Effects of Cold Working on Precipitation in Age-hardenable Alloys

Abstract

Experiments have been carried out to examine the effects of cold working on the rate of precipitation in high purity copper alloys and aluminium alloys with measurements of electrical resistance, micro-hardness and thermo-electromotive force, X-ray Laue photographs and small angle scatter method. The correlation between cold working and the excess vacancies retained in quenching was investigated in water quenched and air cooled specimens. The experiments provided evidence that cold working accelerates the process at higher ageing temperatures, and that at lower ageing temperatures the rate of precipitation of cold worked specimens is less than that of unworked materials. In Al-Ag and Al-Cu alloys, the formation of zones was hindered and the precipitation of the more stable intermediate phases γ´ or θ´ was accelerated. The suppressive effect of cold working is thought to be a general phenomenon in age-hardenable alloys, because the effect is also found in alloys such as Cu-Cr alloys in which the precipitation sequence is simple. The mechanism of retarding by cold working is due to two effects : one is the sweeping out of the quenched-in vacancies, which are necessary for solute diffusion, by the motion of jogs in dislocations during cold working, and the other is the formation of many smaller clusters of enriched solute atoms which were formed by the stronger binding interaction energies between solute atoms and the lattice defects introduced by cold working.