P.F. Browning

Elevated-Temperature Creep-Fatigue Crack-Growth Behavior of HAYNES®188 Superalloy

The creep-fatigue crack-growth behavior of HAYNES® 188, a cobalt-based superalloy, was studied at the temperatures of 649, 816, and 927 oC under isothermal conditions. Various hold times at the maximum load were introduced to study the effects of hold time and temperature on the crack-growth behavior. The experiments were conducted under constant stress-intensity-factorrange control modes. Crack lengths were measured by a direct current potential method. The introduction of hold times led to an increase in the cyclic crack-growth rate. As the temperature increases, the time-dependent crack-growth behavior was dominant.

Low-Cycle Fatigue of Nickel-Based Superalloy Hastelloy X at Elevated Temperatures

The fully-reversed total strain-controlled low-cycle fatigue tests with and without hold times, as well as stress-relaxation tests, were conducted at 816°C and 927°C in laboratory air on a nickel-based superalloy, HASTELLOY X. The influence of temperatures and hold times on low-cycle fatigue behavior of the alloy was investigated. At both temperatures of 816°C and 927°C, the alloy exhibited initial cyclic hardening, followed by a saturated cyclic stress response or cyclic softening under low-cycle fatigue without hold times. For low-cycle fatigue tests with hold times, however, the alloy showed either cyclic hardening or cyclic stability, which is closely related to the test temperature and the duration of the hold time. It was also observed that the low-cycle fatigue life of the 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. However, for the tests without hold times, the test temperature seems to have little influence on the fatigue life of the alloy at the test temperatures used in this investigation. The stress relaxation tests show that at the beginning of strain hold, the stress drops very quickly and then decreases very slowly with prolonging time. In addition, the fracture surfaces of the fatigued specimens were observed using scanning electron microscopy to determine the crack initiation and propagation modes. The fatigue life was predicted by the frequency modified tensile hysteresis energy method. The predicted lives were found to be in good agreement with the experiment results.Copyright