Fatigue crack initiation in as forged Ti–6Al–4V bars with macrozones present

• Texture index is proposed as a measurement for macrozone identification.
• Fatigue crack is found initiated from the interface between two macrozones.
• Schmid factor distributions are used for primary slip system identification.
• Slip trace analysis is used for activated slip system identification.
• Misorientation crack factor is used for quantifying the effect of macrozones.

Macrozones are regions containing grains with similar orientations, which in titanium alloys are detrimental to the fatigue properties. In this study, an investigation regarding the effect of the macrozone characteristics on the crack initiation under four-point bending fatigue cycling tests on an as-forged Ti–6Al–4V bar with macrozones present has been carried out. The macrozone characteristics studied in this paper include the Schmid factors of grains within the macrozone, the misorientation between two macrozones or a macrozone and its adjacent area and the angle of the interface versus loading axis. In this study, fatigue crack was initiated within two adjacent macrozones or within a macrozone and its adjacent region with high Schmid factors. Misorientation crack factors are introduced to combine all the above influencing factors and crack initiation on the interface. Fatigue crack initiation mechanism around macrozones has been studied and discussed. The texture index is introduced as a single value to represent the sharpness of the microtexture of a macrozone.