Unraveling Deformation Mechanisms in CP-Ti via Crystal Plasticity: Direction-Dependent Surface Roughness Evolution

Abstract

This study investigates the evolution of surface roughness and the underlying deformation mechanisms in ultra-thin commercially pure titanium (CP-Ti) sheets, which are attracting increasing attention as candidate materials for metallic bipolar plates in fuel cells. Under uniaxial tension along the rolling direction (RD), the sheets developed markedly rough surfaces with pronounced creases aligned with the loading direction. In contrast, loading along the transverse direction (TD) produced lower roughness and a more uniform, nearly isotropic surface morphology. Crystal plasticity finite element modeling reproduced these observations and attributed the direction-dependent roughness evolution to differences in the activation of slip and twinning systems. Tensile loading along RD was dominated by prismatic ⟨a⟩ slip, restricting through-thickness deformation. Conversely, tensile loading along TD activated multiple deformation systems, enabling more distributed deformation in multiple directions. These mechanisms were further supported by deformation microstructures revealed through electron backscatter diffraction. Taken together, these findings clarify the origin of direction-dependent roughening and provide mechanistic insight into heterogeneous through-thickness deformation behavior and its role in surface roughness evolution.