Anisotropic Size Effect on the Plastic Deformation Behavior of α-Ti

α-Ti sheets are widely used in manufacturing miniaturized components for the outstanding specific strength, corrosion resistance, and formability. However, the size effect brings severe challenges to the deformation and the service of micro parts, and the plastic anisotropy of α-Ti makes the micro-forming process more complicated. Therefore, a series of in-situ experiments and full-field crystal plasticity simulations were carried out to investigate the coupling effect of geometric size and plastic anisotropy of α-Ti. Anisotropic size effect on the plastic deformation was observed, and Diagonal Direction (DD) specimen shows a higher size effect sensitivity than that of RD (rolling direction) and TD (transverse direction) specimens. The flow stress and fracture strain decrease as the geometric size decreases, and the DD specimen shows a higher fracture strain reduction. Evident surface roughing and a higher proportion of hard-oriented grains in DD4 results in stronger intergranular strain heterogeneity, which leads to the formation of through-thickness shear bands as the geometric size decreases. The fracture mechanism in α-Ti transforms from a micro-void coalescence ductile mode to a brittle one as the geometric size decreases. These findings provide a fundamental understanding of the anisotropic size effect on the tensile behavior of α-Ti.