The effect of grain orientation on fracture morphology during high-cycle fatigue of Ti–6Al–4V Original Research Article

Three different product forms of Ti–6Al–4V, unidirectionally rolled and cross-rolled plate and forged bar, have been cyclically loaded within the high-cycle fatigue regime to investigate fatigue crack initiation. The fracture surfaces of fatigued specimens contained large regions of neighbouring facets. The majority of facets examined had a near-basal fracture plane. It was shown that grains favouring crack initiation were primarily those with misorientations between 15° and 40° from the loading direction, whereas other orientations served as crack growth paths. This implies that the formation of a facet requires a combination of a moderately high Schmid factor for basal slip coupled with a tensile component perpendicular to the basal plane. The large regions of neighbouring facets on the fracture surface were found to be a consequence of cracking within a macrozone unfavourably oriented for slip found within the microstructure, i.e. one with its c-axis near to the loading direction.