Size effects on tensile properties and deformation mechanism of commercial pure tantalum foils

The tensile properties and deformation microstructure of commercial high pure tantalum foils with thickness ranging from 30 mu m to 200 mu m have been investigated. The origin foils show a large texture intensity of gamma fiber. The yield strength and ultimate tensile strength gradually increase with the decrease of thickness due to the change of the scale factor of the surface grains and the increase of volume fraction of passivation film. The constitutive model based on the surface layer theory is well established. Meanwhile, the anomalous yield point phenomenon can be seen. The deformation microstructure is closely related with the orientations. Two sets of microbands are developed in most of the grains. The density of microbands is the largest in the grains with tensile direction // < 110 >, which belong to a fiber. Microbands are usually concentrated near the grain boundaries, which frequently incline at 25-45 degrees with respect to the tensile direction. The trace of microbands is usually consistent with the slip planes of {110} or {112} with largest or second largest Schmid factor. Therefore, the appearance of wavy slip traces on the fracture is attributed to the dislocation cross-slip on these different kinds of slip planes.