Modeling thermomechanical fatigue life of high-temperature titanium alloy IMI 834

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 ЊC, 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 stressfree 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 ЊC, 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.

Modeling of Environmental Degradation in Fatigue‐Life Prediction of Near‐α Titanium Alloy IMI 834 under Complex High‐Temperature Loading Conditions

“…Moreover, extensive SEM studies [16] revealed that the fracture surfaces formed during in-phase TMF loading applying cycles with T max ³ 600 were very similar to those obtained in isothermal tests conducted at T = T max . Therefore, oxidation can be assumed to be the only environmental contribution to fatiguecrack growth under in-phase loading conditions.…”

“…[16] Equation 1 can be integrated from an initial crack size a 0 to the final crack length a f in order to calculate the fatigue life. Note that the result of this life prediction is not very sensitive to the value used for a f .…”

“…For this purpose microhardness data obtained on stress-free annealed samples as well as on cyclically deformed ones were compared and mathematically described. As shown in detail earlier, [16] the following expression was deduced as an approach to characterize the oxygeninduced environmental effect on FCGR…”

“…[18] Expression 3 takes into account that plastic deformation accelerates the uptake of oxygen and that a parabolic rate law connects the depth of the embrittled zone in front of the crack tip with the cycle period. Furthermore, the losing significance of environmental contributions with increasing crack growth rate is considered via the proportionality of (da/dN) ox and the reciprocal square root of N. [16] Assuming that environmentally driven crack growth rate (da/dN) env is governed solely by oxygen-induced effects, (da/dN) env = (da/dN) ox , the use of Equations 2,3 should yield the life prediction for tests conducted in dry air, where exclusively pure fatigue and oxidation damage are expected. ) into account.…”

Linking Machining Response of Titanium Billet to Upstream Thermomechanical Processing

“…The authors have recently shown in currently used commercial titanium alloys that conventional and emerging machining practices impart substantial subsurface microstructural damage, such as mechanical twinning [3], [4]. Surface and subsurface twins in structure critical components have the potential to reduce service life [5]- [8]. The objective of this paper is to begin to identify and determine the critical conditions required to generate them.…”