Deformation twinning in fatigue crack tip plastic zone of Ti-6Al-4V alloy with Widmanstatten microstructure

Titanium alloys with the Widmanstatten microstructure usually exhibit high resistance to crack propagation and high fracture toughness. However, this has rarely been connected to mechanical twins. In this study, deformation twinning, which is popular in the fatigue crack tip plastic zone of alpha/beta titanium alloys with the Widmanstatten microstructure, was systematically investigated with the electron backscatter diffraction technique. With the propagation of the fatigue crack, large-scale twins were generated due to the periodic loading and the crystallographic feature of the Widmanstatten microstructure, and they ultimately consumed the major volume of the parent alpha colony. Twin development with various ranks of Schmid factors (SF) was characterized, while ordinary deformation twins with relatively high rank SF grew most, extraordinary twins with very low rank SF were also activated, although restricted by the ordinary twins. The occurrence of extraordinary twins (including primary and secondary twins) was mainly attributed to the non-uniform local stress distribution induced by the plastic deformation compatibility between the neighboring units. Lastly, the influence of deformation twinning on the mechanical properties concerning crack propagation in hexagonal alpha-Ti alloys was discussed.