| The density of TiAl alloy is half that of nickel-based superalloy.In the temperature range of 600～900℃,TiAl alloys are expected to replace the superalloy in the preparation of some aerospace structures and rotating or reciprocating structures for ground dynamic systems.With the unremitting efforts in the past four decades,TiAl alloy was successfully applied in the last two stages of low-pressure turbine(LPT)blades in 2012.However,the service temperature of TiAl alloy is limited to 700℃.Highly textured TiAl alloys with aligned lamellar structure could potentially resolve the dilemma.Unfortunately,directionally solidified TiAl alloy prepared by optical floating zone or Bridgeman technology can’t be applied to engineering at present.On the basis of 40 g vacuum suction-casting equipment for preparing PST TiAl seed crystal,a 1.5 kg vacuum suction-casting equipment has been successfully developed in this work.Subsequently,related technology has been developed.After solving a series of problems caused by size effect,the success rate of suction-casting is more than 80%.The die optimization was carried out based on the solidification characteristics of Ti-48Al-2Cr-2Nb(at.pct.4822)alloy.Then,Ti-48Al-2Cr-2Nb(SC4822)polycrystals billets featuring similarly oriented lamellae with fine lamellar spacing have been successfully prepared.The scatter angle of lamellar orientation is within 30°,and the mean lamellar spacing is 219 nm.The tensile properties of SC4822 alloy were investigated.When loading axis is parallel to the lamellar interfaces,SC4822 alloy achieves the synergistic enhancement of strength and plasticity.The relationship between yield strength and lamellar spacing follows the Hall-Petch formula.The thin lamellae are responsible for the enhanced strength.The improved plastic is attributed to the aligned lamellar structure.The creep properties of SC4822 alloy were investigated.Compared with TiAl alloys having randomly oriented full lamellae,SC4822 alloy has better creep resistance.The stress exponent(n)at 800℃ in the stress range of 200-350 MPa is measured to be 5.1.And the apparent activation energy(QC)under 300 MPa in the temperature range of 775～825℃ is measured to be 397.18 kJ/mol.On the basis of stress exponent,apparent activation energy as well as microstructure observation,the creep deformation is dominated by dislocation climb.Besides,dislocations slipping,dynamic recrystallization as well as twinning also play a part in the creep deformation.The effect of Si on creep properties of SC4822 alloy was studied.The results show that Si can significantly improve creep resistance of SC4822 alloy.At 800℃,the application-stress potential of SC4822 alloy increases by 50 MPa after adding 0.5 at.%Si.At 300 MPa,the application-temperature potential of SC4822 alloy was increased by 50℃ after adding 0.5 at.%Si.The main reasons for improving the creep property of Si are as follows:firstly,Si element precipitates at the lamellar interface in the form of Ti5Si3,which effectively enhances the lamellar stability by hindering dynamic recrystallization;Secondly,the silicide has a significant pinning effect on the dislocation movement.Thirdly,Si can significantly improve the oxidation resistance of SC4822 alloy and slow down the formation of oxidized brittle layer on the surface of creep specimen.TiAl alloys featuring oriented lamellae with fine lamellar spacing have been prepared in this study.The good room-temperature ductility and excellent hightemperature performance at high temperature were achieved in such TiAl alloys.Such kind of TiAl alloys have the potential to expand the application range,which is suitable for potential application in components such as sheets,rings,cylinders and shells. |
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