An in-situ synchrotron diffraction study of stress relaxation in titanium


A schematic of the sample and process

A team of researchers from Professor Ed Tarleton's and Professor Angus Wilkinson's groups addressed the long-standing technical problem of stress dwell during cyclic loading at room temperature in Ti (which causes a drastic fatigue life reduction).  

They incorporated the influence of operating temperatures and common alloying elements on cold dwell fatigue to address the issue.  

In their work, published in Acta Materialia, the characterisation of the time dependent plastic behaviour of two commercially pure titanium samples (grade 1 and grade 4) with different oxygen content at four different temperatures (room temperature, 75oC, 145oC and 250oC) were performed during stress relaxation using synchrotron X-ray diffraction.

The key parametres, which govern the dislocation motion, were determined for the major prismatic and basal slip systems as a function of temperature and oxygen content, by calibrating a crystal plasticity finite element model with the measured lattice strain relaxation responses.  

From the temperatures assessed, 75oC was found to be the worse-case scenario, where the mascroscopic plastic strain accumulation was significant during a relaxation cycle due to the greatest activity of both prism and basal slip systems.  As the temperature increased, the contribution of thermal energy became greater than the mechanical energy for dislocation glide.  

Oxygen was found to have a stronger strengthening effect on prism slip over basal slip, through a significant change in their respective critical resolved shear stresses.  This effect became more significant in high oxygen content commercially pure Ti.