Microstructure and resistance to thermal fatigue cracking of dilute lead–copper sheet alloys Part 1–Characterisation of lead-copper hypoeutectic structure and dispersion of second phase copper particles

Hypoeutectic microstructures in the non1aceted/faceted Pb–Cu system solidified under various cooling conditions are investigated. The results are consistent with the semiquantitative scheme proposed by other workers to classify regular and irregular eutectic microstructures. The morphology of the copper phase changes from lamella/flake–fibre with increased growth velocity, and becomes fully fibrous when growth velocity exceeds ∼0·5 mm S^−l. The microstructual development is influenced by the cooling conditions. Lead dendritic growth is dominant except when the undercooling is small and the temperature gradient in front of the solid/liquid interface is positive and large, when the interface becomes cellular. In rolling, copper particles break up during rolling deformation owing to load transfer from the lead matrix, which increases with increased particle aspect ratio and flow stress of the lead matrix. Fully fibrous copper produced by rapid solidification breaks up into very fine and well dispersed particles.

MST/3030