Low cycle fatigue behavior of nickel-based superalloy GH4145/SQ at elevated temperature

Abstract

Low-cycle fatigue (LCF) behavior of nickel-based precipitation-strengthened superalloy GH4145/SQ was investigated systematically under fully reversed total strain amplitude control conditions at 538degreesC in laboratory air. It was shown that the alloy exhibited a pronounced initial fatigue hardening followed by continuous fatigue softening to failure at high strain amplitudes, while at low strain amplitudes the initial hardening was followed by a well-defined saturation stage. Microstructure observations using optical and transmission-electron microscopy revealed that slip band density increased with increasing total strain amplitudes and the size of the y' precipitates tended to reduction due to repeated shearing by the dislocations with continuous cyclic straining. The above changes in microstructures during cycling were essentially responsible for the fatigue hardening-softening behavior of the alloy. Both optical and SEM micrographs indicated that at low strain amplitudes crack propagation was basically transgranular, while at high strain amplitudes crack propagation was essentially intergranular. Bilinear behavior with a change of slope at a plastic strain amplitude of about 0.2 pct was observed in cyclic stress-strain (CSS), Coffin-Manson (C-M) and plastic strain energy-life plots. A change in the number of operating slip systems and the fracture modes with the strain amplitude employed was used to explain this two-slope characteristic. In both the companion specimens test (CST) and the incremental step test (IST) methods the alloy exhibited non-masing type behavior. It was suggested that, for the first approximation, the half-life plastic strain energy could be calculated directly according to the equation without reckoning in the non-masing behavior. (C) 2004 Elsevier B.V. All rights reserved.