Morphology and structure of tin lamellae formed by cellular precipitation

Lamellae of β-tin precipitated at room temperature from a Pb-Sn (5.5 at. %) alloy were isolated by dissolution of the lead matrix in an acetic acid-hydrogen peroxide solution. The tin plates were not detectably dissolved or etched even after long exposure to this reagent. The morphology and structure of the residual lamellae were studied by electron transmission microscopy and diffraction and by optical microscopy. The plates were predominantly blade-like in form with numerous rifts and holes. They were thin enough to transmit 80 kV electrons and the thickness of a typical plate was shown to be less than 750 Å. The major surfaces of most of the plates were parallel to (010) planes. This result requires that the orientation relation between the precipitated tin and lead, which we reported earlier, be corrected to: (010)_sn//(111)pb and [001]_sn//[110]_pb. Dislocations and twin and grain boundaries were observed in the plates. Small angle grain boundaries were often formed by the rejoining of parts of a plate which had separated during growth. No extensive overlapping of separated parts of any plate was observed. From this we conclude that there was little deviation from the habit relation during growth and that the lamellae within any given colony are not extensively interconnected. These observations alone do not rule out the possibility of filamentary connections between lamellae which fractured in the dissolution process.