TiAl–Nb melt interaction with AlN refractory crucibles

The paper considers contamination of cast TiAl-based alloy, related microstructure evolution and chemical contact interaction when using refractory aluminium nitride crucibles/moulds as an alternative to the traditional Al2O3-, ZrO2- and Y2O3-based oxide ceramics. A series of melting tests has been performed in resistive SiC electro-furnace with Ti–46Al–8Nb (at%) alloy in 99.99% purity AlN boat crucibles with fixed melt superheating times 5, 12 and 25 min at 1670 °C and consequent quenching with high-purity Ar gas flow. As-cast samples were examined by X-ray diffractometry, SEM, SIMS, EDX, EBSD and vacuum fusion analysis of O, N, C and S interstitials content with respect to the initial material. The key features of TiAl–Nb melt interaction with AlN ceramics are revealed. As a result of slow thermal dissociation AlN → Al + N, and the reaction of nascent nitrogen with the melt, a solid continuous TiN-based reaction layer is formed up to 6.4 μm in thickness, together with an enriched Al liquid film between it and the crucible wall. It causes perfect wetting of crucible with the melt and easy removal of the solidified sample. The partial suitability of AlN crucible is restricted by 12 min of the melt superheating from the point of view of invariable (α2 + γ)-microstructure and reasonable contamination of the as-cast samples.