Interactions between Nb-silicide based alloy and yttria mould during directional solidification

Pure yttria moulds have been prepared for directional solidifications of Nb–22Ti–16Si–6Hf–2Al–2Cr (at. %) alloy with a withdrawal rate of 1 × 10–4 m s–1 at 1850 °C and 1900 °C. This study demonstrates that a mild interface reaction occurred between the yttria mould and hafnium, the most reactive element in the alloy. A sequential reaction layer of HfO2 and Y2O3 was formed at metal–mould interface, and its thickness depended on both heating temperature and holding time. HfO2–Y2O3 inclusions were dispersed in the metal matrix, and the majority of them concentrated around the solid–liquid interface rather than in steady state growth region. Some inclusions were merged with each other. The reaction mechanism between Nb-silicide based alloy and yttria mould also has been described.

► Yttria mould was prepared for directional solidification of Nb-silicide based alloys.
► A mild interface reaction occurred between hafnium and yttria moulds.
► The reaction extent depended on both the heating temperature and holding time.
► The majority of inclusions concentrated around the solid–liquid interface.
► Yttria is found to be a promising suitable refractory material for Nb-silicide based alloys.