Freckles in remelted niobium containing superalloys

Abstract

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)/Energy Dispersive Spectrometry (EDS) and Differential Thermal Analysis (DTA). The compositions and the transformation temperatures of the freckled areas were 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 alloys. Quantitative calculation results of the alloys using the criterion are presented and discussed in terms of the experimental measurements.