Effect of fluid flow and hafnium content on macrosegregation in the directional solidification of nickel base superalloys

Nickel base superalloys of the MAR-M200 type with additions of hafnium from 0 to 2.6 pct were directionally solidified in investment molds with an abrupt change in cross-section. Segregation patterns of several alloying elements (Hf, W, Ti, and Co) were determined and related to partition ratios, hafnium content, and casting geometry. The segregation is more pronounced near the section change, and hafnium substantially increases the segregation tendency of these alloying elements. Variation in eutectic volume percent was related to the observed macrosegregation. A model based on the flow of interdendritic liquid to feed solidification shrinkage was Actapted to describe the macrosegregation behavior of these complex superalloys. The model accurately predicts the effect of casting geometry on macrosegregation.