Anisotropic plasticity and fracture of alpha titanium sheets from cryogenic to warm temperatures

Abstract

Titanium is promising for manufacturing high-performance components in aerospace, marine, energy and healthcare. Whether the forming potential of alpha titanium can be excavated under cryogenic and warm working conditions and how the anisotropic plasticity and fracture evolve over a wide temperature range are the premise and basis for accurate control of inhomogeneous deformation of the material and invention of new forming technologies. In tandem with this, the anisotropic plasticity and fracture of alpha titanium sheets over a wide temperature range of -180 to 200 °C were explored regarding behaviors, mechanisms and modeling. 1) From a series of characterization experiments under different temperatures, compared with room temperature, the tension hardening exponent and fracture elongation of the material are increased by 133% and 48% at -180 °C, and by 7% and 50% at 200 °C. The anisotropy and asymmetry in flow stress are decreased with the temperature increasing from -180 to 200 °C, while the fracture displacements present a significant anisotropy over the given temperature range. 2) Via the macro- and micro-observations regarding dislocation, texture, micro-void and fractography and the viscoplastic self-consistent (VPSC) based slip/twinning analysis, at cryogenic temperature, the synergistic effects of slip and twinning make the dislocation motion ability increase and the grains refined. This causes the large strain hardening ability and inhibits the growth of micro-voids, which results in a relatively large fracture elongation. With increasing temperature, the increased slip systems and slip activities suppress the activation of twinning. At warm temperature, the enhanced dislocation motion ability by thermal activation reduces dislocation pile-up and inhibits void initiation, making the fracture elongation increase. 3) By introducing the temperature/strain related interpolation approach and the linear transformation of stress tensor, an integrated modeling framework was proposed, and the evolution of anisotropic yielding-necking-fracture loci with temperature was established. The results indicate that the necking strain limit is significantly improved at cryogenic temperature and the fracture strain limit is dramatically increased at warm temperature.