Thermal extraction of Pb from the bulk to the surface in Al doped with Pb

Abstract

According to most recent data the solubility of Pb in Al is extremely small (1). In a systematic investigation into the influence of material purity and specific impurities on grain boundary motion we found (2-4) that an addition of 20 ppm Pb to pure Al drastically affects grain boundary mobility. The temperature dependence of grain boundary mobility in AL20ppmPb revealed that there is a temperature, at which both the migration activation enthalpy and preexponential mobility factor rise steeply. We intepret this result as a wetting phase transition in the grain boundary and attribute the unstable motion of grain boundaries in the low temperature regime to a boundary motion dragged by small drops of liquid lead (3). On this basis, the temperature dependence of grain boundary mobility proves that even at concentrations as low as 20 ppm Pb is not soluble in Al, but contained as small inclusions (particles) which are liquid at temperatures higher than the melting point of Pb. This causes interesting phenomena and side effects, when such a material is subjected to a heating cycle as will be reported and discussed in this paper.