EFFECT OF COPPER AND ZIRCONIUM UPON THE HIGH-TEMPERATURE MECHANICAL-PROPERTIES OF ALMGZN ALLOYS

High temperature mechanical properties of AlMgZn alloys are investigated by impression creep tests and the strain rate sensitivities and activation energies obtained are presented. It is found that the steady-state impression creep rate is strongly influenced by Cu and Zr addition. The simultaneous addition of Cu and Zr results in a fine-grained microstructure due to which the material exhibits superplastic properties at high temperatures. Together with the superplastic behaviour the occurrence of a threshold stress is also proved. On the contrary, if the deformation is not superplastic, threshold stresses are not observable. It is shown that the relatively high apparent activation enthalpy obtained for the superplastic region of deformation is due to the strong temperature dependence of the threshold stress, and taking into account this fact a true activation enthalpy corresponding to that of the bulk diffusion can be assessed.