Papers by Author: Jens Ribbe

Abstract: Grain boundary (GB) diffusion of 59Fe in high purity polycrystalline copper was measured using the radiotracer technique and precision parallel sectioning in an extended temperature interval. The results of diffusion measurements below 900 K are consistent with Harrison's C kinetics and yield the GB diffusion coefficient of Fe in Cu with the pre-exponential factor of 5.610-6 m2/s and the activation enthalpy of 121 kJ/mol. Unexpectedly strong GB segregation of Fe in Cu hindered reliable determination of the Fe diffusivity in the B kinetics. Additionally, unconventional penetration profiles were measured for GB diffusion of 59Fe in Cu at high temperatures. Molecular dynamics simulation with the literature Finnis-Sinclair type interatomic potentials was performed to shed light into the observed features. A strong effect of the Fe coverage on GB structure and kinetics in pure Cu is predicted at increased temperatures above 900 K.

Abstract: Kinetic characteristics of internal interfaces in ultrafine grained (UFG) materials represent an aspect that is central to their potential applications. Interface diffusion is strongly affected by ultrafine crystallinity and incorporated defects. In the present study the radiotracer technique was applied for measuring grain boundary (GB) diffusion of Ni in UFG Cu-1wt.%Pb alloy. The UFG specimens were prepared by equal channel angular pressing at room temperature. GB diffusion was investigated at room temperature in Harrison’s C kinetic regime under conditions in which Ni diffusion - even along general high-angle GBs in pure Cu - would hardly be measurable by the mechanical sectioning (as the corresponding penetration depths would be less than 100 nanometers). The present study highlights the existence of ultra-fast diffusion paths in severely deformed material. An atypical time dependence of the room-temperature diffusion data indicates a quite involved nature of these “fast” diffusion paths in the Cu-1wt.%Pb alloy.

Abstract: The radiotracer technique was applied for measuring grain boundary diffusion of Ni in ultrafine grained (UFG) copper materials with different nominal purities and in a Cu—1wt.%Pb alloy. The UFG specimens were prepared by equal channel angular pressing at room temperature. The stability of the microstructure was studied by focused ion beam imaging. Grain boundary diffusion of the 63Ni radioisotope was investigated in the temperature interval from 293 to 490K under the formal Harrison type C kinetic conditions. Two distinct short-circuit diffusion paths were observed. The first (relatively slow) path in the UFG materials corresponds unambiguously to relaxed high-angle grain boundaries with diffusivities which are quite similar to those in the respective coarse-grained reference materials. The second path is characterized by significantly higher diffusivities. The experimental data are discussed to elucidate the contribution of nonequilibrium grain boundaries in the deformed materials. Alternative contributions of other shortcircuit diffusion paths cannot be ruled out, particularly for the Cu-Pd alloy.