Transmission electron microscopy investigation of the dislocation structure in a Ti-6Al-4V alloy subjected to an early stage of cyclic deformation

Abstract

A detailed investigation was conducted of the dislocation structure formed within both the α and β phase of a Ti-6Al-4V alloy subjected to an early stage of cyclic deformation. The cyclic loading-unloading was carried out in tension between applied stress levels of 1100 and 100 MPa, the macroscopic yield stress (σ02) being about 1205 MPa, at a strain rate of 2×10−3 s−1 to 300 cycles, as part of an in-situ synchrotron experiment. The dislocation type, arrangement and density were examined post-mortem using transmission electron microscopy, complemented by the automatic determination of individual crystallite orientations through precession-enhanced nanobeam diffraction. The deformation took place entirely through dislocation glide and no evidence of deformation twinning was found within either of the constituent phases. The dislocation structure within the α phase was fairly similar to that formed during monotonic straining and no presence of periodic planar dislocation networks suggested in the literature was detected. Prismatic and basal slip were the dominant observed deformation modes, based on the calculated Schmid factor and critical resolved shear stress values reported in the literature. The <a>−type dislocations present after straining typically displayed a large screw component, with a majority of them having a pure screw character. There was also limited presence of <c+a> dislocations within the α phase and these were largely of a mixed type. Burgers vectors of the dislocations remaining after deformation within the β phase were also largely associated with the potential slip systems having high Schmid factor values and these dislocations mainly displayed a large screw component.