Adiabatic shear localization in α-titanium: experiments, modeling and microstructural evolution

Abstract

The localization of dynamic shearing deformations in α-titanium is examined using a novel experimental technique (the compression–torsion Kolsky bar) that allows the recovery of specimens within which an adiabatic shear band has been grown as the result of a single torsional pulse. The specimens are circumferentially notched thick-walled cylinders that are subjected to simultaneous, independently controlled, dynamic compression and torsion. Explicit finite element computations are performed to obtain the stress, strain, temperature and pressure distributions within the specimens under the measured boundary conditions. The constitutive behavior input to the computational simulations is obtained from independent high-strain-rate experiments (involving only homogeneous deformations) on the same material. Shear band growth and microstructural evolution in the specimens are investigated by sectioning the specimens at different depths from the outer radial surface. TEM observations across the shear bands reveal the following microstructural evolution: (a) planar dislocation motion and twinning; (b) grouping of dislocations into cells; (c) formation of elongated subgrains along the shear direction, and (d) development of equiaxed nanocrystalline grains 50– 200 nm in diameter. The microstructures observed are analogous to those reported for severely cold-rolled metals.