High Temperature Oxidation Resistance Of TiAl Based Alloy With High Nb Content

The as-cast and heat treated microstructure of B, W, Hf, Y micro-alloyed TiAl basedalloy with high Nb content was investigated. The scale characterization of Surfacemorphologies were studied and oxidation kinetics curves were drawn at900℃in static air forhigh Nb-containing TiAl alloys,then the action mechanism of alloy elements on oxidationprocess were revealed.The research results show that the effects on the refinement of as-cast microstructure ofTi-45Al-8Nb based alloy with adding Hf and Y is more apparent than adding respectively.and the alloy with Hf addition has the most uniform lamellar microstructure and the smallestlamellar space after heat treatment. Adding Y can effective refine the lamellar structure, andgranular Y2O3exist in grains and grain boundaries. Not only formed Y2O3and Hf-rich yttriumoxide with the addition of Hf and Y into the high-Nb alloy,but also a small amount of β(B2)phase appeared.Based on the Ti-45Al-8Nb-0.5Hf-0.1Y to add B(0.2at%), it is found thatlamellar structure refined effectively.The microstructure with B addition were nearly lamellarafter heat treatment,and Y2O3particles, Hf-rich Y2O3phase, Nb, Hf-rich TiB2and few β(B2)phase were present. However, when adding W(0.2at%) in Ti-45Al-8Nb-0.5Hf-0.1Y alloy,there were more β(B2) phase left than the alloy with B(0.2at%) addition, besides it formedsome W, Hf rich yttrium oxide.The result shows that Y micro-alloying TiAl based alloy with high Nb content reducesthe grain size of the oxides and facilitates the formation of dense and uninterrupted A12O3film, which contributes to enhanced adhesion between oxide film and substrate, to someextent it can improve high temperature oxidation resistance of high Nb-TiAl alloy. Adding asmall amount of Hf, could chang the oxide film formation mechanism of high Nb-TiAl alloyand reduce the oxidation rate to a certain extent. With the addition of Hf and Y significantlyincrease the adhesion between substate and oxide layer, and form two A12O3layers whichwere relatively continuous and dense, compared to no addition of high Nb containing TiAlalloy the oxide film is thicker, at the same time, due to the formation of stable Y2O3and Hf rich yttrium oxide, alloy with Hf and Y suffer severe internal oxidation. Adding a trace of Bcan be quite effective in reducing oxidation rate of Ti-45Al-8Nb-0.5Hf-0.1Y, and form fineand compact Al2O3film. To some extent W although could slow down alloy oxidation rate, itleads to oxidation cracking. Oxidation resistance is related with the microstructure andoxidation method. The microstructure combined with adding the Hf, Y, W(B) is nearlylamellar and a small number of β(B2) phase exist in it, these internal defects increased thealloy rapid oxidation channel, accelerated the oxidation rate. This structural state has anadverse effect on the high-temperature oxidation resistance.