Rate-dependent thermomechanical behavior and shear localization in a BCC metal

This work seeks to examine the susceptibility of body centered-cubic metals to adiabatic shear localization and to determine how the thermomechanical properties characteristic of the BCC lattice influence the process of localization. Vanadium was chosen for this study because of all the BCC metals it has thermophysical properties closest to those of alpha-titanium, a hexagonal close-packed (HCP) metal that is known to exhibit adiabatic shear localization at large strains.;The rate-dependent thermomechanical behavior of vanadium was determined using low strain rate (10-4 to 100 s-1) servo-hydraulic compression testing, high strain rate (∼4 x 10 +3 s-1) compression Kolsky bar testing, and very high strain rate (10+5 s-1) pressure-shear plate impact. In addition, a new technique for performing high temperature experiments in the compression Kolsky bar was developed in order to determine the high rate thermal softening behavior of vanadium from 27 to 800°C.;Shear localization experiments on vanadium were performed using a notched thickwalled specimen in the compression-torsion Kolsky bar. Experiments were run in both compression-torsion recovery and pure torsion modes, and the deformed microstructures were analyzed using optical and transmission electron microscopy. The shearing deformations developed at the notch tip were found to be diffuse and stable, and showed no evidence of void formation. TEM revealed elongated cells, elongated subgrains oriented parallel to the shearing direction, and equiaxed, highly-misoriented subgrains.;The high strain rate thermomechanical data obtained for vanadium was used to construct a finite element model of the shear localization experiment using ABAQUS/Standard. Comparisons of experimental observations and the numerical results show that the model accurately reflects the stability of the shearing deformations and the resistance of vanadium to shear localization. These results are compared with a similar simulation of alpha-titanium, which predicts the development of a well-defined adiabatic shear band.;A comparison of the strengthening mechanisms of vanadium and titanium using dimensionless quantities clearly illustrates that unstable shearing deformations are likely to develop in HCP titanium, whereas evolving shearing deformations in BCC vanadium remain stable. In addition, plotting shear localization parameters as functions of temperature indicates that evolving material properties (i.e. strain hardening, thermal softening, strain rate hardening) in the current deformation state are more important than the initial material properties in the reference state when predicting the development of adiabatic shear localization.