Abstract
The literature on the processing, microstructure and oxidation properties of TiAl, TiAlCr and TiAlV-based intermetallics materials is briefly discussed. The microstructure and oxidation performance of seven gamma TiAl based alloys was studied using thermogravimetric analysis, electron probe micro-analysis, scanning electron microscopy and X-ray diffractometry. Conventional melt solidification, by skull melting in a water-cooled, copper hearthwas used to produce binary and ternary TiAl-based intermetallic alloys. The following alloys were studied: Ti-50Al, Ti-55Al, Ti-40Al-10Cr, Ti-50Al-10Cr, Ti-50Al-20Cr, Ti-49Al-2V and Ti-45Al-5V (at.%). The microstructure of the alloys was studied prior to oxidation. The alloys all contained the gamma-phase to some degree in the as-formed condition with the exception of the Ti-50Al-20Cr. This alloy did appear to transform at high temperatures to a Laves + gamma structure. In the binary alloys, we found that at lower temperatures, a larger proportion of the gamma-phase improved oxidation resistance but at higher temperatures this was not the case. This was attributed to the presence of nitrogen during oxidation favouring the formation of titania nodules within the oxide structure. In the alloys with ternary additions of Cr, we found that alloys favouring the Laves phase showed extremely good oxidation resistance at temperatures up to 1173K. The alloy containing a microstructure of r + Laves + (betaCr) performed as well as the alloy containing the gamma-phase. Where the Laves phase was not present, severe oxidation was observed. In the alloys with ternary additions of V, we found that vanadium additions had a detrimental effect on oxidation. With only small additions, oxidation similar to that of the binary alloys was observed. With 5 at.% vanadium, breakaway oxidation was observed, attributed to the increased amount of in the microstructure. Evidence for the Z-phase was also observed in all alloys at 1173K although this was sometimes very weak. Magnetron sputter deposition was used to produce a Ti-50Al-10Cr coating on a Ti-50Al substrate which was subjected to cyclic oxidation in air at 1173K, together with a reference sample of Ti-50Al. Where the coating was continuous, the oxidation of the sample was reduced. However the coating had failed in places and severe oxidation occurred at these sites. Further work must be done to improve the adhesion and coverage of these coatings but they appear to be good candidates for further study.