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

To identify the size effects on the mechanical behavior of microparts is essential for micro-forming. Here, the size effect on the tensile properties of commercial pure titanium foils with thickness ranging from 5 µm to 200 µm has been investigated. The tensile strength, yield strength and work-hardening rate reach a minimum value at the thickness of 30 µm because the main deformation microstructures including dislocations, twins and deformation induced face-centered cubic titanium (fcc-Ti) are different in different foils. The density of deformation twins decreases as the thickness decreases. However, the density of fcc-Ti increases and reaches a maximum value at the thickness of 30 µm. The dominant deformation mechanism is dislocation slip when the thickness is below 20 µm. The yield strength decreases with the decrease in thickness because of the strengthening effect of fcc-Ti being smaller than that of twins when the thickness is above 30 µm. When the ratio of the thickness to grain size is below 5, the yield strength increases with the decrease in thickness due to the large volume fraction of oxides layer on the surface. Based on these, two kinds of constitutive equations have been well established in consideration of the grain size or the ratio of the thickness to grain size.