Effect of Temperature, Microstructure, and Stress State on the Low Cycle Fatigue Behavior of Waspaloy

Abstract

The low-cycle fatigue (LCF) behavior of Waspaloy was studied in uniaxial and torsional loading for two heat treatments at 24 and 649°C. It was shown that for both heat treatments deformation and failure mechanisms were independent of stress state at 24°C. Deformation occurred by precipitate shearing and the formation of intense shear bands for the coarse-grain/small-precipitate (CG-SP) condition and by precipitate looping and loosely defined shear bands for the fine-grain/large-precipitate (FG-LP) condition. Failure in both microstructures was associated with the formation of shear cracks. At 649°C deformation and failure mechanisms for the FG-LP condition were independent of stress state, and the mechanisms were similar to those observed at 24°C. For the CG-SP condition, the situation was different. Failure occurred on principal planes when tested in torsion and on shear planes when tested in uniaxial tension. The mechanism transition is interpreted in terms of deformation mode and microstructural instability. In general, there was a more pronounced decrease in life with increasing temperature for uniaxial specimens than for torsional specimens. This result is interpreted in terms of an environmental interaction that is accelerated by a comparatively significant dilatational strain component.

Cyclic deformation characteristics and fatigue crack growth behaviour of a newly developed aerospace superalloy Haynes 282

“…Furthermore, this relatively short duration of the initial cyclic hardening is followed by gradual cyclic softening (decrease in stress amplitude) over the subsequent life of the specimens. This phenomenon has been observed in the stress response behaviour of other precipitation strengthened superalloys [11][12][13][14][15][16]. The possible causes of the cyclic hardening have been reported to be due to: formation of cottrell-lomer locks, accumulation of forest dislocations, and increase in the density of slip bands at the beginning of cycling [13].…”

“…Precipitate shearing has been predominantly used to explain cyclic softening in ␥ or ␥ precipitation strengthened nickel-base superalloy systems [11][12][13]15,16,19]. Considering the extent of the strain involved during the test, it is most likely that the softening observed during the cyclic loading is due to precipitate shearing.…”

“…Considering the extent of the strain involved during the test, it is most likely that the softening observed during the cyclic loading is due to precipitate shearing. Moreover, it has been reported that precipitate shearing is responsible for cyclic softening in Waspaloy [12,19], a nickel-base superalloy that has microstructural features similar to that of Haynes 282.…”

Fatigue Crack Growth in Nickel‐based Superalloys at 500‐700°c. I: Waspaloy

“…In the present work, the effects of hold-time and cyclic frequency on high-temperature fatigue crack growth in Waspaloy have been studied with the aim of obtaining a better understanding of the relative importance of the above effects. The basic aspects of high-temperature deformation, fatigue, and creep in Waspaloy are fairly well established [2,8,9], and Waspaloy is therefore also a good "model" material to investigate the effects of variables such as hold-time, cyclic frequency and temperature. The effects of these variables on fatigue crack growth in Direct Aged 718, a nickel-based superalloy with a much finer grain size and more complex microstructure than Waspaloy, is described and discussed in Part I1 in the light of the results for Waspaloy.…”