The high-strain-rate response of alpha-titanium: Experiments, deformation mechanisms and modeling

The high-strain-rate mechanical response of cc-titanium is examined in terms of the underlying deformation mechanisms that govern its macroscopic behavior. The mechanical behavior of cc-titanium has been evaluated using quasistatic (10(-5) to 10(0)s(-1)) compression testing with servohydraulic machines, dynamic (10(-2) to 10(4) s(-1)) compression testing with a compression Kolsky bar, and high-rate (10(-4) to 10(5) s(-1)) shearing under pressure using the pressure-shear plate impact technique. At the macroscopic level, alpha-titanium displays substantial rate sensitivity of the flow stress and pronounced strain hardening. The strain-hardening is greater al high strain rates than at low strain rates, and increases with strain al low strain rates. In an effort to determine the deformation mechanisms underlying this macroscopic behavior, the microstructures developed after low-rate and high-rate deformations have been characterized using both optical and transmission electron microscopy (TEM). At the microscopic level, both dislocations and twins are observed: the density of twins increases with both strain and strain rate and is shown to be a unique function of the flow stress (but not vice versa). Although dislocation motion accounted for the majority of plastic deformation, twin-dislocation interactions play an important role in strain hardening. The Kocks-Mecking model is used in order to describe the mechanical response as a function of the strain, strain rate and temperature; while the model is able to predict the monotonic behavior fairly accurately, ii is unable to capture the experimental behavior observed in load-unload-reload tests. (C) 1998 Acta Metallurgica Inc.