The influence of crystallographic orientation and strain rate on the high-temperature low-cyclic fatigue property of a n
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INTRODUCTION
MUCH work has been carried out to investigate the influence of orientation and strain rate on the mechanical property of nickel-base single-crystal superalloys. In particular, the anomalous yielding behavior, tension/compression asymmetry, and orientation dependence have been observed and investigated widely under monotonic and cyclic loading.[1] Lall, Chin, and Pope (LCP)[2] modified the model proposed by Takeuchi and Kuramote[3] and predicted satisfactorily the unusual phenomenon in the flow stress of L12 intermetallic compounds. The LCP model can also give a good prediction of the nickel-base single-crystal superalloys.[4,5] The influence of strain rate on the flow stress has been studied by Milligan and Antolovich on [001] oriented single crystals of the nickel-base superalloy PWA1480. A model was developed.[6] Fatigue resistance is another important property.[7,8,9] For high cyclic fatigue (HCF), because the deformation is in the elastic region, the dependence of HCF behavior on orientation is due primarily to Young’s modulus of elasticity. It is not enough to consider only the difference in Young’s modulus for low-cyclic fatigue (LCF), because cyclic inelastic deformation is a major factor in governing LCF life. The purpose of this investigation was to obtain an improved understanding of the influence of crystallographic orientation and cyclic strain rate on LCF behavior at 950 7C.
Z.F. YUE, Professor of Engineering Mechanics, Department of Applied Mechanics, and Z.Z. LU, Associate Professor of Aircraft Engineering, Department of Aircraft Engineering, are with Northwestern Polytechnical University, Xian, 710072, People’s Republic of China. Manuscript submitted July 8, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
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MATERIALS AND EXPERIMENTAL PROCEDURES
The DD3 single crystals used in this study are oriented in [001], [012], [112], [011], and [114] directions to within 5 deg. The chemical composition in weight percent is given in Table I. Standard bar specimens with 10.0-mm diameter and 10.0-mm gage length shown in Figure 1 were tested at 950 7C in a Shimadzu servohydraulic testing machine (type ED-100KN-20L). The machine has a capacity of 5100 kN and can be controlled in load, position, or strain. A ‘‘PC-486’’ microcomputer was used to generate a triangular waveform command signal. Loads were measured by a load cell in the machine crosshead. Strain was measured using a high-temperature axial extensometer. All tests were carried out under fully reversed total strain control, i.e., R 5 εmin/εmax 5 21, with a constant cyclic strain rate. Throughout the tests, a continuous record was made of the load-time and strain-time history to monitor the cyclic hardening/softening response. Hysteresis loops were also recorded regularly throughout the tests using an X-Y recorder. Specimen heating was by an induction 10 kW RF induction heater. The coil diameter and spacing were found by experiment to give temperature gradients within 53 7C over the specimen gage length. Temperature was measured using a
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