Hamburg 2001 – scientific programme
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M: Metallphysik
M 36: Nanoskalige Materialien IV
M 36.4: Fachvortrag
Friday, March 30, 2001, 11:45–12:00, S5.3
Ordering of nanocrystalline Fe3Si, FeAl, and NiAl by vacancy migration — •K. Reimann, L. Pasquini, A. Rempel, and H.-E. Schaefer — Universität Stuttgart, Institut für Theoretische und Angewandte Physik, Pfaffenwaldring 57, 70550 Stuttgart
The nanocrystalline intermetallic alloys FeAl, NiAl, and Fe3Si (with 5 % Nb addition) were prepared by cluster condensation or ball milling resulting in a crystallite size between 20 and 40 nm. The crystallites become chemically disordered by plastic deformation, whereas the disordering is stronger for the compounds with lower ordering energy. Furthermore, in the strongly ordered compound NiAl more vacancies are formed than anti-site atoms. The range of homogenity for the B2 phase could be extended for FeAl and NiAl to Al-rich compositions which may be stabilized by the small crystallite size. The interfaces (grain boundaries and surfaces) do not inhibit the ordering significantly in these compounds with relative high ordering energies, i.e., high critical ordering temperatures. The reordering by annealing can be well described by a vacancy mediated mechanism. In FeAl and NiAl vacancies are introduced during the preparation whereas in Fe3Si thermal vacancies have to be formed as detected by positron annihilation studies. In nanocrystalline FeAl, which is almost completely disordered, the vancancy migration energy decreases by about 0.6 eV with respect to 1.7 eV in the fully ordered state. In NiAl the vacancy migration is slower than in FeAl. In Fe3Si full D03 order sets in when thermal vacancies are formed. A comparision with diffusion data shows that Si atoms control the ordering kinetics. This work was supported by the EC (INTAS99-1216) and the DFG (Scha428/25-1)