Calori- and Resistometric Analyses of Ageing and Precipitation in Aluminum-Copper Alloys

Abstract

Specific heat versus temperature curves have been obtained on polycrystalline and single crystal specimens of Al-4 wt% Cu alloys during reheating after various age-hardening treatments. The results have been analysed in comparison with those of the hardness tests, dilatometric and X-ray structural measurements of the previous workers; the results are useful in providing further information on the low temperature ageing or the first stage of the ageing and its relation to the high temperature ageing. The reversion phenomena in the alloys aged at low temperatures are attributed to the dissolution of the G.P. zone. The effects of the ageing temperature and ageing time upon both the reversion temperature and the heat of reversion are examined from the heat absorption, and it is confirmed that the reversion temperature rises with ageing temperature. The incubation period for formation of the G.P. zone has been determined as a function of ageing temperature. The results represent a C-curve in good agreement with that determined by the X-ray study. An evidence has been obtained for the formation of the G.P. zone at 190°C after the θ″ and θ′ precipitations.
The kinetics of precipitation processes at higher ageing temperatures reaching close to the solvus has been investigated by the electrical resistivity measurement. It has been recognized that there are two distinct, independent precipitation processes at temperatures higher than 200°C with the apparent activation energies of 30.1 and 25.4 kcal/mol, respectively. These processes seem to correspond to the θ and θ′ precipitations, respectively.