Solidification of tin droplets embedded in an aluminium matrix

Abstract

The solidification behaviour of tin droplets embedded in an aluminium matrix in a rapidly solidified Al-5 wt % Sn alloy has been investigated by a combination of transmission electron microscopy and differential scanning calorimetry. Detailed transmission electron microscopy shows that rapidly solidified Al-5 wt % Sn consists of about 5 μm diameter columnar aluminium grains, with a fine-scale distribution of 20–300 nm sized tin particles embedded within the aluminium grains, and 100–400 nm sized tin particles at the aluminium grain boundaries. The tin particles exhibit two different orientation relationships with the aluminium matrix and a variety of different faceted shapes: {1 1 1}_Al∥{1 0 0}_Sn and 〈¯2 1 1〉_Al∥〈0 1 0〉_Sn, with the main facet parallel to {1 1 1}_Al, and {1 0 0}_Sn; and {1 0 0}_Al∥{1 0 0}_Sn and 〈0 1 1〉_Al∥〈0 1 1〉_Sn, with the main facet parallel to {1 0 0}_Al and {1 0 0}_Sn.In situ heating in the transmission electron microscope shows that the different tin particle shapes are not affected by heat treatment in the solid state, but change into a truncated octahedral shape bounded by {1 1 1}_Al and {1 0 0}_Al facets when the tin particles melt. The {1 0 0}_Al-liquid Sn interfacial energy is about 9% larger than the {1 1 1}_Al-liquid Sn interfacial energy just above the tin particle melting point, and the {1 0 0}_Al/{1 1 1}_Al interfacial energy anisotropy decreases gradually as the temperature increases above the melting point. Differential scanning calorimeter experiments show that the liquid tin droplets solidify in three stages. Firstly, the larger tin droplets at the aluminium grain boundaries solidify by nucleation on catalytic trace impurities, over a temperature range of 170–140 °C. Secondly and thirdly, the smaller tin particles embedded within the aluminium grains solidify by catalytic nucleation on the {1 0 0}_Al and {1 1 1}_Al facets, over the two temperature ranges of 140–128 °C and 128-115°C. Catalytic nucleation of the solidification of tin takes place at special sites such as steps or dislocations on the {1 0 0}_Al and {1 1 1}_Al facets with contact angles of 55° and 59°.