Regensburg 2007 – scientific programme

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MM: Fachverband Metall- und Materialphysik

MM 12: Growth

MM 12.6: Talk

Monday, March 26, 2007, 17:30–17:45, H6

Kinetics of grain growth in nanocrystalline Fe at low annealing temperatures — •Heiko Paul and Carl Krill — Institute of Micro and Nanomaterials, Ulm University, D-89081 Ulm

At low annealing temperatures the rate of grain growth observed in nanocrystalline materials can be orders of magnitude smaller than would be expected from an extrapolation of high-temperature growth recorded in coarse-grained counterparts. One possible explanation for this observation rests on the drag force exerted on moving grain boundaries by triple junctions (TJ), the migration rate of which is independent of the average grain size ⟨ R⟩ [1]. Since the curvature-driven speed of a grain boundary (GB) varies inversely with ⟨ R⟩, there must be a critical grain size below which TJ migration becomes the rate-controlling step for grain growth. Likewise, if the activation enthalpy for triple-junction migration is higher than that of grain boundaries, then low annealing temperatures should favor TJ-controlled growth. We have searched for the latter by carrying out long-term in situ investigations of coarsening in high-purity nanocrystalline Fe at temperatures as low as 470^∘C. Grain growth was measured in a laboratory x-ray diffractometer equipped with a high-temperature chamber and position-sensitive detector. An automated analysis routine was developed to extract the average grain size and microstrain from wide-angle diffraction scans, yielding isothermal grain-growth curves of unprecedented accuracy and duration for a nanocrystalline specimen.

[1] U. Czubayko, V. G. Sursaeva, G. Gottstein and L. S. Shvindlerman, Acta mater. 46 (1998) 5863–5871.