TRACER DIFFUSION AND CRYSTALLITE GROWTH IN ULTRA-FINE-GRAINED PD PREPARED BY SEVERE PLASTIC-DEFORMATION

After a brief review of diffusion in nanocrystalline materials, the pa per reports on studies of the Fe-59-tracer diffusion in submicrocrysta lline Pd prepared by severe plastic deformation. Interface diffusion o ccurs at relatively low temperatures and is accompanied by a substanti al recovery and grain growth as detected by means of positron lifetime spectroscopy, transmission-electron and optical microscopy, Vickers-h ardness measurements, and calorimetry. Lower limits of the tracer diff usivities in the crystallite interfaces are derived which are similar to or slightly higher than the extrapolated diffusivities in conventio nal grain boundaries. The tracer-diffusion measurements on severely pl astically deformed Pd and comparative studies on Pd after conventional plastic deformation show that the recovery processes and the crystal growth occurring in a main recovery stage at 500 K are triggered by at omic defects in the crystallites which become mobile in this temperatu re regime and by the onset of diffusion in the interfaces. For the cry stallite growth an activation enthalpy H-G = 1.1 eV is obtained.