INFLUENCE OF RE-MELTING ON PURITY OF TiAlNb ALLOYS

It has been shown that a vacuum induction re-melting of TiAl–base intermetallic alloys in refractory oxide crucibles influences microstructure and final properties of cast parts. The present study investigates reactions between Ti-46Al-7Nb (at. %) melt and 99,99% purity Y2O3 ceramic crucibles. A series of vacuum re-melting experiments (route VIM using furnace BALZERS VSG 02) was performed at melting temperatures of 1680oC and 1730oC. At each temperature a set of exposure times ranging from 5 to 30 min was examined. An interface separating the solidified alloy and the refractory was investigated using a technique of metallographic cross-sections. These cross-sections were characterized by means of optical and scanning electron microscopy. The purity of the solidified alloy was analyzed using inert gas fusion and diffraction techniques. A quantitative assessment of microstructure was based on an Adaptive Contrast Control (ACC) software used to estimate a volume fraction of ceramic phases in the melt. It has been shown that both, the increasing temperature and exposure time result in a non-negligible oxygen pick-up. Moreover, the content of ceramic phases in the solidified intermetallic exhibits the same evolution. Based on these results, recommendations relevant for a cost effective melting and casting route are given. Abstrakt