Modeling thermomechanical fatigue life of high-temperature titanium alloy IMI 834 |
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Authors: | H J Maier R G Teteruk H -J Christ |
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Affiliation: | (1) the Lehrstuhl fur Werstoffkunde (Materials Science), Universitat Paderborn, D-33098 Paderborn, Federal Republic of Germany;(2) the Institut fur Werkstofftechnik (Materials Technology), Universitat Siegen, D-57068 Siegen, Federal Republic of Germany |
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Abstract: | A microcrack propagation model was developed to predict thermomechanical fatigue (TMF) life of high-temperature titanium alloy
IMI 834 from isothermal data. Pure fatigue damage, which is assumed to evolve independent of time, is correlated using the
cyclic J integral. For test temperatures exceeding about 600 °C, oxygen-induced embrittlement of the material ahead of the advancing
crack tip is the dominating environmental effect. To model the contribution of this damage mechanism to fatigue crack growth,
extensive use of metallographic measurements was made. Comparisons between stress-free annealed samples and fatigued specimens
revealed that oxygen uptake is strongly enhanced by cyclic plastic straining. In fatigue tests with a temperature below about
500 °C, the contribution of oxidation was found to be negligible, and the detrimental environmental effect was attributed
to the reaction of water vapor with freshly exposed material at the crack tip. Both environmental degradation mechanisms contributed
to damage evolution only in out-of-phase TMF tests, and thus, this loading mode is most detrimental. Electron microscopy revealed
that cyclic stress-strain response and crack initiation mechanisms are affected by the change from planar dislocation slip
to a more wavy type as test temperature is increased. The predictive capabilities of the model are shown to result from the
close correlation with the microstructural observations. |
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