TRANSMISSION ELECTRON-MICROSCOPIC OBSERVATION OF PRECIPITATES IN AN AGED PB-0.1WT.PERCENT-CA-0.3WT.PERCENT-SN ALLOY

Samples of a Pb-0.1wt.%Ca-0.3wt.%Sn alloy were cast and either air-coo led or ice-water-quenched. They were then age-hardened for 400 days at ambient temperature and examined by thin foil transmission electron m icroscopy. In the air-cooled and aged condition, the alloy exhibits a sluggish ageing response, while a rapid ageing response occurs in the water-quenched and aged samples. In both types of sample, precipitate dispersions were observed. These are tentatively identified as Pb3Ca w ith an ordered L1(2) crystal structure. In the air-cooled and aged con dition, the particles were about 100 nm in diameter and occurred in pl anar arrays consistent with precipitation on migrating grain boundarie s. In the water-quenched and aged samples, the particles were about 10 nm in diameter and uniformly distributed with a smaller interparticle spacing, which is probably due to nucleation on tangle dislocations. The more rapid ageing kinetics in the water-quenched and aged conditio n are attributed to the excess vacancy concentration produced by quenc hing.