract

The effect of carbon ranging from 0.014 to 0.071 wt.% on the solidification and microstructure of a Ni-based superalloy with high Al and Ti contents was studied. The results show that the increase in carbon addition significantly increases the size and volume fraction of MC carbides and promotes the change of their morphology from blocky to elongated shape. However, the carbon addition obviously decreases the size and volume fraction of eutectic (γ + γ′) and reduces η phase and borides formation. The change in carbide characteristics is mainly because of the increasing carbide-forming element and carbides precipitation temperature with the increase in carbon which favors the growth of them along the interdendritic liquid film. MC carbides are formed at an earlier solidification stage than the eutectic (γ + γ′). The increased carbide formation consumes more Ti, which delays and reduces the eutectic (γ + γ′) precipitation. The delay of eutectic (γ + γ′) precipitation leads to a deeper undercooling, which significantly decreases the critical Ti concentration for its precipitation. This, in turn, lowers Ti/Al ratio in residual liquids ahead of the eutectic (γ + γ′) and hence reduces η formation subsequently. B and Zr are slightly enriched in the carbides, which are considered during discussing how carbon influences the eutectic (γ + γ′) precipitation.