High-cycle fatigue of nickel-based superalloy ME3 at ambient and elevated temperatures: Role of grain-boundary engineeri
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NICKEL-BASED superalloys are widely used in turbines for both aerospace and land-based power-generation applications, due to their exceptional elevated-temperature strength, high resistance to creep, oxidation, and corrosion, and good fracture toughness. However, a critical property of these alloys is their resistance to fatigue-crack propagation, particularly at service temperatures. In engine applications, there are often two components to this problem: (1) low-cycle fatigue, which results from relatively large cycles associated with the stopping and starting of the turbine, and (2) highcycle fatigue (HCF), associated with vibrational loading during service. The HCF, in particular, has been recognized as the single largest cause of engine failures in military aircraft.[1] It results in rapid, and often unpredictable, failures due to the propagation of fatigue cracks in blade and disk components under high-frequency loading, where the cracking initiates from small defects, in many instances resulting from fretting or foreign-object damage.[2] Due to the high vibrational frequencies involved, even cracks growing at slow per-cycle velocities can propagate to failure in short time periods, possibly within a single flight segment. Consequently, HCF-critical turbine-engine components must be operated below the fatigue-crack initiation or growth thresholds, such that cracking cannot occur within 109 cycles. To address this problem, significant research efforts have been directed in recent years to developing HCF design and life-prediction methodologies for titanium- and nickel-based alloys; these studies have resulted in an extensive database YONG GAO, Graduate Student, and R.O. RITCHIE, Professor, are with the Department of Materials Science and Engineering, University of California, Berkeley, CA 96720. Contact e-mail: [email protected] MUKUL KUMAR, Member of the Scientific Staff, is with the Lawrence Livermore National Laboratory, Livermore, CA 94550. R.K. NALLA, Postdoctoral Research Fellow, is with the Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720. Manuscript submitted April 7, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
on HCF,[1,3,4] which has been exclusively directed to typical blade and disk microstructures.[5–8] However, the question as to whether these microstructures can be optimized to promote HCF resistance has rarely been addressed. One approach to enhancing microstructural resistance to fracture has been through the notion of grain-boundary engineering, where the “character” of the grain boundaries is changed by thermomechanical treatment.[9] Specifically, the grain-boundary character distribution (GBCD) is controlled principally by promoting a high proportion of so-called “special” grain boundaries. These boundaries are characterized by a particular misorientation and high degree of atomic matching; they are described geometrically by a low “sigma number” (1 29), which is defined in terms of the coincident-site lattice (CSL) model[10] as the reciproc
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