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Düsseldorf, Germany

Rademacher T.,University of Gottingen | Al-Kassab T.,King Abdullah University of Science and Technology | Deges J.,MPI fur Eisenforschung GmbH | Kirchheim R.,University of Gottingen
Ultramicroscopy | Year: 2011

Addition of ternary elements to the D03 ordered Fe3Al intermetallic phase is a general approach to optimise its mechanical properties. To understand the physical influences of such additions the determination of the probability of site occupancies of these additions on the lattice site and ordering parameters is of high interest. Some common experimental techniques such as X-ray diffraction or Atom Location by Channelling Enhanced Microanalysis (ALCHEMI) are usually applied to explore this interplay. Unfortunately, certain published results are partly inconsistent, imprecise or even contradictory. In this study, these aspects are evaluated systematically by atom probe tomography (APT) and a special data analysis method. Additionally, to account for possible field evaporation effects that can falsify the estimation of site occupancy and induce misinterpretations, APT evaporation sequences were also simulated. As a result, chromium occupies most frequently the next nearest neighbour sites of Al atoms and local ordering parameters could be achieved. © 2010 Elsevier B.V. Source


Dovbenko O.,MPI fur Eisenforschung GmbH | Stein F.,MSI Materials Science International Services GmbH | Palm M.,MPI fur Eisenforschung GmbH | Prymak O.,MPI fur Eisenforschung GmbH
Intermetallics | Year: 2010

Isothermal sections of the Co-Al-Nb system at 800, 1000, 1150, 1200, and 1250°C were experimentally determined by combining results of light-optical as well as scanning electron microscopy, electron-probe microanalysis, and X-ray diffraction measurements with Rietveld analysis of heat-treated alloys and adding some results obtained from solid/liquid diffusion couples. Three polytypes of Laves phases, which are the cubic C15 and the hexagonal C14 and C36 variants, occur as stable phases in the central part of the system. The phase equilibria involving these phases were in the focus of the present study. The composition-dependence of the crystallographic properties of the Laves phases as well as the crystal structure parameters of all other occurring intermetallic phases was investigated. As in case of the binary Co-Nb system, the hexagonal Laves phases C14 and C36 are only stable at off-stoichiometric Nb contents. The preferential site occupations of the Co-, Al-, and Nb-atoms in the hexagonal lattices of the C14 and C36 Laves phase polytypes were determined by Rietveld analyses. For all binary phases except the Nbrich phase Nb3Al, the solubility for the third element was investigated and the extensions of all phase fields were established. Besides the three Laves phases, the most striking features of the Co-Al-Nb phase diagram are the occurrence of the slowly forming L21-type Heusler phase Co2AlNb and the extended phase field of the μ phase. The existence of a μ' phase, which was reported in the literature, is not confirmed. © 2010 Elsevier Ltd. All rights reserved. Source

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