Research On The Effect Of Microstructure On The Oxidation Behavior Of ?-TiAl Alloys

Due to their low density,high elastic modulus,superior creep strength and good corrosion resistance,TiAl-based alloys are regarded as the most promising high temperature structural materials in aerospace and automobile industries.As high temperature structural materials,the oxidation resistance is an important factor determining the application temperature range.TiAl alloys exhibit good oxidation resistance even after thousands hours of hot exposure at 700?.However,the new generation engines which have higher thrusttoweight ratio requires the materials can serve at much higher temperatures,and hence the oxidation resistance of TiAl alloys cannot be fulfilled.When the temperature is higher than 800?,the continuous oxidation layer which protecting the matrix from further oxidation cannot form at the surface of TiAl alloys,and the oxidation resistance of TiAl alloys degrades significantly.Therefore,to enhance the oxidation resistance of TiAl alloys is one of the most critical issues nowadays.Recent years,there have been a large number of researches concerning the oxidation resistance of TiAl alloys,which are mainly focused on coating,alloying modification and halogenations treatments.However,there are only a few attentions have been paid the effect of microstructures on the oxidation behavior of TiAl alloys.Hence in this study two typical TiAl alloys,Ti-48Al-2Cr-2Nb and Ti-42Al-8Nb-0.2W-0.2B-0.1Y,have been selected,the oxidation behavior of which have been systematically studied.In order to investigate the effect of microstructure on the oxidation behavior,thermo-mechanical treatments were performed to adjust the microstructures.The main results are shown below.To clarify the oxidation behavior of typical TiAl alloys with fully lamellar structure and reveal the formation mechanism of oxidation layer,the oxidation kinetics,morphologies,constituents and chemical compositions of oxidation layer have been characterized.It was found that their oxidation kinetics were quite different.In the early stage of oxidation,the exponent of oxidation rate for Ti-48Al-2Cr-2Nb alloy is about 1.With the oxidation continuing,the oxidation kinetic curve roughly followed the parabolic law.However,the oxidation kinetics of Ti-42Al-8Nb-0.2W-0.1Y alloy accorded with the parabolic law from beginning.After 100 h oxidation,the weight gain of Ti-48Al-2Cr-2Nb was 7.7 g/m²,while it was 9.0 g/m² for Ti-42Al-8Nb-0.2W-0.2B-0.1Y alloy at 900?.The oxidation product was a mixture of TiO₂ and Al₂O₃ for both alloys.In the early stage of oxidation,the minority TiO₂ exhibited island-like distribution and the granular Al₂O₃ particles dispersed uniformly on the surface.With the oxidation proceeding,the amount of TiO₂ increased and became the main constituent.In comparison with that of Ti-48Al-2Cr-2Nb alloy,the oxides of Ti-42Al-8Nb-0.2W-0.2B-0.1Y were smaller in size and denser.By means of heat treatments,the Ti-48Al-2Cr-2Nb alloys with duplex and near-lamellar microstructures have been prepared.After oxidation testing at 800?,it was found that the oxidation resistance of the alloy with duplex microstructure is better than that with near-lamellar microstructure.The oxidation layer can be divided into three sub-layers.The inner layer is the matrix,the outer layer mainly consisted of TiO₂particles and the middle layer was continuous mixture of Al₂O₃ and TiO₂.The oxidation kinetics of both microstructures was consistent with the parabolic law.It was deduced that in the steady state of oxidation,the diffusion of Ti and Al ions across the oxidation layer was the main factor controlling the oxidation.Two fully lamellar TiAl alloys with the same composition?Ti-42Al-8Nb-0.2W-0.1Y?but with different colony size has been prepared by thermo-mechanical treatments,and then oxidized at 900?.It was found that the larger colony size,the worse oxidation resistance.The alloy with a colony size of 85?m behaved impressive oxidation resistance due to the low oxidation rate and good adhesiveness of oxidation layer.Besides,near-gamma Ti-42Al-8Nb-0.2W-0.2B-0.1Y alloys with three different grain sizes?50?m,23?m,15?m?are fabricated to examine the grain size effect on the oxidation rates of high Nb-TiAl alloys.The results showed that,the oxidation kinetics of all the three specimens accorded with the parabolic law.After 100 h oxidation,the weight gain is 6.75 g/m²,8.33 g/m² and 6.48 g/m²,respectively for the three specimens.The oxides of the alloy with relatively small grain size are finer and denser.Therefore,it can be concluded that the high Nb-TiAl alloys with smaller grain size possessed better oxidation resistance at high temperatures.