Effect of oxidation kinetics on the near threshold fatigue crack growth behavior of a nickel base superalloy
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INTRODUCTION
HIGH temperatures
can greatly accelerate effects of corrosive environments on fatigue crack growth behavior of metals. Environmental effects depend on the rate at which reactants can be transported to the crack tip and on the creation of new crack surfaces by advancement of the crack. They can either accelerate or decelerate fatigue crack growth. ~,2 Accelerating and decelerating mechanisms may be viewed as competitive mechanisms that can act concurrently. Hydrogen embrittlement is one corrosion fatigue mechanism that can accelerate the intermediate and near threshold fatigue crack growth rates in high strength steels 3'4 and aluminum alloys. 2'3'5 In this mechanism, hydrogen, which is produced by a cathodic reaction, by absorption of H2, or by reaction with water, enters the lattice and erabrittles the metal ahead of the crack tip. On the other hand, decelerating mechanisms may involve corrosion products accumulating in the crack resulting in enhanced crack closure (oxide-induced crack closure),6,7 fracture surface roughness enhancing crack closure (roughness-induced crack closure), 8'9 crack tip blunting, ~~ or crack branching. 12'~3 Recently, there has been interest in understanding the near threshold fatigue crack growth behavior of a wide range of metallic alloys. 1-6'14-~8Limited near threshold data exist for Alloy 718, a y" strengthened nickel base superalloy, at elevated temperatures and in an air environment. Because corrosion rates at elevated temperatures are greatly enhanced, they are expected to have a significant influence on the decelerating fatigue crack growth mechanisms. The purpose of this study is to determine the influence of temperature on the near threshold fatigue crack growth behavior of Alloy 718. In particular, the environmental effects are discussed in terms of their effect on the oxide-induced crack closure mechanism. Roughness-induced crack closure will also be considered. J.L. YUEN, formerly with General Electric Company, AdvancedNuclear TechnologyOperation, Sunnyvale,CA 94086, is now with Rockwell International, RocketdyneDivision, Canoga Park, CA 91304. P. ROY is with General Electric Company,AdvancedNuclearTechnologyOperation, Sunnyvale,CA 94086. W~D. NIX is with StanfordUniversity, Department of Materials Science and Engineering, Stanford, CA 94305. Manuscript submitted May 16, 1983. METALLURGICALTRANSACTIONSA
MATERIAL
The Alloy 718 was received as a plate 19 mm thick in the solution annealed condition. The chemical composition is listed in Table I. Compact tension specimens 12,7 mm thick were fabricated from the plate in the T-L orientation. The specimens were solution annealed at 954 ~ for one hour and air cooled to room temperature. They were then precipitation hardened by aging at 718 ~ for eight hours, furnace cooled to 621 ~ and held at 621 ~ for a total aging time of eighteen hours before cooling to room temperature. An average grain diameter of 22 /xm was obtained. Tensile properties at 24 ~ and 538 ~ are listed in Table II.
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EXPERIMENTAL PR
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