Effect of Zr on solidification and microstructure of a Ni-based superalloy with high Al and Ti contents

The total content of Al and Ti in advanced Ni-based wrought superalloys is up to 7.5wt.%, which makes it easier to form harmful nonequilibrium eutectic (gamma+gamma ') and eta phase. It has been reported that the addition of certain amount of Zr can modify precipitation of the nonequilibrium phases obviously, but the mechanism is still controversial. The effect of Zr ranging from <0.0006wt.% to 0.150wt.% on solidification behavior, segregation and microstructure of a Ni-based superalloy with high Al and Ti contents was investigated, eliminating the interferences of C and B. Results show that increase in Zr content significantly promotes the formation of eutectic (gamma+gamma '), eta and Zr-rich phase in the interdendritic region. Besides the Zr-rich phase, Zr dissolves slightly in the eutectic gamma ' and obviously in the eta phase. An interesting phenomenon is discovered that the Zr addition significantly increases the area fraction of liquid pools and enlarges the forming range of gamma dendrites, which suggests that Zr markedly retards the solidification. Zr affects the eutectic (gamma+gamma ') and eta formation mainly due to the retard of solidification and dissolution of Zr in them. The retard of solidification obviously increases the residual liquid fraction and undercooling. Zr can serve as a forming element for the eutectic (gamma+gamma ') and eta phase, and the obvious dissolution of Zr in eta phase significantly decreases the critical concentration of Ti for its precipitation.