Abstract
Milled lead sheet exhibits a recrystallised microstructure, the grain size of which decreases with increasing copper content; copper is present as particles of ∼1–5μm in length, formed from the initial needles or plates of the cast billet by spheroidisation, or by particle break-up during rolling. A comparison of materials produced by several manufacturers indicates that the copper distribution and final grain size are dependent on the thermomechanical history of the sheet. Direct method sheet exhibits a cast cellular structure within grains which usually extend through the full thickness of the cast sheet; copper is present as a fine dispersion (particles ∼0·5 μm dia.) at cell boundaries. The copper distribution in both materials was stable with prolonged heating (100 h at 200°C), but some grain growth occurred in the milled sheet. It was not possible to obtain a fully dispersed eutectic microstructure at the eutectic composition; primary lead dendrites (or cells) were always present. This is thought to be a result of the difficulty of nucleating a copper particle in the very low copper concentration alloys used in this work.
MST/1065
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