Effect of Ge on the heterogeneous nucleation of Pb solidification by Al

Abstract

Melt-spun hypermonotectic Al-5wt%Pb alloys containing ternary additions of 0–2 wt% Ge have been examined by a combination of transmission electron microscopy and differential scanning calorimetry, in order to determine the effect of Ge on the heterogeneous nucleation of Pb solidification by Al. The melt-spun alloy microstructures consist of 5–150 nm diameter Pb particles embedded in an Al(Ge) matrix, with additional Ge precipitates at Ge concentrations greater than or equal to 1.5 wt%. At Ge concentrations less than or equal to 1 wt%, Pb particle solidification is catalytically nucleated with a cube-cube orientation relationship by the {111} planes of the sorrounding Al(Ge) matrix. As the Ge concentration in the Al(Ge) matrix increases from 0 to 1 wt%, the lattice disregistry between the Pb particles and the surrounding Al(Ge) matrix decreases slightly, from 18% to 17%, but the nucleation undercooling increases linearly from 22 to 43 K, and the cosine of the contact angle θ decreases linearly from 0.93 to 0.84. In other words, chemical factors dominate over structural factors in determining the catalytic efficiency of Al(Ge) as a heterogeneous nucleant for the solidification of Pb. At Ge concentrations greater than or equal to 1.5 wt%, the solidification of Pb particles not in contact with Ge precipitates is catalytically nucleated by the {111} planes of the sorrounding Al(Ge) matrix, at an undercooling which increases from 53 to 60 K as the Ge concentration increases from 1.5 to 2.0 wt%. However, the solidification of Pb particles in contact with Ge precipitates is catalytically nucleated by the Al(Ge) matrix-Ge precipitate interfaces at a constant undercooling of 45 K.