Mechanistic basis of temperature-dependent dwell fatigue in titanium alloys

Abstract

The temperature-dependent dwell sensitivity of Ti-6242 and Ti-6246 alloys has been assessed over a temperature range from −50∘C to 390 °C using discrete dislocation plasticity which incorporates both thermal activation of dislocation escape from obstacles and slip transfer across grain boundaries. The worst-case load shedding in Ti-6242 alloy is found to be at or close to 120 °C under dwell fatigue loading, which diminishes and vanishes at temperatures lower than −50∘C or higher than 230 °C. Load shedding behaviour is predicted to occur in alloy Ti-6246 also but over a range of higher temperatures which are outside those relevant to in-service conditions. The key controlling dislocation mechanism with respect to load shedding in titanium alloys, and its temperature sensitivity, is shown to be the time constant associated with the thermal activation of dislocation escape from obstacles, with respect to the stress dwell time. The mechanistic basis of load shedding and dwell sensitivity in dwell fatigue loading is presented and discussed in the context of experimental observations

Micromechanical approaches to understand dwell fatigue: from titanium a-b microstructures to disc thermal alleviation

“…The yellow zone represents the strain rate sensitive temperatures, and if the basal stress developed is located in this region, even if its current magnitude is lower than the critical stress, it should be treated as a threat because the basal stresses are ratcheting up (due to cold creep in the adjacent soft grain during dwell) to exceed the critical stress of 1200MPa with further loading cycles. The material behaviour relevant to the yellow zone has been studied in some detail [22] in which thermally activated dislocation escape from obstacles during stress dwell is shown via discrete dislocation analyses to lead to dislocation pile-up at the hard-soft grain boundaries, leading to a gradual cycle by cycle increase in the hard grain basal stress, supporting the CP studies reported here. The green zone in Figure 8 indicates the safe zone for which the loading is not anticipated to give rise to dwell fatigue.…”

“…Superimposed on the map are the thermo-mechanical loadings associated with the first cycle only of each of four loading scenarios considered: the three rig tests (A, B and C) and the inservice (labelled Redline) conditions. The criteria defining the regions are that a basal (hard grain) stress greater than 1200MPa is taken to be a necessary condition for facet nucleation (determined from rig spin tests [3]), that a temperature in excess of 200°C inhibits facet nucleation [20,22], and that soft grain slip is also a prerequisite for facet nucleation (which defines the elastic region shown). However, the diagram…”

“…This is important in demonstrating that it is not the chemistry (or more particularly the molybdenum) which gives rise intrinsically to the differing dwell behaviours of alloys Ti-6242 and Ti-6246. Zheng et al [22] using discrete dislocation plasticity (DDP) investigated the temperature sensitivity of dwell fatigue in Ti-6242 and Ti-6246 alloys assuming homogenised α-HCP polycrystal representations in both cases. Similar behaviour of temperaturedependent load shedding as for Ti-6Al alloy was found for Ti-6242, and alloy Ti-6246 was found to show dwell sensitivity at higher temperatures of 300 o C. When the α-β basketweave microstructure is fully accounted for in the latter material, it is anticipated that the alloy would not be expected to show any dwell sensitivity.…”

The rupture life prediction in cold dwell fatigue of Ti-6Al-4V based on the creep deformation

“…Titanium alloys are used for a wide variety of members and parts because they have high specific strength as well as high corrosion resistance. However, it has been found that the fatigue strength of Ti alloys decreases due to cold dwell fatigue (CDF) at room temperature, and there have been reports on cases of damage due to decrease fatigue life [1][2][3][4][5][6][7][8][9][10][11][12]. Generally, most of metals are deformed by creep phenomenon at higher temperature (T/Tm > 0.3, where Tm is melting point).…”

Effect of Rise and Fall Time on Dwell Fatigue Behavior of a High Strength Titanium Alloy

“…The dwell fatigue in titanium alloys has drawn great attention [1][2][3][4][5][6] since it was detected in Rolls-Royce RB211 engines on Lockheed Tristar aircraft in the early 1970s [7,8]. It has been shown that the dwell period at the peak stress significantly reduces the fatigue life of titanium alloys in comparison with the fatigue life under the conventional fatigue loading and that the crack initiation region has the characteristic of cleavage or quasi-cleavage facets [9][10][11].…”