Reduced Dwell-Fatigue Resistance in a Ni-Base Superalloy After Short-Term Thermal Exposure
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ne of the major challenges for high-temperature structural materials is the environmentally induced grain boundary embrittlement.[1,2] The focus of the present paper is the coupling of microstructure and dwellfatigue resistance in Allvac 718Plus (hereafter 718Plus). Previous results have shown that 718Plus is indeed sensitive to dwell-time fatigue,[3–6] although less than the main contender, Alloy 718. The effect of long-term thermal exposure on the dwell-fatigue crack growth is not as well studied, although it has been shown that 718Plus actually acquires improved dwell-fatigue resistance during exposure.[4–6] This was attributed to the loss of creep resistance with increasing size of the hardening precipitates, resulting in a more rapid relaxation of the crack tip stresses.[4] The exposure studies of 718Plus have all been carried out in an accelerated manner, where aging temperatures of 1003 K to 1033 K (730 C to 760 C), well above the typical upper usage limit, and exposure times of 100 to 1000 hours have been used.[4–6] While it was stated in Reference 6 that the accelerated aging has been verified against experiments at 977 K (704 C), the underlying data has so far not been disclosed (to the best of the authors’ knowledge) and the range of validity for the verification in terms of aging times was not included. Contrary to this, it has MAGNUS HO¨RNQVIST, Research Associate, is with the Department of Applied Physics, Chalmers University of Technology, Gothenburg, Sweden, and also with the R&T Centre, GKN Aerospace Engine Systems, Trollha¨ttan, Sweden. Contact e-mail: [email protected] LEIF VISKARI, formerly Doctoral Student with the Department of Applied Physics, Chalmers University of Technology, is now Development Engineer with AB SKF, Gothenburg, Sweden. Manuscript submitted December 18, 2013. Article published online April 8, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
been shown that there can actually be a reduction in the dwell-fatigue resistance in 718Plus during the early stages of exposure.[3] The present study aims to further investigate the effect of short-term thermal exposure on the dwell-fatigue crack growth in 718Plus, and relate the behavior to the evolution of the microstructure. The investigated material, 718Plus (nominal composition in wt pct: Ni-18Cr-10Fe-9.0Co-5.4Nb-2.8Mo1.5Al-1.0W-0.7Ti-0.02C-0.004B-0.006P), was obtained from forged rings with grain size ASTM 5. The material was subjected to pre-solution soaking followed by solution treatment and conventional two-step aging (condition A). A number of specimens from each material were subjected to a second heat treatment cycle consisting of 140 hours at 973 K (700 C) and 460 hours at 948 K (675 C) (condition E). This cycle was selected to mimic the thermal history of an aero engine structural component during service. The selection of times and temperatures were based on a larger test program regarding the effects of thermal exposure on the mechanical properties of 718Plus of which the present study was a part. To compare with
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