Low temperature carbide precipitation in a nickel base superalloy
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I.
INTRODUCTION
THEpresence of carbide phases has long been recognized as an important strengthening mechanism in nickel base superalloys. Carbides which are typical of the commercial nickel base superalloy family include the MC, M23C6, and M6C forms, l'z Historically, carbide strengthening has been associated with the M23C6 and M6C carbides. When these intergranular carbides are present as discrete particles, they are reported to "pin" grain boundaries and thereby inhibit grain boundary sliding at elevated temperatures. 3'4'5 Conversely, when grain boundary carbides are present in continuous morphologies they provide easy paths for crack propagation and thereby degrade high temperature mechanical properties. Examples of detrimental carbide morphologies are common in the literature, 6 and are usually related to alloy chemistry and/or heat treatment. The formation of the M23C6 and MaC carbide is reported to be accompanied by the decomposition of the MC carbide. 7'8'9 Evidence of this reaction includes reports that the MC carbide distribution decreases when the M23C 6 carbide forms and the observation of ,/' and M23C6 precipitation around MC carbides. This reaction may be written as follows: MC + y
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