Abstract
Non-isothermal oxidation is one of the important issues related to the safe application of high-temperature titanium alloys, so this study focuses on the non-isothermal oxidation behavior and mechanism of near-α titanium alloys. The thermogravimetry-differential scanning calorimetry (TGA/DSC) method was used to study the non-isothermal oxidation behavior of TA29 titanium alloy heated from room temperature to 1450 °C at a heating rate of 40 °C/min under pure oxygen atmosphere. The results show that non-isothermal oxidation behavior can be divided into five stages, including no oxidation, slow oxidation, accelerated oxidation, severe oxidation and deceleration oxidation; for the three-layer TiO2 scale, Zr, Nb, Ta are enriched in the intermediate layer, while Al is rich in the inner layer and Sn is segregated at the oxide-substrate interface, which is related to their diffusion rates in the subsurface α case. The oxidation mechanism for each stage is: oxygen barrier effect of a thin compact oxide film; oxygen dissolution; lattice transformation accelerating the dissolution and diffusion of oxygen; oxide formation; oxygen barrier effect of recrystallization and sintering microstructure in outer oxide scale. The alloying elements with high valence state and high diffusion rate in α-Ti are favorable to slow down the oxidation rate at the stage governed by oxide formation.
Highlights
Compared with nickel-based superalloys, high temperature near-α titanium alloys have advantages in specific strength, low-cycle fatigue property and fatigue crack propagation resistance in the range of 500~600 ◦ C, based on which, new advanced aeroengines are in urgent need of the near-α titanium alloys for the purpose of weight reduction and thrust-weight ratio increase [1,2,3,4].The typical near-α titanium alloys in the world are IMI834 from UK, Ti-1100 from USA, BT36 and BT41 from Russia, among which IMI834 has been successfully applied in EJ200, TRENT series, PW305 and PW150 engines [2]
The mass-gain curve, mass-gain rate curve and heat flow curve of TA29 titanium alloy during non-isothermal oxidation are shown in Figure 2, among which Figure 2b is a local enlarged diagram of Figure 2a
The oxide scale on TA29 titanium alloy after non-isothermal oxidation is a three-layer structure of TiO2
Summary
Compared with nickel-based superalloys, high temperature near-α titanium alloys have advantages in specific strength, low-cycle fatigue property and fatigue crack propagation resistance in the range of 500~600 ◦ C, based on which, new advanced aeroengines are in urgent need of the near-α titanium alloys for the purpose of weight reduction and thrust-weight ratio increase [1,2,3,4].The typical near-α titanium alloys in the world are IMI834 from UK, Ti-1100 from USA, BT36 and BT41 from Russia, among which IMI834 has been successfully applied in EJ200, TRENT series, PW305 and PW150 engines [2]. Compared with nickel-based superalloys, high temperature near-α titanium alloys have advantages in specific strength, low-cycle fatigue property and fatigue crack propagation resistance in the range of 500~600 ◦ C, based on which, new advanced aeroengines are in urgent need of the near-α titanium alloys for the purpose of weight reduction and thrust-weight ratio increase [1,2,3,4]. TA29 titanium alloy is a near-α titanium alloy developed by China for the service characteristics and performance requirements of the advanced aeroengines with high thrust-weight ratio. Owing to its excellent thermal strength, good fracture toughness, plasticity and thermal stability, TA29 titanium alloy could be applied in disks, blades, blisks and casings of high-pressure compressors under long-term service temperature up to 600 ◦ C, indicating that TA29 alloy is a new high-temperature titanium alloy with good application potential in aeroengines [3,4]. The research on high temperature oxidation of titanium alloys is mostly concerned with their long-term isothermal oxidation behavior at service temperature or slightly higher than service temperature [7,8,9,10]
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