Mechanical alloying of immiscible elements in the Fe-Mg system

Mechanical alloying of immiscible elements in the Fe-Mg system (less than or equal to32 at % Mg) is achieved by grinding in a planetary ball mill. The process is studied by x-ray diffraction, Mossbauer spectroscopy, and magnetic measurements. The results attest to the formation of supersaturated solid solutions of Mg in alpha-Fe. The highest Mg content of the solid solutions is 5-7 at %. Mossbauer results are used to evaluate the changes in the hyperfine magnetic field at Fe nuclei associated with the presence of one Mg atom among the nearest neighbors and next nearest neighbors of Fe in the solid solutions: DeltaH(1) = -1760 kA/m and DeltaH(2) = -800 kA/m, respectively. Increasing the Me, content of the starting mixture reduces Mg solubility in Fe. Thermodynamic analysis indicates that the driving force for the formation of solid solutions may be associated with the excess energy of coherent interfaces in the Fe-Mg nanocomposite resulting from mechanical alloying. The elastic strain arising from the lattice mismatch between Fe and Mg facilitates incorporation of Mg into a-Fe. Above a certain Mg content, no coherent interfaces are formed, and the thermodynamic driving force for Mg dissolution disappears. As a result, the system becomes immiscible.