Berlin 2008 – scientific programme

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MM: Fachverband Metall- und Materialphysik

MM 13: Growth

MM 13.3: Talk

Monday, February 25, 2008, 15:15–15:30, H 0111

Kinetics of grain growth in nanocrystalline Fe at low annealing temperatures — •Heiko Paul and Carl E. Krill III — Institut für Mikro- und Nanomaterialien, Universität Ulm, Albert-Einstein-Allee 47, D-89081 Ulm

The migration rates of curved grain boundaries (GB) and triple junctions (TJ) manifest different dependencies on the average grain size ⟨ R⟩, implying that there may be a critical grain size ⟨ R⟩ _c below which the overall growth kinetics are controlled by TJ migration. If the activation enthalpy for TJ migration is significantly higher than that for GB migration, then a linear growth law is expected when ⟨ R⟩ < ⟨ R⟩ _c, whereas the well-known parabolic growth behavior should still obtain when ⟨ R⟩ > ⟨ R⟩ _c. To test this hypothesis, we carried out long-term in situ annealing experiments on highly pure samples of nanocrystalline Fe. Grain growth was measured in a laboratory x-ray diffractometer (XRD) equipped with a heating stage and a position-sensitive detector. By fitting linear and parabolic growth models to isothermal grain-growth curves recorded at temperatures between 409^∘C and 482^∘C, we were able to extract activation enthalpies for initial and late-stage coarsening, observing a decrease suggestive of a transition between TJ-controlled and GB-controlled growth. Additional microstructural analysis was performed to verify the average grain-size values calculated from the XRD measurements.