THE SUPERHEATING AND THE CRYSTALLOGRAPHY OF EMBEDDED PB PARTICLES IN FCC AL, CU AND NI MATRICES

Al-1.4 at.%Pb, Cu-3.2 at.%Pb and Ni-3 at.%Pb have been rapidly solidified to obtain a nanoscale dispersion of Pb particles embedded within the higher melting point Al, Cu and Ni matrices. Each Pb particle in Al and Cu matrices is a single crystal and faceted. The shape of the Pb particles in Al and Cu is a truncated octahedron bounded by 111 and 100 facets. The second phase Pb particles in Ni are not well faceted and the shape is a roughened truncated octahedron. A well defined orientation relationship is observed between the second phase Pb particles and the matrices. The melting behaviour of these particles is studied using a differential scanning calorimeter. The majority of the second phase particles in all the matrices start melting below the equilibrium melting point of the Pb with a broad endothermic peak. In Al and Cu matrices, a few Pb particles are seen to be superheated while in the Ni matrix superheating of the Pb particles is not observed. It is shown that the observed difference in the melting behaviour is not due to the size dependent behaviour of melting of the second phase. Our results suggest that the shape of the second phase particle strongly influences the superheating.