Effect of the presence of macrozones on short crack propagation in forged two-phase titanium alloys

Macrozones, which are regions containing grains with similar orientations, are commonly reported to be present in two-phase titanium alloys and to be detrimental to their fatigue resistance. The effect of macrozones on fatigue crack initiation is established, but their effect on short crack propagation remains unknown. In this study, the effect of the presence of macrozones on short crack propagation is investigated under four-point bend fatigue testing on both as-forged and powder hot isostatic pressed Ti-64. Micronotches were machined into the samples as crack initiation sites and the short crack propagation behaviour was recorded by replica. It was found that the crack propagation in macrozones can be faster than in random texture because most of the crack planes in the grains of the same macrozone are aligned. Furthermore, macrozones oriented favourably for basal 〈a〉 slip systems are less resistant to short crack propagation than macrozones oriented favourably for prismatic 〈a〉 slip systems. This is because the basal planes of the grains within the same macrozone are aligned, while the prismatic planes of the grains within the same macrozone may not be aligned. This study provides an important fundamental understanding of the effect of macrozones on short crack propagation in forged two-phase titanium alloys and thus on fatigue resistance of titanium alloys and could be useful for the titanium industry in predicting the effects of the presence of macrozones in their products.