Transmission electron microscopic observation of precipitates in and aged Pb---0.1wt.%Ca---0.3wt.%Sn alloy

Samples of a Pb---0.1wt.%Ca---0.3wt.%Sn alloy were cast and either air-cooled or ice-water-quenched. They were then age-hardened for 400 days at ambient temperature and examined by thin foil transmission electron microscopy. 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 with an ordered L12 crystal structure. In the air-cooled and aged condition, the particles were about 100 nm in diameter and occurred in planar arrays consistent with precipitation on migrating grain boundaries. 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 condition are attributed to the excess vacancy concentration produced by quenching.