FORMATION OF THERMODYNAMICALLY UNSTABLE SOLID-SOLUTIONS IN THE CU-CO SYSTEM BY MECHANICAL ALLOYING

Cu-Co has been chosen as a model system to study phase formation durin g mechanical alloying in systems with components exhibiting a positive heat of mixing. Phase formation is monitored by scanning electron mic roscopy, x-ray diffraction, and calorimetric measurements. Although th e occurrence of a single fcc Bragg pattern cannot definitely prove the alloy formation on an atomic scale, the quantitative agreement of the heat release upon decomposition of the alloy with the heat of mixing, calculated from the available thermodynamic data of the system, is re garded as clear evidence for the formation of an fcc solid solution. A s a mechanism of alloy formation the dissolution of Cu and Co particle s during milling is discussed. It is proposed that the formation of a homogeneous solid solution is energetically favored if the particle si ze is reduced below a critical value, which amounts to about 1-2 nm, d epending on the composition of the alloy and the temperature. The driv ing force is provided by the chemical contribution of the Cu/Co interf ace enthalpy and by the high configurational entropy of the randomly d istributed atoms in the solid solution.