The complete development of the cellular precipitation structure at selected grain boundaries in a Pb-3.0 wt% Sn alloy during aging at 34°C and 23°C was investigated by light microscopy. The cellular structure in this alloy consists of lamellae of solute depleted lead solid solution and lamellae of ß tin precipitate. The cellular precipitation reaction begins by local migration of a grain boundary with concomitant grain boundary precipitation of ß tin. The grain boundary bows around the ß tin grain boundary precipitates which then begin to lengthen in the direction of boundary migration thus forming the precipitate lamellae of the cell. The α\u27 lamellae always preceed the ß tin lamellae. At early stages of cell development, the lamellae may grow cooperatively and an orientation and habit plane relationship between the lamellae forms as the cell structure stabilizes. Since the coherence between the lamellae is weak, the direction of cell growth is not confined to the initial habit plane of the ß tin lamellae --Abstract, page iii
