"Thermal Alleviation Effects on the Dwell Fatigue Life of Ti-6Al-4V" 

Joel Davis, MSE PhD Candidate 

Advisor: Professor Michael Sangid

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ABSTRACT

Cold dwell fatigue is a failure mechanism present in titanium alloys that stems from creep at low temperatures in grains well oriented for crystallographic slip. Dwell fatigue is a temperature dependent phenomenon and at elevated temperatures no reduction in fatigue life is observed. Material exposed to elevated temperatures in this range for a portion of its loading cycle do not exhibit a reduction in fatigue life, due to a thermal alleviation (TA) process that has not been previously studied in the open literature. In this research project, the effect of TA on the dwell fatigue properties of Ti-6Al-4V (Ti-64) will be investigated. In this work, fatigue testing was performed on material that exhibited large microtextured regions and a correspondingly large dwell debit. Each fatigue specimen was exposed to a single mid-life TA event which increased the fatigue life of the specimen when compared to specimens without such treatment. TA was found to temporarily reduce the rate of plastic strain accumulation in the test specimens during the dwell periods. A phenomenological model was developed to characterize the change in the rate of plastic strain accumulation due to TA as a function of the applied stress as well as the temperature and duration of the TA event. The model parameter representing the critical temperature necessary for TA was fit to the empirical data and found to agree with the maximum temperature for dwell fatigue proposed in literature for Ti-64. The number of cycles for reduced plastic strain accumulation, representing the TA effect, was found to decrease as applied stress increased and as temperature decreased. There was no observed difference in subsequent dwell behavior between the TA durations examined and it is theorized that the dislocation recovery process activated by TA operates on a short time scale. Measurement of intragranular misorientation using EBSD, as a proxy for geometrically necessary dislocation content, demonstrated a reduction in dislocation density for temperatures corresponding to large reductions in the rate of plastic strain accumulation. At temperatures near the critical TA temperature a change in intragranular misorientation was not detected with TA. It is therefore likely that the mechanistic basis for TA is due to an evolution of the dislocation structure in response to time at elevated temperature. With the model developed in this thesis work, it is now possible to predict the degree and frequency of TA events necessary to mitigate dwell fatigue susceptibility in Ti-64.