Cold dwell fatigue of titanium
Keywords: time dependent plasticity
stress-relaxation experiment
synchrotron diffraction
titanium
crystal plasticity
dwell fatigue

There is a long-standing technological problem in which a stress dwell during cyclic loading at room temperature in Ti causes a drastic fatigue life reduction. It is thought that localised time dependent plasticity in ‘soft’ grains oriented for easy plastic slip leads to load shedding and an increase in stress within a neighbouring ‘hard’ grain that is poorly oriented for easy slip. Quantifying this time dependent plasticity process is key to successfully predicting the complex cold dwell fatigue problem. Incorporating the influence of operating temperatures and common alloying elements on cold dwell fatigue will be beneficial for future alloy design to address this problem.
Firstly, stress relaxation tests were performed at four different temperatures on three major alloy systems: commercially pure titanium (two alloys with different oxygen content), Ti-6Al-4V (two microstructures with differing beta phase fractions) and Ti-6Al-2Sn-4Zr-xMo (two alloys with different Mo content x=2 or 6 and portion of beta phase). Key parameters controlling the time dependent plasticity were determined as a function of temperature from the macroscopic stress relaxation results. It was found that the dwell fatigue effect is more significant by oxygen alloying but is suppressed by the addition of Mo. The presence of the beta phase did not strongly affect the dwell fatigue, however, it was suppressed at high temperature due to the low strain rate and strain rate sensitivity.
To understand the mechanism in greater detail, synchrotron X-ray diffraction technique during stress relaxation experiments was utilised to characterise the time dependent plastic behaviour of two commercially pure titanium samples (grade 1 and grade 4) with different oxygen content at 4 different temperatures (room temperature, 75C, 145C and 250C). Such experiments enable direct assessment of lattice strains from grain families with common grain orientations from polycrystal sample. By monitoring the lattice strain response, it is possible to infer the behaviour of different slip systems. Lattice strains were measured by tracking the diffraction rings radii changes from multiple plane families (21 diffraction rings) as a function of their orientation with respect to the loading direction. The critical resolved shear stress, activation energy and activation volume were established for both prismatic and basal slip modes by fitting a crystal plasticity finite element model to the lattice strain relaxation responses measured along the loading axis for five strong reflections.
It was found that cold dwell fatigue highly depends on crystallographic orientation, where accumulation of plastic strain during a stress relaxation period was found to be higher in ‘soft’ grains over ‘hard’ grains, which results from the higher strain rate sensitivity of these ‘soft’ grains. The prism slip parameters correspond to a stronger strain rate sensitivity compared to basal slip. This slip system dependence on strain rate has a significant effect on stress redistribution to ‘hard’ grain orientations during cold dwell fatigue.
Oxygen was found to enhance the cold dwell effect, as plastic accumulation and strain rate sensitivity were both found to be higher in the higher oxygen content CP-Ti grade 4 samples. Among the four temperature assessed, 75C was found to be the worst-case scenario, where the macroscopic plastic strain accumulation was significant during a relaxation cycle due to the greatest activity of both prism and basal slip systems.
Digital image correlation (DIC) and crystal plasticity simulation were utilised to study cold dwell behaviour in a coarse grain Ti-6Al alloy at 3 different temperatures up to 230C. Strains extracted from large volume grains were measured during creep by DIC and were used to calibrate the crystal plasticity model. Stress along paths across the boundaries of two grain pairs, (1) a ‘rogue’ grain pair and (2) a ‘non-rogue’ grain pair, were determined at different temperatures. Load shedding was observed in the ‘rogue’ grain pair, where a stress increment during the creep period was found in the ‘hard’ grain. At elevated temperatures, 120C was found to be the worst-case scenario as the stress difference at the grain boundaries of these two grain pairs were found to be the largest among the three temperatures. Local stress state plays a more important role in the activation of slip systems compared to grain orientations with respect to macroscopic load direction in large polycrystals. As a result, the ‘soft’ and the ‘hard’ grain would dynamically vary depending on the applied stress and operating temperature.