On the nature of freckles in nickel base superalloys

Abstract

Freckles are chains of equiaxed grains that have previously been observed in macroetched ingots of both iron and nickel base alloys formed by the consumable arc process. These defects have been observed in nickel base superalloys in both directionally solidified polycrystalline and single crystal ingots. Such ingots exhibit a relatively periodic distribution of vertical freckle trails around their circumference which are particularly suitable for study. This paper investigates the nature of “Freckles,” examines the conditions under which they occur and specifies a set of consistent observations which any successful model for freckling must explain. Visual, chemical, electron probe, metallographic, and X-ray diffraction data are presented which establish the appearance, composition and orientation differences between freckled and normal regions in single crystals of the nickel base superalloys Udimet 700 and Mar-M200. It is shown that the freckle lines are linear assemblies of small random equiaxed grains which are enriched in all but the inversely segregated solute species. Excessive interdendritic porosity and feeding shrinkage are observed in the vicinity of a freckle line. Some correlations of the tendency toward freckling and the geometric distribution of freckle lines are made with growth rate, thermal gradient, and interface shape. The dependence of freckling tendency on composition is examined in the Ni-Al system.