Interactions between high Nb containing TiAl alloy and yttria facecoat during investment casting

Using X-ray diffraction, hardness evaluation and scanning election microscopy, the interaction
between a Ti–46Al–8Nb–B alloy and an yttria facecoat was analysed after investment casting with
different cast bar diameters. Results showed that the interaction between an yttria facecoat and
alloy can cause the oxygen in the facecoat to diffuse into the alloy to form an interaction layer at the metal/shell interface. Aluminium was observed to evaporate at the alloy surface, and the remaining Al ions at the interaction layer interacted with oxygen to form Al2O3 whiskers.Meanwhile, some Nb rich precipitates were found at the metal/shell interface. By increasing the
casting bar diameters, the interaction layer thicknesses were increased with the associated growth of Nb rich precipitates