TiAl–Nb melt interaction with pyrolytic boron nitride crucibles

This paper studies the chemical interaction on contact, and the related evolution of composition and microstructure of cast TiAl-based alloy, when using refractory oxygen-free pyrolytic boron nitride (pBN) crucibles/moulds instead of traditional oxide ceramic ones. We extend the research begun with AlN crucibles and reported in [4]. Three melting tests were performed in resistive SiC electro-furnace with Ti–46Al–8Nb (at%) alloy in 6N-purity pBN boat crucibles with fixed melt superheating times of 5, 12 and 25 min at 1670 °C and subsequent quenching with high-purity Ar gas flow. As-cast samples were examined by optical microscopy/metallography, X-ray diffractometry, SEM-BSE, EDX and EBSD micro-analyses, and compared to the initial material. Unlike the AlN it was shown that dissolved BN reacts with the melt, forming Ti2AlN and (Ti,Nb)B micro-precipitates at the expense of selective destruction of the α2-Ti3Al major intermetallic phase in the solidified alloy: Ti3Al + (Nb) + BN → Ti2AlN + (Ti,Nb)B. The key features and temporal dynamics of this interaction are studied.