New knowledge about 'White Spots' in superalloys

In April 1991, the first in a series of workshops was held to discuss ways in which the gas turbine industry could better understand defects in nickel-base superalloys. The group's primary objective was to better define, and expand knowledge about, segregation in superalloys such as Alloy 718 and Waspaloy,with emphasis on light-etching areas referred to as solute-lean defects or 'white spots'. This 'White Spots Committee' formed four subcommittees to focus efforts on classification, inspection, mechanisms, and mechanical properties. Completion of the tasks that these subcommittees have undertaken should greatly improve the gas turbine industry's understanding of the physical and mechanical nature of white spots. The primary purpose of this article is to formalize the characterization and classification of white spots in high-strength superalloys so that the metallurgical community can begin to use a common vocabulary when referring to them. An overview of formation mechanisms is presented along with a brief description of detection methods. Also discussed are preliminary test results, which should help shed light on the effects of solute-lean microstructures on tensile and fatigue properties. Although white spots are not limited to any single superalloy or class of superalloy, Alloy 718 is emphasized because it is so widely used,more » and because its relatively large solidus-liquidus temperature interval ([approximately]75 C, 135 F) and high niobium content ([approximately]5.3% Nb) make it prone to segregation. Three distinct types of white spots have been identified and named by the committee: discrete, dendritic, and solidification white spots.« less