Dislocation structures in a Ti–6Al–4V alloy subjected to cyclic tensile deformation

Abstract

The present work investigates dislocation structures developed within both the α and β phase of a Ti–6Al–4V alloy undergoing cyclic tensile straining. The cyclic loading-unloading was performed using the stress levels of 830 and 580 MPa (R∼0.7) at a strain rate of 2 × 10-3 s-1 to 15125 cycles, as part of a synchrotron in-situ experiment. The macroscopic yield stress was about 950 MPa. The dislocation arrangement, type and density were studied post-mortem employing transmission electron microscopy, together with the automatic determination of crystal lattice orientations via precession nanobeam diffraction. The deformation took place exclusively through dislocation slip and no presence of deformation twins was detected within either of the studied phases. The α phase dislocation structure displayed some similarity to that reported for monotonic deformation and no evidence of the formation of regular planar dislocation networks, proposed in the literature, has been found. The prevalent deformation modes were prismatic and basal glide, based on the experimentally determined Burgers vectors together with the global Schmid factor and reported critical resolved shear stress values. The <a>-type dislocations observed after deformation generally possessed a large screw component, with most of them being of a pure screw type. There were also some <c+a> dislocations present within the α phase and these typically displayed a mixed character. Dislocation Burgers vectors present after straining within the β phase frequently deviated from the Schmid factor rule and these dislocations were often wavy and largely possessed a significant screw component.