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Cold creep of titanium: Analysis of stress relaxation using synchrotron diffraction and crystal plasticity simulations

DOI: 10.1016/j.actamat.2020.08.010 DOI Help

Authors: Yi Xiong (University of Oxford) , Phani Karamched (University of Oxford) , Chi-toan Nguyen (Safran SA) , David M. Collins (University of Birmingham) , Christopher M. Magazzeni (University of Oxford) , Edmund Tarleton (University of Oxford) , Angus J. Wilkinson (University of Oxford)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Acta Materialia

State: Published (Approved)
Published: August 2020
Diamond Proposal Number(s): 17222

Abstract: 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 that is 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 performed 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. 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 three strong reflections. Prismatic slip was the easier mode having both a lower critical resolved shear stress ( MPa and MPa) and activation energy ( and ). 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.

Journal Keywords: Dwell fatigue; Titanium; Synchrotron diffraction; Stress-relaxation experiment; Crystal plasticity

Subject Areas: Materials, Engineering

Instruments: I12-JEEP: Joint Engineering, Environmental and Processing

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