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NICKEL-BASE superalloys are used in gas turbines because of their superior elevated-temperature mechanical properties. By using threshold data and fatigue-crackpropagation (FCP) curves to predict fatigue lives, failures in turbine engines under low-cycle fatigue (LCF) conditions have been minimized in recent years. Because of this success, the single largest cause of failure in military aircraft gas turbines is now high-cycle fatigue (HCF).[1] The reason that this remains a pervasive problem may be related to the highly empirical approach, based on Goodman diagrams, which is currently employed to estimate HCF capability. A fracture-mechanics, threshold-based HCF approach would be, therefore, highly desirable. Mechanisms of crack arrest associated with FCP thresholds are very complex, and there is a large discrepancy between results predicted from theoretical threshold models and experimentally determined values.[2] These models deal exclusively with either intrinsic A. SHYAM, Graduate Student, and W.W. MILLIGAN, Professor, are with the Department of Materials Science and Engineering, Michigan Technological University, Houghton, MI 49931. Contact e-mail: milligan@ mtu.edu S.A. PADULA II, formerly Graduate Student, Department of Materials Science and Engineering, Michigan Technological University, is Research Scientist with NASA–Glenn Research Center, Cleveland, OH 44135. S.I. MARRAS, formerly Graduate Student, Department of Mechanical Engineering and Engineering Mechanics, Michigan Technological University, is serving in the Greek military. Manuscript submitted March 6, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A

or extrinsic factors affecting the threshold, without taking into account their synergistic interactions. To address these problems, data under engine HCF conditions (very high frequencies in the Kilohertz range and high R values) is necessary. In this work, elevated-temperature long-crack FCP tests were performed at high frequencies (up to 1 kHz) in a Ni-base turbine disk alloy under a variety of experimental conditions. Four important factors affecting threshold (temperature, frequency, microstructure, and load ratio) were systematically varied in order to understand the relative importance of individual factors and the level of interaction between them. Similar work at room temperature for the same alloy has been reported earlier.[3] II. MATERIALS A polycrystalline Ni-base superalloy designated KM4[4] was studied in this program. It was developed by General Electric (GE) Aircraft Engines specifically to resist creepfatigue crack propagation and is similar to the commercial alloys Rene95 and IN100 in its microstructure and basic mechanical properties.[5] This disk alloy was produced by a process route involving the consolidation of atomized powders by hot extrusion, followed by forging. The nominal composition of the alloy is given in Table I, and the details of its microstructure and deformation characteristics have been reported elsewhere.[5] The specimens studied in this work were obtained from a