FLOW-STRESS OF OFE COPPER AT STRAIN RATES FROM 10-3 TO 104S-1 - GRAIN-SIZE EFFECTS AND COMPARISON TO THE MECHANICAL THRESHOLD STRESS MODEL

The results of electromagnetically launched expanding-ring experiments and conventional compression tests with 10- to 200-mu-m-grain-sized oxygen-free electronic (OFE) copper are reported and analyzed in terms of the mechanical threshold stress (MTS) model. The activation energy for slip and the strain-rate pre-exponential are taken from the literature, but other model parameters are derived from the present data. We find that the work hardening, d-sigma/d-epsilon, varies significantly with the strain rate but is independent of the grain size. This implies that the grain-size dependence, which is well described by a Hall-Petch relationship, must be assigned to the structure independent, or "athermal," component of the MTS description of the flow stress. The model parameters associated with dynamic recovery are critical to the description of the high-temperature, high-strain-rate ring data, and we find values slightly different from those in the literature. Overall, however, the MTS model provides an excellent description of the material behavior over a wide range of grain size, strain rate, and temperature. In particular, it successfully reproduces the interplay between temperature and strain rate that occurs during ring expansion.