Anisotropic Deformation Behavior of Rolling Textured Alpha Titanium

Abstract

The present dissertation aims to clarify underlying mechanisms that cause the deformation anisotropy of rolling textured alpha titanium. In addition, a modified rolling process is proposed to enhance the formability at room temperature by controlling texture. This study consists of three parts as follows. Part 1: Mechanisms causing such a yielding anisotropy were identified by evaluating the activation stress. The results revealed that the specific crystallographic orientation of the material, caused by rolling texture, influences the activity of each deformation mode by affecting the SF and this significantly varies with the loading direction, thereby inducing a change of a deformation mode dominating the material yielding. Prismatic <a> slip governed the yielding in the RD and TD, while basal <a> slip was responsible for the yielding in the ND. Part 2: Plastic anisotropy and associated deformation mechanisms of rolling textured high purity alpha phase titanium were investigated by carrying out uniaxial compression tests along the three featured directions, the rolling direction (RD), normal direction (ND) and transverse direction (TD), in combination with an electron backscatter diffraction measurement and a Schmid factor (SF) analysis. The results revealed that the specific crystallographic feature of the material, caused by rolling texture, influences the activities of dislocation slips and twinning by affecting their SF, and this significantly varies with the loading direction, consequently leading to anisotropic deformation. As the material deformed beyond the yielding point, deformation twins took place and played a decisive role in the deformation. The twinning characteristics including types of twins, morphology, twin area fraction with strain and related texture modification significantly varied with the loading direction, and this had a completely different effect on strain hardening behavior, thereby causing deformation anisotropy. Part 3: A noticeable enhancement in the stretch formability of commercially pure titanium sheets at room temperature was achieved by employing a two-step cold rolling process. The sheets manufactured by the two-step cold-rolling process exhibited a lower split basal texture intensity and other basal texture components where the c-axis is distributed in the rolling-transverse plane, and their Erichsen values were remarkably high compared with the conventionally cold-rolled sheet. The enhanced formability is attributed to the higher deformation capability of sheet thinning, evidenced by the low Lankford value.