Design of an Eta-Phase Precipitation-Hardenable Nickel-Based Alloy with the Potential for Improved Creep Strength Above
- PDF / 1,961,129 Bytes
- 9 Pages / 593.972 x 792 pts Page_size
- 29 Downloads / 255 Views
RODUCTION AND BACKGROUND
PRECIPITATION strengthened nickel-based alloys are being evaluated for high-temperature applications in Advanced Ultrasupercritical (A-USC) coal-fired steam power plants.[1] Operation of A-USC power plants with steam temperatures of 973 K to 1033 K (700 C to 760 C) will decrease effluents emissions (including CO2) by about 20 pct compared to the US baseline efficiency.[1] To achieve these operating temperatures, the nickel-based alloys under consideration rely on gamma-prime (c¢-Ni3Al, L12 structure) precipitation to achieve hightemperature creep resistance. However, the amount of gamma prime must be balanced by the needs to process, fabricate, and weld large boiler components. Inconel 740 and Nimonic 263, with a maximum of 20 pct gamma prime, have been identified as having the requisite strength and fabrication behavior for these applications.[3,4] The maximum operating temperature of these alloys is limited to 1023 K (750 C), as higher temperatures coarsen and dissolve the c¢-strengthening phase leading to reductions in creep strength.[4,5] Given the temperature limitation of c¢-strengthened alloys, one strategy for expanding the operating range of nickel-based alloys is to identify strengthening phases MATTHEW J. WONG, PAUL G. SANDERS, Professor, and CALVIN L. WHITE, Professor Emeritus, are with the Department of Materials Science & Engineering, Michigan University, 1400 Townsend Dr., Houghton, M&M Building 512, Houghton, MI 49931. Contact e-mail: [email protected] JOHN P. SHINGLEDECKER, Program Manager, is with the Electric Power Research Institute, Charlotte, NC. Manuscript submitted January 29, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
that are stable to higher temperatures. One phase that is thermodynamically stable at these temperatures is the hexagonal Ni3Ti g-phase (D024 structure). Although little is known about the role of g-phase in these alloys, it is often described as a phase that forms at the expense of the c¢-strengthening phase.[5,6] A study by Evans et al., however, indicated that the g-phase may aid in pinning grain boundaries during creep.[5]
II.
APPROACH
To study the effectiveness of the eta-phase as a hightemperature-strengthening phase in nickel-based alloys, this study used Nimonic 263 as a baseline. In Nimonic 263, the more thermodynamically stable g-phase grows at the expense of c¢ at temperatures above 1023 K (750 C).[6] The g-phase is known to form in Ni-, Ni-Fe-, and Ni-Co-based alloys having a Ti:Al ratios above 3:1, and has been observed to precipitate in both cellular and Widmansta¨tten morphologies.[6] The Widmansta¨tten plates form from c¢ precipitates at grain boundaries, consuming the c¢ as they grow toward the grain centers. A cellular morphology of g-phase has also been observed in Ni-Fe alloys when exposed to temperatures in the 873 K to 1123 K (600 C to 850 C) range for extended aging times.[7,8] These two morphologies have also been observed at the same time in Cu-Ti alloys, where growth kinetics determine which microstructural form will be a
Data Loading...
