The impact of particle morphology on the melting characteristics of matrix-embedded Pb nanocrystals

The melting behaviour of Pb nanoparticles embedded in an Al matrix (0.5 at.% Pb) synthesized by high-energy ball milling is investigated. By differential scanning calorimetry it is shown that, as the material is cycled repeatedly through the melting transition, the particles melt reversibly at temperatures T-f below the melting temperature of bulk Pb. However, after a suitable heat treatment the nanoparticle morphology is modified such that a significant fraction of the Pb particles now melts at increased temperatures T-f > T-f(0). Transmission electron microscopy gives strong indications as to the origin of the change in melting behaviour; strains in the as-prepared material do not recover during cycling through T-f, and resolidification of the molten particles in the strained matrix brings the particles back to the initial structure with curved interfaces and small, but noticeable, misalignments relative to the cube-on-cube orientation relationship. Annealing leads to strain relief and to faceted Pb particles. The relation between strains and particle morphology is discussed.