Melting and freezing behavior of embedded nanoparticles in ball-milled Al–10 wt% M (M=In, Sn, Bi, Cd, Pb) mixtures

Nanometer-sized In, Sn, Bi, Cd, and Pb particles were homogeneously embedded in an Al matrix through ball milling of powder mixtures of pure immiscible elements. The melting and freezing behavior of the embedded nanoparticles were systematically investigated using differential scanning calorimetry. It was observed that the melting temperature as well as the latent heat of fusion of the embedded particles decreased significantly relative to their bulk values. Both the melting point depression and heat of fusion reduction are inversely proportional to the size of the embedded particles. The melting behavior is interpreted using a thermodynamic model and found to depend not only on the particle size but also on the structure and excess enthalpy of the particle/matrix interface. Solidification of the nanoparticles, presumably via heterogeneous solid nucleation at the Al/particle interfaces, exhibited significant undercooling. The degree of undercooling increased with decreasing particle size. A thermodynamic analysis based on the classical nucleation theory indicates that the large undercoolings observed result from the energy barrier for solid nucleation. The effect of particle size on undercooling is attributed to the size dependence of the melting point.