Abstract: Extensive smooth-bar low-cycle-fatigue (LCF) and fatigue crack growth (FCG) experiments on three solid-solution-strengthened superalloys HASTELLOY X, HAYNES 230, and HAYNES 188 have been conducted at 816 and 927 C. Limited tests were run at 649 C, 871 C, and 982 C to study the temperature effect. The LCF tests were performed under a total-strain-range-control mode at Solar Turbines Inc., Metcut Research Inc., and The University of Tennessee (UT). The FCG tests were done under both the constant-load-range and constant-stress-intensity-factor-range modes at Westmoreland Testing Inc. and UT. Various hold times were imposed at the maximum strain or load in both the LCF and FCG tests, respectively, to investigate the hold-time effect. For the LCF tests, the influence of the total strain range and hold time on the cyclic stress response was determined at temperatures ranging from 816 to 982 C.
At the temperatures considered, the HASTELLOY X alloy exhibited initial cyclic hardening, followed by a saturated cyclic-stress response or cyclic softening under LCF without hold times. For LCF tests with hold times, however, the alloy showed cyclic hardening, cyclic stability, or cyclic softening, which is closely related to the test temperature and the duration of the hold time. It was also observed that the LCF life of the X alloy considerably decreased due to the introduction of strain hold times. Generally, a longer hold time would result in a greater reduction in the fatigue life. For the tests without hold times, the test temperature seems to have little influence on the fatigue life of the X alloy at the test temperatures from 816 to 927 C. However, when the test temperature increased to 982 C, the fatigue life clearly shortened. The effects of heat-to-heat variation on the cyclic stress response were illustrated. A parameter based on the hysteresis energy was proposed to rationalize the LCF life data with and without hold times. In general, the fatigue life of HAYNES 230 alloy decreased as the temperature increased. However, at total strain ranges higher than 1.0% and without a hold time, the LCF life was longer at 927 C than at 816C. This “abnormal” behavior was found to result from the smaller plastic strain amplitude at half-life at 927 C than that at 816 C. The introduction of a hold time led to a decrease in the fatigue life. At both 816 and 927 C, the material exhibited a cyclic hardening/softening behavior at higher total strain ranges and a cyclic hardening/saturation behavior at lower total strain ranges. An increase in the temperature and/or the introduction of a hold time decreased the hardening rate and increased the softening rate. The introduction of a hold time and/or the increase of the test temperature progressively changed the fracture mode from the transgranular to mixed trans/inter-granular, then to intergranular feature. Within the two phases of the fatigue process, crack initiation was more severely influenced by the change of the hold time and/or temperature.
For the LCF of HAYNES 188 alloy, in the…