Flow stress of F.C.C. polycrystals with application to OFHC Cu

Based on the concept of dislocation kinematics and kinetics, paralleled with a systematic experimental investigation, a physically-based model is developed for f.c.c. polycrystals, using OFHC copper for illustration. First, the concept of the motion of dislocations and the barriers that they must overcome in their motion, is used as an underlying motivation to obtain general expressions which include a number of free constitutive parameters. These parameters are then evaluated by direct comparison with experimental data. High strain-rate compression experiments are performed using UCSD's recovery Hopkinson technique (see Nemat-Nasser, S., Isaacs, J. B. and Starrett, J. E., Proc. R. Soc., 1991, 435A, 371; Nemat-Nasser, S., Li, Y. F. and Isaacs, J. B., Mech. Mater., 1994, 17, 111; Nemat-Nasser, S. and Isaacs, J. B., Acta Metall., 1997, 45, 907). Strains close to 100% are achieved in these tests, over a temperature range of 77-1100 K, and strain rates of 10^-3 to 8000 s^-1; the quasi-static tests are performed using an Instron machine. For low-temperature tests, both the as-received and annealed samples are tested. With few free constitutive parameters, good correlation between the theoretical predictions and experimental results is obtained, over the entire range of strain rates and temperatures. The orders of magnitude of several of these parameters are first estimated based on the underlying structure of the material. Experimental results are then used to tune the final values of these parameters. It turns out that the structure of the constitutive relations and the value of a number of the constitutive parameters are essentially the same for commercially pure tantalum (b.c.c. metal) and OFHC copper. The relation between the two cases is examined and the similarities and differences are discussed.