Microstructure and composition of cast Ti–47Al–2Cr–2Nb alloys produced by yttria crucibles

Abstract

Abstract In this work, second-generation γ-TiAl alloys with chemical compositions of Ti–47Al–2Cr–2Nb (at.%) with an initial oxygen concentration of 0.05 wt% were melted with different melting parameters (temperatures and times) and allowed to solidify in yttria crucibles. The microstructure and composition of the cast alloys were investigated, using back-scattered scanning electron microanalysis (BSE), transmission electron microscopy (TEM), energy dispersive spectrometry (EDS) and chemical composition analysis. The microstructure observations and composition analysis showed that β phase was the primary solidification phase and there were three microsegregations in the metal matrix, i.e. single γ phase, remaining β phase and yttria inclusions. There was not a clear change in the morphology of β phase and the lamellar spacing as melting parameters were changed, while the size and morphology of the yttria inclusions depend on melting parameters. Lower melt temperature and shorter interaction time result in the formation of near-equiaxed yttria particles. Higher melt temperature and longer interaction time lead to the formation of coarser and elongated ribbon-like particles. The oxygen content and the volume fraction of yttria inclusions were found to proportionally increase with increasing melt temperature and interaction time. A kinetic view of the metal–crucible interactions and transport processes was given. The activation energy for increase of oxygen content was calculated to be Q o = 366.8 kJ/mol and a rate equation for such increase during melting was provided, which can be used to predict the oxygen pick up of TiAl melts. Decreasing melting temperatures and reducing melting times help to reduce the alloy contamination.