Freckle defects can be found in several niobium-containing nickel base superalloys. The freckles in VAR or ESR ingots of 718. 706, 625 and other model alloys were analyzed using Scanning Electron Microscopy (SEM) 1 Energy Dispersive Spectrometry (EDS) and Differential Thermal Analysis (DTA). The compositions and the transformation temperatures of the freckled areas \\ere determined It is found that for most of the alloys the composition and melting temperature of the freckle body usually correspond to a solidification moment when about 70% liquid has transformed into solid. though in some alloys the correspondent volume fraction was found to be higher. Freckle criterion considering the compositional effect is proposed for the evaluation of the freckle potential of allojs. Quantitative calculation results of the alloys using the criterion are presented and discussed in terms of the experimental measurements.

A macrosegregation defect, commonly termed freckle, has limited the maximum diameter of remelted ingots of some Nbstrengthened nickel base superalloys. Two aspects of freckle formation were studied in three commercially important superalloys 718, 706 and 625: 1) the segregation characteristics which provide the driving force for freckle formation; 2) the solid development in the mushy zone which provides resistance to the flow inducing the defect. A Rayleigh number form freckle criterion that considers the effects of composition is derived. Freckle tendency of the alloys was evaluated using the experimental results.