Unidirectional solidification and single crystal growth of Al-rich Ti-Al alloys
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Unidirectional solidification and single crystal growth of Al-rich Ti-Al alloys Anne Drevermann1, Georg J. Schmitz1, Günther Behr2, Elke Schaberger-Zimmermann3, Christo Guguschev2 1 ACCESS e.V., Intzestr. 5, 52072 Aachen, Germany 2 Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden, Helmholtzstr. 20, D-01069 Dresden, Germany 3 Foundry Institute, Intzestr. 5, 52072 Aachen, Germany ABSTRACT To investigate the basic mechanical and thermomechanical properties of TiAl alloys with high Aluminium content sufficiently large single crystalline domains are required. To fabricate these samples undirectional solidification in Bridgman Stockbarger furnaces and optical floating zone devices were used. Focus of investigation were grain selection and impurity contamination. Both processes allow for growth of single crystal domains of some millimetres diameter and a few centimetres length. However in a Bridgman Stockbarger furnace the long contact times with the crucible proved detrimental to oxidation issues whereas in the optical floating zone device oxidation is negligible due to containerless processing. INTRODUCTION TiAl alloys with high aluminium content are considered as promising candidates for new materials and components. Compared to today’s conventional gamma-Titaniumaluminides they offer increased oxidation resistance at high temperatures with simultaneously decreased weight [1]. In previous research single crystal material was produced with float zone technique [2] and travelling solvent / flux method [3] for TEM investigations of long-period superstructures in Al-rich TiAl. In this work unidirectional solidification in Bridgman-Stockbarger furnaces and in floating zone devices was used to generate single crystalline domains with diameters of a few millimetres diameter and a few centimetres length for mechanical and thermomechanical testing. The objectives of the present research were (i) optimisation of the grain selection process, (ii) minimisation of stresses and strains leading to cracking of the samples and (iii) minimisation of impurities resulting from crucible materials. Samples were processed in Bridgman-Stockbarger furnaces and floating zone devices. Parameters for the Bridgman-Stockbarger experiments comprised solidification velocity, temperature gradient, crucible material and the use of a self-seed [4]. Furthermore the processing of Al-rich TiAl in a Bridgman-Stockbarger process gives first insights into the problematics of processing in a crucible and provides cooling rates that are nearer to future industrial applications. Floating zone experiments complemented these investigations with a very high temperature gradient and processing without crucible.
EXPERIMENT Several kilograms of master alloys were produced by Vacuum Arc Remelting. Precursor rods for the directional solidification and the floating zone experiments were produced by remelting these alloys in a cold-wall induction crucible and subsequent centrifugal casting into a permanent Niobium mould [5]. For the present ex
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