EVOLUTION OF MICROSTRUCTURE AND SHEAR-BAND FORMATION IN ALPHA-HCP TITANIUM

The evolution of the microstructure generated by high strain-rate plastic deformation of titanium was investigated. A testing geometry generating controlled and prescribed plastic strains under an imposed stress state close to simple shear was used; this testing procedure used hat-shaped specimens in a compression Kolsky bar which constraints the plastic deformation to a narrow region with approximately 200 mum width. Within this band, localization sets in, initiated at geometrical stress concentration sites, at a shear strain of approximately 1.4. The shear-band widths vary from 3 to 20 mum and increase with plastic strain. High strain-rate deformation induces, at lower plastic strains (gamma < 1.4), planar dislocation arrays and profuse twinning in titanium. In the vicinity of the shear band, elongated cells are formed, which gradually transform into sub-grains. The break-up of these sub-grains inside the band leads to a microstructure composed of small grains (approximately 0.2 mum) with a relatively low dislocation density. The combined effects of plastic strain and temperature on the microstructural recovery processes (dynamic recovery and recyrstallization) are discussed. The experimental results are compared with predictions using a phenomenological constitutive equation and parameters obtained from compression experiments conducted over a wide range of strain rates. The experimental results indicate that the formation of shear bands occurs in two stages: (a) instability, produced by thermal softening and the enhancement of the thermal assistance in the motion of dislocations; (b) localization, which requires softening due to major microstructural changes (recovery and recrystallization) in the material. The calculated temperature rises required for instability and localization are 350 K and 776 K, respectively. Whereas instability may occur homogeneously throughout the entire specimen, localizatkon is an initiation and propagation phenomenon, starting at geometrical (stress concentration sites) or microstructural inhomogeneities and propagating as a thin (3-20 mum) band.