Dresden 2009 – wissenschaftliches Programm

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MM: Fachverband Metall- und Materialphysik

MM 11: Growth

MM 11.1: Vortrag

Montag, 23. März 2009, 14:45–15:00, IFW D

Tales of the abnormal: nanocrystalline grain growth at low temperatures — •Heiko Paul and Carl E. Krill III — Institute of Micro and Nanomaterials, Ulm University, Ulm, Germany

Unlike in coarse-grained polycrystalline materials, which manifest grain-boundary-controlled growth kinetics, nanocrystalline specimens are presumed to exhibit a coarsening behavior that is affected by their high number density of triple junctions and quadruple points. The consequences of such microstructural features on the nature of grain growth are potentially complex and still not well understood, but most current grain-growth models predict that the rate-controlling mechanism depends on the average grain size, with the dependence extending to larger sizes at lower annealing temperatures. Using a combination of x-ray diffractometry and electron microscopy, we have investigated the low-temperature evolution of microstructure in nanocrystalline Fe prepared by ball milling. The initial stage of grain growth agrees qualitatively with models positing a transition from triple-junction-controlled boundary migration to the standard grain-boundary-controlled case, but a quantitative examination of various moments of the grain-size distribution points to a different explanation: the samples appear to coarsen abnormally at first but to resume a normal growth mode at larger average grain sizes. The striking similarity of this behavior to the room-temperature growth observed in nanocrystalline Pd [1] suggests that abnormal grain growth may not be so “abnormal” after all!

[1] M. Ames et al., Acta mater. 56 (2008) 4255–4266.