Local deformation patterns in Ti–6Al–4V under tensile, fatigue and dwell fatigue loading

We have used EBSD orientation mapping and digital image correlation-based strain mapping to investigate
inhomogeneous deformation of Ti–6Al–4V in tension, fatigue and cold-dwell fatigue. Strong strain inhomogeneities were found in all loading modes and in each case the pattern of high and low strain is established relatively early in the tests. Comparing the orientation and strain maps shows that grain–grain interactions are the primary cause of strain concentration. Surface grains with the crystallographic c-axis parallel to the loading direction showed very low strain levels, and neighbouring grains showed exceptionally high strain levels. In both fatigue and dwell fatigue, these regions of high strain concentration were observed to act as sites for crack nucleation. Strain evolution was found to be significantly different in each loading mode; in particular, deformation in dwell fatigue appears to have similarities with creep deformation.