Heterogeneous nucleation of In particles embedded in an AI matrix

Abstract

A hypomonotectic alloy of Al-7 wt% In has been manufactured by melt spinning and the resulting microstructure examined by transmission electron microscopy. The rapidly solidified hypomonotectic Al-7 wt% In exhibits a homogeneous distribution of facetted In particles embedded in an Al matrix, with average In particle sizes of 20–40nm depending upon the wheel speed during melt spinning. The In particles exhibit an orientation relationship with the f.c.c. Al matrix, which can be described as {111}_A1 ‖ {111}_in and (110)_Al II (110)_in, using a face-centred tetragonal representation for In. The In particles have a truncated octahedral shape bounded by {111}_Al and {100}_A1 facets. The equilibrium In particle shape, and therefore the anisotropy of solid Al-solid In and solid Al-liquid In surface energies, have been monitored as a function of temperature during in situ heating in a transmission electron microscope. The anisotropy of the solid Al-solid In surface energy is constant between room temperature and the In melting temperature, with the {100}_Al surface energy being on average about 36% greater than the {111 }_Al ‖ {111}_In surface energy and 25% greater than the other {lll}_Al surface energy. When the In particles melt, the {100}_Al facets disappear and all the {111}_Al surface energies become equal. The {111}_A1 facets do not disappear when the In particles melt, and the anisotropy of the solid Al-liquid In surface energy decreases gradually from 1^.25 to 1 with increasing temperature above the In particle melting temperature. The kinetics of In particle solidification have been examined by heating and cooling experiments in a differential scanning calorimeter. The In particle solidification is nucleated catalytically by the surrounding Al matrix on the {111}_Al facetted surfaces, with an undercooling of 13 K and a contact angle of 27°.