Anisotropic and asymmetrical yielding and its evolution in plastic deformation : titanium tubular materials

Abstract

The coupling effects of low asymmetry of HCP structure and transient non-uniform stress/strain states during multi-pass deformation processing cause a great variation in crystallographic orientation of titanium tubes, which may induce anisotropic/asymmetrical behaviors and affect the formability and service performance of the materials. The unique plastic deformation and mechanisms under 3D stress need to be accurately and fully understood for integrated design of fabrication and forming of titanium tubular products achieving shape forming and property tailoring simultaneously. How to address this eluded and tantalized issue, however, is still a bottleneck issue. In tandem with this, taking high strength titanium tube (HSTT) as a case, by using macro/meso scaled hybrid methodology, a correlation among loading condition, distorted plasticity and texture evolution of the material is established and articulated: 1) Via Knoop indentation, tension/compression and EBSD, the distorted plasticity of HSTT is identified, and the coupling of three slip and two twinning modes are found to dominate the inhomogeneous deformation, which must be introduced in viscoplastic self-consistent (VPSC) crystal plasticity; 2) A compression-based orthogonal inverse method is used to calibrate the crystal plasticity parameters and validated from various aspects, and the VPSC-based computation is conducted for tubular materials with six typical initial textures under six fundamental loadings; 3) The remarkable distorted plasticity and evolution in strain hardening, strain flow and yield loci are observed for most cases; The interactions among slipping/slipping, slipping/twinning and twinning/twinning coordinate the anisotropic/asymmetrical behaviors and hence induce the distorted evolution of plasticity; The relationship between deformation modes (strain vectors) and texture evolution is constructed, and the tubes with the desired textures and bespoke properties can be tailored in cold rolling by allocating spatial plastic flow.