Effect of twinning on deformation of textured commercially-pure Ti sheets under plane stress states

Abstract

Pure titanium sheets have been processed with laboratory scale to develop four different textures; the basal pole of the preferentially oriented grains is aligned at angles (a) 0, (b) π/6 and (c) π/2 from the sheet normal around the rolling direction, and (d) the basal pole of grains is distributed widely. The, effects of texture and twinning on deformation behavior in uniaxial tension, plane strain compression and hydraulic bulge have been studied. In addition, the yield loci at various strain levels have been constructed on the assumption of the constant level of plastic work. Strain hardening in each deformation is strongly influenced by twinning frequency and the orientation of twinned grains, and in many cases, a 2-stage strain hardening process is found with a higher n value above a critical strain. The change in the n value under the plane strain state occurs at smaller strain than under other stress states, because of higher twinning frequency. The yield locus of each textured sheet expands with a similar form up to a strain of 0.1, and then varies in its form at larger strains, reflecting high strain hardening under plane strain loading. As the basal pole of the preferred orientation deviates from the sheet normal around the rolling direction, the difference between the principal axes of stress and plastic anisotropy is increased. It is shown that the formability of the textured pure Ti sheets is reasonably predictable from their yield loci.