Chemical compatibility of a TiAl-Nb melt with oxygen-free crucible ceramics made of aluminum nitride

The problem of uncontrolled oxygen contamination of intermetallic TiAl ingots is considered for the application of crucibles and molds based on traditional oxide ceramics. A synthesized Ti-45.9Al-8Nb (at %) alloy is solidified in alternative oxygen-free crucibles made of high-purity aluminum nitride (99.99% AlN) upon holding at 1670°C for 5, 12, and 25 min and subsequent quenching in a high-purity argon atmosphere. The initial material and the solidified ingots are studied by scanning electron microscopy, optical microscopy, X-ray diffraction, electron-probe microanalysis, and gas-content chemical analysis. The key features of the interaction of the TiAl-Nb melt with AlN ceramics are revealed. Partial thermal dissociation of the crucible material according to the reaction AlN → Al + N and the reaction of atomic nitrogen with the melt lead to the formation of a solid 6.4-μm-thick TiN coating on the ingot surface and provide perfect wettability of the crucible by the melt and easy removal of solidified casting items from the mold. The TiN coating serves as a diffusion barrier that hinders the diffusion of nitrogen and residual oxygen from the pores in the crucible toward the melt. As a result, no oxide particles are detected in the ingots. However, few single microprecipitates of two nitride phases ((Ti,Al) _x N _y , NbN) are detected in the near-bottom region, 300 μm thick, in the alloy after holding at 1670°C for 25 min. The total oxygen contamination in a two-phase α₂ + γ ingot does not exceed 1100 wt ppm, which is 1.5–2 times lower than that obtained in the experiments performed with modern advanced oxide crucibles made of yttrium ceramics Y₂O₃. AlN is shown to be a promising crucible material that can be considered as an alternative to oxide ceramics in the metallurgy of TiAl intermetallics.