Hydrogen and fatigue behaviour in a near alpha titanium alloy

It is believed that the near-α titanium alloy Ti-685 was the first to demonstrate dwell sensitive fatigue behaviour whilst under engineering service as a gas turbine fan disc material. New fatigue data, micro-textural characterisation and fractography from a recent study which utilised remnant Ti-685 specimens originally manufactured for previous laboratory campaigns based at Swansea dating back to the late 1980s are now reported. In isolation, the present results failed to demonstrate a significant dwell effect in either aligned or basketweave microstructural variants. On this occasion, the limited sensitivity to dwell loading was explained by the combination of heat treatment artifacts and selection of a relatively small test specimen geometry. However, the present data were also reviewed in the context of the wider fatigue database generated by multiple studies over the past fifty years. In combination, stress-life (SN) curves defined the magnitude in applied peak stress necessary to demonstrate a dwell effect in this alloy. Ultimately, this facilitated a fundamental definition of dwell sensitivity that is relevant to the engineering community.

In this paper, the creep-fatigue failure mechanism and damage behavior of a Ti-6Al-3Nb-2Zr-1Mo titanium alloy at room temperature were quantitatively investigated with a series of fatigue and creep-fatigue tests. Based on experimental data and continuum damage mechanics theory, a phenomenological creep-fatigue damage model applicable to Ti-6Al-3Nb-2Zr-1Mo titanium alloy at room temperature is proposed that can describe the stress dependence of the interaction between fatigue damage and creep damage. The proposed model can consider the mutual effects of fatigue damage and creep damage at room temperature. Remarkably, the previous damage models proposed based on creep-fatigue experiments at high temperatures cannot accurately characterize the creep-fatigue damage behavior of titanium alloys at room temperature. Creep damage accumulates at a fast rate and dominates the damage evolution in the initial stage, and fatigue damage predominates in the late stages. The fatigue life is significantly reduced by creep damage at room temperature. Moreover, fatigue and creep damage promote each other.

Cold dwell fatigue of titanium alloys used in aero-engine components has threatened flight safety for over five decades. This phenomenon was encountered in Al-containing titanium alloys, occurring at relatively low temperatures, where load hold at applied peak stress result in reduction in number of cycles by an order of magnitude or more (the so-called dwell debit). The present paper reviews the recent advances in dwell-fatigue of titanium alloys with the primary motivation being to understand the dwell-fatigue damage mechanism relating microstructural configurations, dislocations and local strain rate sensitivity. Numerous studies have focused on the identification of fatigue-critical microstructural configurations, such as microtextured regions, rogue grain pairs and (0001) twist grain boundaries. As an example of dwell resistant alloy design, Timetal 575 is a candidate alloy for aero-engine fan disc applications with both superior static and cyclic strengths and low dwell sensitivity.

Different components of deep-sea submersibles, such as the pressure hull, are usually subjected to intermittent loading, dwell loading, and unloading during service. Therefore, for the design and reliability assessment of structural parts under dwell fatigue loading, understanding the effects of intermittent loading time on dwell fatigue behavior of the alloys is essential. In this study, the effects of the intermittent loading time and stress ratio on dwell fatigue behavior of the titanium alloy Ti-6Al-4V ELI were investigated. Results suggest that the dwell fatigue failure modes of Ti-6Al-4V ELI can be classified into three types, i.e., fatigue failure mode, ductile failure mode, and mixed failure mode. The intermittent loading time does not affect the dwell fatigue behavior, whereas the stress ratio significantly affects the dwell fatigue life and dwell fatigue mechanism. The dwell fatigue life increases with an increase in the stress ratio for the same maximum stress, and specimens with a negative stress ratio tend to undergo ductile failure. The mechanism of dwell fatigue of titanium alloys is attribute to an increase in the plastic strain caused by the part of the dwell loading, thereby resulting in an increase in the actual stress of the specimens during the subsequent loading cycles and aiding the growth of the formed crack or damage, along with the local plastic strain or damage induced by the part of the fatigue load promoting the cumulative plastic strain during the dwell fatigue process. The interaction between dwell loading and fatigue loading accelerates specimen failure, in contrast to the case for individual creep or fatigue loading alone. The dwell fatigue life and cumulative maximum strain during the first loading cycle could be correlated by a linear relationship on the log–log scale. This relationship can be used to evaluate the dwell fatigue life of Ti alloys with the maximum stress dwell.

Cold dwell fatigue has become one critical issue in titanium industry over the past decades. Though some reports have emphasized the importance of time-dependent plastic strain on cold dwell effect in titanium alloys, the mathematical relationship between plastic strain and fatigue life has never been discussed because plastic strain changes continuously no matter in normal cyclic fatigue or dwell fatigue. In this paper, a series of fatigue tests under constant plastic strain is designed to investigate the mathematical relationship between plastic strain and fatigue life in titanium alloys at room temperature in air. Results show that plastic strain has a definitely detrimental effect on fatigue life in titanium alloys. Different from the traditional qualitative mechanisms, one semi-quantitative explanation for cold dwell effect in titanium alloys is proposed by introducing hysteresis energy density.

The prior studies have investigated the influence of internal hydrogen on dwell-fatigue behavior of near-α titanium alloys primarily in the lamellar microstructural condition. In the current study, the effects of internal hydrogen, in the range 10–230 ppm (by weight), on the dwell-fatigue behavior of Ti-6242Si alloy were investigated. The examined alloy had a bimodal microstructure comprising approximately 70 vol% primary α grains and 30 vol% transformed β regions. The dwell-fatigue life generally increased with increasing hydrogen content. The dwell-fatigue lives were longer by a factor of as high as 6 for high (≥150 ppm) hydrogen contents than for the low (<60 ppm) hydrogen contents, which is reported for a near-α titanium alloy for the first time. The crack-initiation site was essentially faceted for all the hydrogen levels examined in current study. The crystallographic orientations of fracture facets at the crack-initiation sites were similar for the low (<60 ppm) and high (>150 ppm) hydrogen contents. Specifically, these facets were inclined at ∼8 – 17° from the basal plane. Therefore, the longer dwell-fatigue lives observed for the alloys with hydrogen contents ≥150 ppm could not be explained on the basis of any differences in crystallography of the facets at crack-initiation sites. The longer dwell-fatigue lives for higher hydrogen contents can be explained within the framework of time-dependent load shedding from the soft microtextured regions (MTRs) to the hard MTRs if the local stress redistribution at the soft MTR/hard MTR boundary due to the hold at maximum load is reduced with increasing hydrogen content.