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 QO = 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 QY = 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.

Research highlights
► Directional solidification of intermetallic Ti–46Al–8Nb (at.%) alloy in Y2O3 crucibles leads to contamination of the directionally solidified (DS) samples by oxygen, yttrium and Y2O3 particles.
► The main mechanism responsible for the contamination is diffusion controlled dissolution of the Y2O3 mould in the melt.
► The oxygen content and volume fraction of Y2O3 particles increase with increasing melt temperature and reaction time.
► The kinetic equations for the increase of oxygen content and volume fraction of Y2O3 particle formation over the studied temperature range and reaction time are defined.
► Vickers microhardness measurements can be used as an indirect evidence of the level of the contamination of the DS samples.