Effect of Y2O3 crucible on contamination of directionally solidified intermetallic Ti–46Al–8Nb alloy

The effect of Y₂O₃ crucible on contamination of Ti–46Al–8Nb (at.%) alloy directionally solidified (DS) in a Bridgman-type apparatus was studied. Directional solidification experiments were performed in dense Y₂O₃ crucibles using different growth rates, melt temperatures and various reaction time between the melt and the crucible. The main mechanism responsible for the contamination of the DS samples is diffusion controlled dissolution of the Y₂O₃ crucible in the melt which leads to an increase of oxygen and yttrium content in γ(TiAl) + α₂(Ti₃Al) matrix and precipitation of non-metallic particles in interdendritic region. Transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD) showed that these particles are Y₂O₃ phase. The oxygen content and volume fraction of Y₂O₃ particles increase with increasing melt temperature and reaction time. The activation energy for increase of oxygen content is calculated to be Q_O = 412.1 kJ/mol and the kinetics of this process is suggested to be controlled by long-range diffusion with the oxygen content exponent of 3. The activation energy for Y₂O₃ particle formation is calculated to be Q_Y = 421.8 kJ/mol and the time exponent is determined to be m = 0.55. Vickers microhardness measurements in lamellar γ + α₂ matrix without Y₂O₃ particles can be used as an indirect evidence of the level of contamination of DS samples with statistically identical α₂–α₂ interlamellar spacing.