Effects of cold working on the aging of Al-Cu alloys

Abstract

Experiments have been made to examine the effects of cold working on the rate of cluster formation and precipitation in Al-4%Cu or Al-1.8% Cu alloys with X-ray Laue Photographs, electron micrographs and measurements of micro-hardness. The correlation between cold woring and the excess vacancies retained in quenching was investigated with water quenched, air cooled and reverted specimens. The effects of cold working after aging at low temperature were also examined here.
The experiments provided evidence that the formation of the less stable G. P. (1) and G. P.
(2) is hindered, while the precipitation of the more stable θ' phase is accelerated.
The rate of precipitationis more rapid in water quenched specimens than in air cooled, or reverted materials. The increased rate is due to the fact that the lattice vacancies which are more than the equilibrium are retained in quenching. The suppressive effects of cold working are most remarkable in water quenched alloys.
The mechanism by which the rate of cluster formation is retarded may be 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 plastic deformation or the increase in the rate of annealing of quenched-in vacancies as a result of cold working; and the other is the formation of many smaller enriched clusters of solute atoms which are formed by the stronger binding interaction between solute atoms and lattice defects introduced by cold working.