Synthesis and characterization of ball-milled nanocrystalline fcc metals

Nanocrystalline fcc metals (Al, Cu, Ni, Pd, Rh, Ir) have been prepared by ball milling. The development of the microstructure is investigated by x-ray diffraction, differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The final grain sizes range from 6 to 22 nm and scale with the melting point and the bulk modulus of the elements: metals with higher melting point and bulk modulus have a smaller final grain size. From this a general relation between the deformation mechanism during ball milling and the ultimate grain size achievable by this technique is inferred. With decreasing grain size the lattice strain is enhanced and deformation enthalpies of up to 40% of the heat of fusion are stored in the material. The contributions of the lattice strain and of the excess enthalpy of the grain boundaries to the stored enthalpies are critically assessed. The kinetics of grain growth are investigated by thermal analysis. In this paper the activation energy for grain boundary migration is derived from a modified Kissinger analysis and estimates of the grain boundary enthalpy are given