On the Activation Energies Observed in Al-based Materials Deformed at Ultrahigh Temperatures

This study examines the exceptionally high activation energies estimated from data obtained from various composites exhibiting high strain rate superplasticity and loaded at ultrahigh temperatures near or slightly above the solidus. It is shown that the activation energies for the 6061 composite and for Al 6061 and 1050 alloys are below 200 kJ/mol at temperatures lower than the incipient melting temperature, T_i, but they become significantly higher at temperatures near or slightly above T_i and this increase is independent of the alloy system, the grain size, whether the material is superplastic and the dominant deformation mechanism at T<T_i. There is a transition from plastic flow in a solid state to a viscous-like flow behavior in the Al-6061 systems at T>610°C and in the Al-1050 alloy at T>650°C. It is concluded that the high values of Q_t have no physical meaning in terms of the rate-controlling diffusive species.