Microstructure and grain growth of nanosized materials

An anomalous dependence of the lattice parameter on the crystallite size of nanocrystalline ball-milled powders of metals was observed: lattice contraction followed by lattice expansion with decreasing crystallite size. These data were determined by application of detailed Xray diffraction measurements. To this end, the lattice parameters of all the metals were accurately determined by correcting for influences of stacking faults in FCC metals as well as by correcting for instrument-related aberrations. The non-monotonic variation of the lattice constant was proposed to be a result of two competing mechanisms: interface-stress induced contraction versus expansion as a result of the stress field generated at the crystallite boundary due to the increased excess free volume in the crystallite boundary upon decreasing crystallite size.