The Effect of Secondary Gamma-Prime on the Primary Creep Behavior of Single-Crystal Nickel-Base Superalloys
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AS the high-temperature (HT) creep behavior of single-crystal Ni-base superalloys has been improved dramatically through increased alloy content, including the increasing use of refractory elements such as rhenium, Re, an interesting behavior developed in some second-generation superalloys during creep at intermediate temperatures (650 C through 850 C) and high loads (greater than 500 MPa).[1–4] Under these conditions, some second-generation alloys, such as PWA 1484 and CMSX-4, have been shown to exhibit nonuniform deformation producing large primary creep strains in only a few hours.[4–6] After this initial surge in elongation, the creep rate drops by as much as two orders of magnitude during the onset of secondary or steady-state creep. Primary creep behavior has been attributed to many different deformation mechanisms and models. Some of the most common models rely on specimen orientation,[1,2,4] the presence of secondary c¢,[4,7] lattice misfit,[8,9] and alloy composition.[4] Mechanistically, large primary creep strains have been linked to the cooperative shearing of c¢ precipitates by ah112i dislocation ribbons.[2] These dislocation ribbons are thought to B.C. WILSON, Graduate Student, and G.E. FUCHS, Associate Professor, are with the Materials Science and Engineering Department, University of Florida, Gainesville, FL 32611. Contact e-mail: [email protected]fl.edu This article is based on a presentation given in the symposium ‘‘Neutron and X-Ray Studies of Advanced Materials,’’ which occurred February 15–19, 2009, during the TMS Annual Meeting in San Francisco, CA, under the auspices of TMS, TMS Structural Materials Division, TMS/ASM Mechanical Behavior of Materials Committee, TMS: Advanced Characterization, Testing, and Simulation Committee, and TMS: Titanium Committee. Article published online September 22, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
originate from a=2h011i dislocations in the c matrix. When sufficient stress causes a matrix dislocation to enter a c¢ particle, the dislocation dissociates into two h112i-type partial dislocations with a stacking fault (SF) in between. As the SF pair moves through the precipitate, it leaves behind an antiphase boundary (APB), due to the ordered nature of the c¢. Thus, a second matrix dislocation must enter and likewise dissociate into partial dislocation in the c¢ precipitate, to lower the energy caused by the APB. Full discussions of this mechanism can be found in Rae et al.,[1] Link and Feller-Kniepmeier,[9] and Knowles and Chen.[10] While the common deformation mechanisms linked to primary creep have been identified, the factors that determine whether it happens in a given alloy are still not entirely clear. Orientation sensitivity has been shown to significantly impact primary creep. This is expected, because primary creep has been linked to slip on a limited number of slip systems. Depending on the orientation of the sample, if fewer slip systems activated due to the sample orientations, less work hardening will be experienced and, subsequently, more primary
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