Flow Behaviour of Aluminium-Based Materials at Ultrahigh Temperatures in the Presence of a Liquid Phase

Experiments were conducted to determine the flow behavior of three materials at ultrahigh temperatures: an Al-6061 composite containing 20 vol% SiC whiskers and unreinforced Al-6061 and Al-1050 alloys prepared by casting. Tensile tests were performed at strain rates up to 5 × 10^-1 s^-1 and over a range of ultrahigh temperatures up to and above the temperatures where there is a small amount liquid phase. High strain rate superplasticity was achieved in the composite material but not in the unreinforced alloys. For all three materials, it is shown that the true activation energy for flow changes from values of < 200 kJ mol^-1 at the lower temperatures where there is no liquid phase to exceptionally high values in the presence of a liquid phase: these values are up to > 1000 kJ mol^-1 for the composite and the Al-105 alloy. It is concluded that exceptionally high activation energies are an inherent feature of flow in materials containing a small amount of discontinuous liquid at temperatures immediately above the onset of partial melting.