Effect of Test Method on Stress-Relaxation Behavior of Alloy 718
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The stress-relaxation behavior of alloy 718 was established at 991 K (718 °C) using two contrasting test protocols. For this purpose, compression and tension samples extracted from two subscale forgings were first solution treated and cooled to room temperature. Each sample was then reheated to 991 K (718 °C), compressed or pulled isothermally to a total strain between 0.006 and 0.02, and then allowed to relax after locking either (i) the extensometer attached to the dies (compression tests) or reduced section (tension tests) or (ii) the ram itself. For the tests in compression, the stress dropped in a monotonic fashion under locked-extensometer conditions, while the locked-ram mode gave rise to a short stress drop, a broad hardening stage, and finally softening at long times. For the tension stress-relaxation tests, monotonic stress drops whose magnitudes varied for the two test modes were observed. The various behaviors were ascribed to the specific level of plastic strain imposed during the different types of tests, its quantitative effect on the level of dynamic strain aging, and thus the increment in hardening which counterbalanced stress-relaxation per se.
Portions of this research were conducted in support of the AFRL Materials and Manufacturing Directorate’s Foundational Engineering Problem (FEP) on bulk residual-stress development in nickel-base superalloys performed under the auspices of the Metals Affordability Initiative, Contract No. FA8650-13-2-5201. The support and encouragement of the FEP Program Managers (B. Song, T.J. Turner, and M.J. Caton) are gratefully acknowledged. The authors also thank Drs. C.S. Lee, G.A. Sargent, J. Cormier, and J.M. Cabrera for insightful technical discussions regarding stress-relaxation testing/concurrent phase transformations and N.C. Levkulich for performing the metallography. One of the authors (R.L. Goetz) was with Rolls-Royce Corporation, Indianapolis, IN during the early portion of the work.
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