Decomposition of a supersaturated solid solution in thin foils of Sn-Bi alloys

The morphology of the surface and the microstructure of the cross section of rapidly solidified foils of the Sn-Bi alloys have been investigated. The initial observations have been conducted 5–10 min after preparation of the foil. The formation of thin needle-like precipitates immediately after solidification has been found both on the surface and inside the foil. The mechanism of the formation of these precipitates is suggested. After continuous decomposition, the decomposition by the discontinuous mechanism was observed in the foils. When the boundary of the decomposition zone passes through needle-like precipitates of Bi, their thickness increases. For comparison, the rapidly solidified foils after homogenizing and quenching in air have been investigated. A distinction is that upon quenching in air the process of discontinuous decomposition begins within some time after the commencement of observations and occurs simultaneously with the continuous decomposition, which consists in the formation of few globural inclusions. The experimentally established (upon observations in situ) rate of growth of cells upon discontinuous decomposition in rapidly solidified and air-quenched foils of the Sn-8 at % Bi alloy varies within 1–4 × 10−9 m/s. The results of X-ray diffraction analysis confirm the occurrence of discontinuous decomposition in the foils.