Grain boundary niobium carbides in Inconel 718
- Lehigh Univ., Bethlehem, PA (United States). Dept. of Mechanical Engineering and Mechanics
A coordinated, multidisciplinary investigation was conducted to determine the mechanisms and rate controlling processes for environmentally assisted crack growth under sustained (static) loading in Inconel 718 at elevated temperatures. The results showed that oxygen had a significant influence on crack growth, increasing the crack growth rate, for example, by nearly four orders of magnitude at 973 K. Based on results from the companion surface chemical and metallurgical studies, it was suggested that the mechanism for crack growth enhancement by oxygen was the formation and fracture of a brittle niobium oxide (Nb{sub 2}O{sub 5}-type) film on the grain boundary surfaces. The Nb came principally from the oxidation and decomposition of NbC (or Nb-rich carbides) at the grain boundaries, and crack growth was controlled by the rate of oxidation and decomposition of these carbides. Because there was extensive oxidation of the primary NbC, these carbides were implicitly considered to be the primary source of Nb. Although these carbides were large and contained substantial amounts of Nb, they were too few and spaced too far apart to be of concern (averaging one NbC for two grain boundary facets and spaced about 25 {micro}m apart). To wit, whether the freed Nb could diffuse over such a large distance and be oxidized to support the postulated mechanism for crack growth? The presence of other Nb-rich carbides elsewhere on the grain boundaries, or other sources for Nb, therefore, needs to be re-examined and is the focus of this study. In this paper, the results of a study of grain boundary niobium carbides and their distribution in Inconel 718 are reported, and are discussed in terms of their contribution to crack growth.
- Sponsoring Organization:
- National Science Foundation, Washington, DC (United States)
- OSTI ID:
- 586023
- Journal Information:
- Scripta Materialia, Journal Name: Scripta Materialia Journal Issue: 12 Vol. 37; ISSN 1359-6462; ISSN SCMAF7
- Country of Publication:
- United States
- Language:
- English
Similar Records
Environmentally enhanced crack growth in nickel-based alloys at elevated temperatures
Surface enrichment and grain boundary segregation of niobium in Inconel 718 single- and poly-crystals