Melting and Possible Amorphization of Sn and Pb in Ge/Sn and Ge/Pb Mechanically Milled Powders

Mixtures of Ge-Sn and Ge-Pb powders were ball-milled to form a fine dispersion. After 32 h of milling the average diameter of the hard Ge particles embedded in the Sn (or Pb) matrix was about 10 nm. As the Ge concentration was increased in each system, the melting point, T_M, and the enthalpy of fusion, ΔH_M, of Sn (or Pb) decreased. Only small changes in ΔT_M and ΔH_M were observed after heating cycles in the DSC to above the melting point. The melting endotherm measured by DSC disappeared for Ge-rich compositions (88 and 95 vol. % Ge for Ge-Sn; 93.5 vol. % Ge for Ge-Pb). It is suggested that atomic disorder/melting is nucleated at the Ge/Sn (or Ge/Pb) interfaces and the melting point and enthalpy of fusion decrease as the interfacial area increases. When the Ge volume reaches a value where essentially all the Sn (or Pb) atoms are adjacent to the Ge particle surfaces, the Sn is in a disordered—perhaps amorphous—state such that no melting transition is observed.