Tracking pores during solidification of a Ni-based superalloy using 4D synchrotron microtomography

Time-resolved in situ microtomography is employed to track the nucleation and growth of individual pores during solidification of a commercial nickel-based superalloy. Three cooling rates (up to 1°C/s) are investigated to evaluate the effect of this key processing parameter on the formation of porosity. Phase contrast obtained with a coherent X-ray beam is used to visualize the evolution of dendritic structures in absence of a sufficient absorption contrast. Two mechanisms leading to shrinkage pores have been identified. The first mechanism (mechanism A) is associated with the coalescence of secondary dendrite arms at temperature during the early stages of solidification. The second mechanism (mechanism B) is related to insufficient liquid feeding in the interdendritic region during the last stages of solidification, at lower temperatures. A variation of cooling rate by a factor 2 does not affect the nucleation rate of pores generated through mechanism B. However, it has a significant impact on the nucleation rate of small pores obtained through the mechanism A. The kinetics of growth for the majority of individual pores can be described using an exponential-like function. This kinetics is faster for mechanism B compared to mechanism A.