Effect of Test Method on Stress-Relaxation Behavior of Alloy 718
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DURING final processing, precipitation-hardenable metallic alloys are typically solution heat treated, cooled rapidly via oil or water quenching, and then aged. The quenching of large components can give rise to spatially non-uniform cooling/small plastic strains which induce residual stresses. Subsequent aging may mitigate the level of residual stress concurrent with an increase in yield strength associated with precipitation per se. Because of the complexity of the thermomechanical phenomena during heat treatment, computer simulations can provide useful insight into the evolution of residual stress. Needless to say, such approaches require detailed descriptions of constitutive behavior, often under transient thermal and microstructural conditions. Nickel-base superalloys are among those materials for which accurate numerical simulations of residual-stress formation would be very useful. This is because of the high cost of these materials, the need to control/limit residual-stress-induced distortion during final machining,
S.L. SEMIATIN is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/RXCM, Wright-Patterson Air Force Base, OH 45433. Contact e-mail: [email protected] P.N. FAGIN is with UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432. R.L. GOETZ and V. VENKATESH are with Pratt & Whitney, East Hartford, CT 06108. M.G. GLAVICIC is with Rolls-Royce Corporation, Indianapolis, IN 46206. Manuscript submitted October 17, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
and the possibly-beneficial effect of bulk residual stress on service properties, provided their spatial disposition and magnitude can be predicted and controlled. Despite compelling economic drivers, only a modest amount of research to simulate residual stress evolution during heat treatment of nickel-base superalloys has been reported in the open literature.[1–5] The early work of Franchet et al.[1] focused on residual-stress evolution during oil quenching of the powder-metallurgy (c-c¢) superalloy Astroloy. Their constitutive formulation comprised visco-plastic behavior at high temperatures and elastic–plastic behavior at low temperatures. Later work by Dye et al.[2] and Rist et al.[3] for wrought alloy 718 examined the evolution of residual stress during water/oil quenching and air cooling. These researchers used constitutive formulations consisting of a temperature-dependent yield strength or a rate-independent elastic–plastic model, respectively. In a review article by Ma et al.[4] two techniques for obtaining constitutive inputs for computer simulation of residual stress were cited: on-cooling tension (or compression) tests and stress-relaxation tests. The former method, used to obtain data to simulate plastic flow during cooling following solution heat treatment, comprises preheating samples at the solution temperature, cooling at a prescribed rate to one of a series of test temperatures, holding for a short period of time for temperature equilibration, and deformation at a desired strain rat
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