Aging acceleration phenomenon at elevated temperatures in Al-Cu-Cd alloys

Abstract

It has been known that a small addition of Cd, Sn, or In to an Al-Cu alloy accelerates artificial aging and raises peak hardness. In Al-Cu-Sn alloys, the present authors found that the acceleration of age-hardening was due to the formation of the Sn rich phase which would act as nucleation sites for θ' precipitates. The present study was conducted to examine whether the similar mechanism was operative in Al-Cu-Cd alloys. Al-3wt%Cu and Al-4wt%Cu alloys with various amounts of Cd (0 to 0.4wt%) were prepared. Aging phenomena of these alloys were examined by hardness, calorimetric and electrical resistivity measurements as well as transmission electron microscopy. The results obtained are as follows:1) Although the Cd addition decreased the solubility of Cu in the α phase, the aging rate and the age hardenability increased as the amount of Cd in the α phase at the solution temperature increased.2) Indication of the various measurements was compared well with the aged structures. The θ' precipitates were finer and more densely distributed as the amount of Cd increased.3) A small dark spot, which was not observed in the binary Al-Cu alloy, was found to exist at a corner of a θ' precipitate in the Al-Cu-Cd alloys. The number of the dark spots and that of the θ' precipitate increased with an increase of the Cd content.4) From the above observations, it was considered that the aging acceleration in the Al-Cu-Cd alloys was caused by the formation of Cd rich phases which aided the nucleation of the θ' precipitates by acting as nucleation sites in the same way as in the Al-Cu-Sn alloys.5) Aging in the Al-Cu-Cd alloys occured in a slightly slower rate than in the Al-Cu-Sn alloys. Transmission microscopy showed that the distribution of the θ' precipitates was coarser and many Cd rich phases existed freely from the θ' precipitates in the Al-Cu-Cd alloys. The latter was not observed in the Al-Cu-Sn alloys. These facts suggest that the precipitation of the Cu rich phases occurs more rapidly than that of the Cd rich phases in the early stage of aging.