Environmentally assisted, sustained-load crack growth in powder metallurgy nickel-based superalloys
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DUCTION
NICKEL-BASED
superalloys, such as INCONEL*
*INCONEL is a trademark of INCO Alloys International, Huntington, WV.
718, which are age hardened by the precipitation of fine gamma prime (␥ ⬘) and gamma double-prime (␥ ⬙) phases in an austenite matrix, are widely used in high-temperature aerospace and other applications. Their sensitivity to environmentally enhanced crack growth (EECG) at operating temperatures from 773 to 1033 K has received considerable attention.[1–4] It has been shown that the environmentally enhanced crack growth rates (EECGRs) in oxygen can be more than four orders of magnitude faster than those in vacuum or inert environments at 973 K.[4] By defining the ratio of crack growth rates in the deterious and inert environments as the material’s environmental cracking sensitivity (ECS), the ECS for a series of alloys in air/oxygen was found by Gao et al.[4] to range from about 1 to over 104. Their findings showed that the alloys exhibited differing degrees of sensitivity to the environment, even though the mechanical properties of some are similar. To improve their performance and to better predict their service lives, it is essential to develop a better understanding of the processes of crack growth in these alloys at high temperatures. In their study of INCONEL 718, Gao et al.[4] suggested that niobium (Nb) played an important role in the enhancement of crack growth by oxygen. They further suggested the formation and rupture of a brittle niobium oxide film along grain-boundary surfaces ahead of the crack tip as a principal mechanism for the observed crack-growth enhancement. These oxides were believed to form from the ZHIFAN HUANG, Project Engineer, is with Eastman Kodak, Rochester, NY 14650. CARL IWASHITA, Electromigration Engineer R&D, is with LSI Logic, Milpitas, CA 95035-7451. IRENE CHOU, Senior Process Engineer, is with Applied Materials, Santa Clara, CA 95054. ROBERT P. WEI, Professor, is with the Department of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA 18015-3085. Contact e-mail: [email protected] Manuscript submitted June 12, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
oxidation and decomposition of niobium-rich carbides at the boundaries; the presence, composition, and structure of the carbides at the grain boundaries have been established.[5] The surface enrichment of Nb after heating a single crystal of INCONEL 718, with preadsorbed oxygen, to 975 K for 1 hour in ultrahigh vacuum (Figure 1) and the evolution of carbon monoxide from such a surface, provided support for the suggested mechanism.[4] The possible contribution from the oxidation of ␥ ⬙ (Ni3Nb) precipitates at the grain boundaries was not examined, but is now affirmed by recent surface-chemistry studies.[6] Data on other nickel-based superalloys in the literature appear to be consistent with the suggested influence of niobium, with niobium-containing alloys (such as RENE* 95 and X-750) showing an ECS of *RENE is a trademark of General Electric Company, Fairfield, CT.
several orders of mag
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