Mechanical properties and deformation mechanisms of a commercially pure titanium

Abstract

The mechanical behavior of a commercially pure titanium (CP-Ti) is systematically investigated in quasi-static (Instron, servohydraulic) and dynamic (UCSD's recovery Hopkinson) compression. Strains over 40% are achieved in these tests over a temperature range of 77–1000 K and strain rates of 10 −3–8000/s. At the macroscopic level, the flow stress of CP-Ti, within the plastic deformation regime, is strongly dependent on the temperature and strain rate, and displays complex variations with strain, strain rate, and temperature. In particular, there is a three-stage deformation pattern at a temperature range from 296 to 800 K, the specific range depending on the strain rate. In an effort to understand the underlying mechanisms, a number of interrupted tests involving temperature jumps are performed, and the resulting microstructures are characterized using an optical microscope. Based on the experimental results and simple estimates, it is concluded that the three-stage pattern of deformation at temperatures from 296 to 800 K, is a result of dynamic strain aging, through the directional diffusion of dislocation-core point defects with the moving dislocation at high strain rates, although the usual dynamic strain aging by point defects segregating outside the dislocation core through volume diffusion is also observed at low strain rates and high temperatures. The microscopic analysis shows that there is substantial deformation twinning which cannot be neglected in modeling the plastic flow of CP-Ti. The density of twins increases markedly with increasing strain rate, strain, and decreasing temperature. Twin intersections occur, and become more pronounced at low temperatures or high strain rates. In sum, the true stress–true strain curves of CP-Ti show two stages of deformation pattern at low temperatures, three stages at temperatures above 296 K, and only one stage at temperatures exceeding 800 K, although all three stages may exist even at 1000 K for very high strain rates, e.g. 8000/s. While the dislocation motion is still the main deformation mechanism for plastic flow, the experimental results suggest that dynamic strain aging should be taken into account, as well as the effect of deformation twinning.