Effect of solidification parameters on the microstructures of superalloy CMSX-6 formed during the downward directional solidification process

The single crystal Ni-base superalloy CMSX-6 was cast by using the downward directional solidification process (DWDS) using withdrawal rates of between 0.0013 and 0.0217 cm/s. The evolutions of as-cast microstructures were characterized as functions of the withdrawal rate. The primary and secondary dendrite arm spacings, λ1 and λ2, decreased with increasing withdrawal rate, which is similar to the experimental results obtained in the conventional Bridgman process. However, the value of λ1 and λ2 measured in the present work is much smaller than that in the Bridgman process. In addition to this, the value of λ1 cannot be reasonably described by the theoretical models for the primary dendrite arm spacing in which the convection effect was not taken into account. In comparison, the theoretical model of Bouchard and Kirkaldy which considers the convection factor can predict the λ1 value well in the present work if the dendrite-calibrating factor (a1) is assumed to be 13.5. The sizes of the γ′ phase in the dendrite and interdendritic regions were also reduced with an increased withdrawal rate. The shape of the γ′ phase was cuboidal in the dendritic regions at all experimental withdrawal rates. This contrasts with the γ′ phase in the dendrite cores which became more rounded at the highest withdrawal rates employed in the present work, due to the low supersaturation and insufficient growth time. With an increased withdrawal rate, significant reduction in the size of the γ/γ′ eutectic island was observed in the samples. Meanwhile, the microsegregation of the alloying elements was reduced and the volume fraction of the γ/γ′ eutectic initially decreased and then increased. The difference in the shape of the γ/γ′ eutectic was also found in those samples processed at low withdrawal rates as well as at high withdrawal rates.