Formation mechanism of high-strain bands in commercially pure titanium

In textured α-Ti, band-like high-strain regions at approximately 45° to the loading direction are commonly observed under tensile deformation. Herein, such high-strain regions were called high-strain bands (HSB), and the reasons for the formation mechanism of HSBs were investigated by a uniaxial tensile test and crystal plasticity finite element (CPFE) analyses. First, we conducted a uniaxial tensile test of commercially pure titanium (CP–Ti) with a TD-split texture, where aggregates of (0001) split and incline in the transverse direction, and changes in strain distributions with deformation were observed by digital image correlation. The strain distributions showed that HSBs of approximately 45° to the loading direction were formed from the initial stage of deformation. Second, a geometric model including crystal orientation information was constructed from the crystal orientation map of the CP-Ti specimen obtained by electron back-scattered diffraction, and the cause of the formation of HSBs was investigated by CPFE analysis. The stress–strain relationship and strain distributions obtained by CPFE analysis correlated well with those obtained experimentally, and the HSBs were successfully reproduced. Finally, CPFE analyses were conducted when the loading conditions, critical resolved shear stress (CRSS), or elastic constants were changed. These results show that the HSBs formed from the initial stage of deformation and the distributions formed by elastic deformation changed depending on the elastic anisotropy. However, the distribution of HSBs after plastic deformation was unchanged by elastic anisotropy. This may be because the directions of easy deformation by elastic and plastic deformation coincided. Elastic anisotropy did not affect the distributions of HSBs formed by plastic deformation, and elastic and plastic deformations were predominantly determined by elastic constants and CRSS, respectively. Curved HSBs were also observed when plastic deformation-resistant regions occupied a large area in the specimen. In this case, the distributions of HSBs, caused by elastic deformation did not coincide with those of the plastic deformation.