Formation of supersaturated solid solutions in the immiscible Ni-Ag system by mechanical alloying

The mechanical alloying process in the immiscible Ni-Ag system with a positive heat of mixing was investigated by x-ray diffraction and differential scanning calorimetry. High energy ball milling of mixed elemental powders, with nominal composition Ni_xAg_100-x (x = 95, 90, 70, 50, and 30), results in the formation of mixtures of supersaturated, nanocrystalline Ni-rich and Ag-rich solid solutions. The solubilities and final grain sizes of these phases depend on the nominal composition of the powder. The maximum solubilities were determined using Vegard's law to be 4.3 at. % Ni in Ag and 6.6 at. % Ag in Ni for samples milled at room temperature. The effect of milling temperature on mechanical alloying was examined in the range -195 to 250°C. Lower temperature milling leads to a larger solubility of Ni in the Ag-rich samples, up to 7.1 at. % for the Ni₃₀Ag₇₀ composition. Indications for the existence of a concentrated solid solution (Ni₃₆Ag₆₄-Ni₄₄Ag₅₆) were also found. Milling at higher temperatures leads to lower solubilities. A study of the thermal stability of supersaturated Ag-rich and Ni-rich phases shows that milling at high temperature can be understood in terms of a competition between mechanical mixing and thermal decomposition. At room temperature, nonequilibrium vacancies are responsible for decomposition. The results give new insight into the general characteristics of the mechanical alloying process in thermodynamically unstable systems.