The role of biaxial stress ratio on the mechanical behavior and deformation mechanisms in HCP α-Ti

The mechanical behavior of hexagonal close-packed (HCP) Ti alloy is closely related to the stress state. Renewing the mechanical behavior and associated mechanisms of HCP α-Ti alloy under biaxial stress states is critical for their manufacturability and serviceability. In this work, biaxial tensile tests with different stress ratios closing to actual service conditions were conducted to investigate the mechanical behavior of commercially pure Ti (CP-Ti). Then, comprehensive microstructural characterizations were performed using electron back-scattering diffraction (EBSD) and transmission electron microscopy (TEM) techniques. The effects of biaxial stress ratio on the evolution of dislocation and deformation twinning throughout plastic deformation were clarified. It was found that the activity of prismatic slip was suppressed while pyramidal slip was promoted, which was attributed to the decrease of external stress component on the prismatic planes and the increase of that on the pyramidal planes with increasing biaxiality. Moreover, the increased stress component in the c-axis direction also encouraged the occurrence of extension twinning at high biaxiality. The decrease of prismatic slip with low critical resolved shear stress (CRSS) and the increase of twinning and pyramidal slip with high CRSS contributed to an increase in deformation resistance and work hardening rate with increasing biaxiality.