Abstract (eng)
Nickel aluminides, like γ’-Ni3Al and β-NiAl, are used in high temperature alloys, which are applied in aircraft and space vehicle engines, power generating turbines and marine propulsion turbines. Nickel aluminides are nowadays used in terms of nickel-base superalloys which usually contain high amounts of chromium, as well as several other transition metals and main group elements, making these materials rather complex. Since applied parts of nickel-base superalloys are moulded in complicated procedures at elevated temperatures, they are very expensive. A successful application of these materials thus requires suitable and cost-effective joining techniques such diffusion brazing (TLP). During this process two substrates with a filler material inserted between them are annealed for a short time. As filler materials, alloys with a lower melting temperature than the two component parts, but similar chemical compositions, are applied, to provide optimal diffusion during the annealing process.
In this work, germanium has been chosen, as melting point depressing element, as it forms deep eutectics with Al and Ni. The aim of the present work was to investigate the ternary phase diagram Al-Ge-Ni, in order to optimize the filler composition and to better understand the interface reactions occurring during the brazing process.
The ternary phase diagram was studied by a combination of optical microscopy, X-ray powder diffraction, Electron probe microanalysis and Differential thermal analysis. Two partial isothermal sections at 400 °C and 700 °C and vertical sections at 10, 20 and 35 at.% nickel as well as one vertical section at constant Al:Ni ratio 1:3 were investigated. The results were hardly comparable with prior results of Yanson et al [46]. At least four new ternary phases have been found to exist in the system. Two ternary intermetallic phases were determined with single crystal diffraction and designated with the empirical formula AlxGe2-xNi (NiGe2-type, τ1) and AlyGe9-yNi13±x (Ga3Ge6Ni13-type, τ3). Furthermore, one phase was identified from its powder diffraction pattern, using DIFFRACplus TOPAS software, and designated as AlxGe2-xNi (CaF2-type, τ4). One additionally phase was determined by EPMA and powder XRD at the composition Al67.5Ge18.0Ni14.5 (τ2).
Moreover phase relations and solubility limits were studied in detail for the Ni-poor part of the system. A partial liquidus projection and a partial reaction scheme were examined. In total twelve invariant reactions (1 degenerated eutectic, 3 peritectic reactions, 7 transition reactions, 1 maximum) were found in the Ni-poor part.
Furthermore, fundamental wetting- and diffusion-brazing experiments were performed with two selected brazing alloys, using AlNi and AlNi3 as substrate materials, at processing temperatures of 900 and 1000 °C, respectively.