MechanicaHy driven alloying and grain size changes in nanocrystalline Fe-Cu powders

Highly supersaturated nanocrystalline Fe,Culcc-X alloys (lO<n<95) have been prepared by mechanical alloying of elemental crystalline powders. The development of the microstructure is investigated by x-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. The results are compared with data for ball-milled elemental Fe and Cu powders, samples prepared by inert gas condensation, and sputtered films. The deformation during milling reduces the grain size of the alloys to 6-20 nm. The final grain size of the powders depends on the composition of the material. Single-phase fee alloys with x<60 and single-phase bee alloys with ~80 are formed even though the Fe-Cu system exhibits vanishingly small solid solubilities under equilibrium conditions. For 6O<x<80, fee and bee solid solutions coexist. The alloy formation is discussed with respect to the thermodynamic conditions of the material. The role of the large volume fraction of grain boundaries between the nanometer-sized crystals, as well as the intluence of internal strains and stored enthalpies introduced by ball milling, is critically assessed.