Cold Creep of Titanium: Analysis of Stress Relaxation Using Synchrotron Diffraction and Crystal Plasticity Simulations

It is well known that titanium and some titanium alloys creep at ambient temperature, resulting in a significant fatigue life reduction when a stress dwell is included in the fatigue cycle. 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 poorly oriented for easy slip. Quantifying this time dependent plasticity process is key to successfully predicting the complex cold dwell fatigue problem. In this work, synchrotron X-ray diffraction during stress relaxation experiments was used to characterise the time dependent plastic behaviour of commercially pure titanium (grade 4). Lattice strains were measured by tracking the diffraction peak shift from multiple plane families (21 diffraction rings) as a function of their orientation with respect to the loading direction. Critical resolved shear stress, activation energy and activation volume were established for both prismatic and basal slips modes by fitting a crystal plasticity finite element model to the lattice strain relaxation responses measured along the loading axis for three strong reflections. Prismatic slip was the easier mode having both a lower critical resolved shear stress (τc basal=252MPa and τcprism=154MPa) and activation energy (∆F basal=10.5×1020 J=0.65eV and ∆F prism=9.0×1020 J=0.56eV). The prism slip parameters correspond to stronger strain rate sensitivity compared to basal slip. This slip system dependence on strain rate will have a significant effect on stress redistribution to hard grain orientations during cold dwell fatigue.