PHYSICALLY-BASED TEMPERATURE-DEPENDENCE OF ELASTIC-VISCOPLASTIC CONSTITUTIVE-EQUATIONS FOR COPPER BETWEEN 20-DEGREES-C AND 500-DEGREES-C

An anisothermal material model has been established for cyclic mechani cal and thermal loading in chill-cast pure copper undergoing creep and cyclic plasticity. The temperature dependences in the model are physi cally based. The material parameters arising in the model have been de termined from a programme of tests carried out on pure copper, enablin g the model to be validated over the temperature range 20-500 degrees C, strain range 0-+/-1.0% and strain rate range 0.006-0.6% s(-1). The variations, with inverse temperature, in the viscoplastic strain rate parameter, the cyclically hardened yield stress, and hardening and sof tening parameters, when considered with respect to logarithmic scales have been shown to be bilinear in nature, over the temperature range c onsidered. The transition temperature, in all cases, was found to be 2 09 degrees C. For the viscoplastic strain rate parameter, an activatio n energy of 200 kJ mol(-1) was determined for temperatures in excess o f 209 degrees C, and 33 kJ mol(-1) for temperatures less than 209 degr ees C. Deformation mechanisms of bulk diffusion, and dislocation glide and climb, are thought to predominate for the two temperature ranges respectively. However, in the case of the cyclically hardened yield st ress, and the hardening and recovery parameters, activation energies o f 34 kJ mol(-1) and 11 kJ mol(-1) were determined for temperatures gre ater than and less than 209 degrees C respectively. The deformation me chanisms predominating are thought to be dislocation glide and climb f or the case of the higher temperature range, and glide only for the lo wer temperature range, because of the dependence of dislocation climb on core diffusion which is thought not to occur significantly in coppe r below about 209 degrees C. The constitutive equations developed have been generalized to multiaxial stress states, and a methodology for t he determination of the material parameters given, which ensures that they are physically meaningful.