Melting of Pb particles embedded in Cu–Zn matrices

The effect is reported of interface structure and in particular the existence of steps at the interface on melting and superheating of nanometre-size Pb particles (5–65 nm) embedded in Cu–Zn f.c.c. matrices (of two compositions, Cu–11% Zn and Cu–36% Zn) using high resolution and in-situ transmission electron microscopy and differential scanning calorimetry (DSC). The particles are shown to exhibit a cube on cube orientation relation. The shape of the Pb particles embedded in the Cu–Zn matrix of low Zn concentration (11 at.%) is found to be a truncated octahedron bounded by 111 and 100 facets. The in-situ microscopy and DSC thermograms show direct evidence of superheating of the Pb particles in this alloy only after repeated thermal cycling. High resolution electron microscopy reveals that the particle–matrix interfaces in the as solidified alloys contain steps of heights ranging from 0.42 to 1.9 nm on the {111} planes. The steps vanish after repeated cycling to yield a perfectly faceted equilibrium shape and can be correlated with the appearance of superheating. On the other hand, the Pb particles dispersed in the matrix of higher Zn concentration in the as melt spun state show three distinct size dependent shapes. In all cases, the bounding planes do not reveal prominent steps and show considerable superheating. A change of melting behaviour is observed in the second and subsequent heating with the majority of the Pb particles melting near the bulk melting temperature. The microstructure after the second cycle shows that many of the particles are sheared by fine plates having a twin relation with the matrix which form in the matrix during thermal cycling between 340 and 410°C leading to a prominent stepped Pb–matrix interface.