Study On Microstructure And Properties And Creep Behavior Of Y₂O₃ Particles Reinforced TiAl Alloys

TiAl alloys are a new lightweight and heat-resistant structural material very suitable for hot components of aircraft engines,due to the advantages of low density,high specific strength and modulus,excellent high-temperature creep resistance and oxidation resistance.However,the disadvantages of poor room-temperature plasticity and insufficient high-temperature strength limit their further engineering application.Thus,it is of great significance to study on material design as well as strengthening and toughening mechanisms of high-performance TiAl alloys.In the present work,the microstructure and mechanical properties of Ti-48Al-2Cr-2Nb and Ti-48Al-6Nb alloys（called T4822 and T486）were optimized and improved by adding nano Y₂O₃particles.A systematical research was conducted on the effect of nano Y₂O₃ addition on solidification path,microstructure,room-temperature and high-temperature tensile properties,especially high-temperature creep properties of the two types of as-cast TiAl alloys.Creep behavior of the Y₂O₃ particles reinforced TiAl alloys was studied,and the microstructure evolution and creep strengthening mechanisms were revealed.Finally,heat treatment was adopted to regulate the microstructure of TiAl alloys,in order to further improve the comprehensive mechanical properties.The effect of Y₂O₃ addition on microstructure and tensile properties of the two types of TiAl alloys was studied.On the one hand,Y₂O₃ addition made the solidification path of investigated alloys move to the high Al side,which induced the high-temperatureαphase to precipitate directly from the liquid phase without peritectic reaction.Therefore,T4822-Y₂O₃ alloy possesses more massiveγphases,and T486-Y₂O₃ alloy shows（α+β）hybrid dendrites rather than cellular dendrites.On the other hand,during the solidification process,Y₂O₃ addition could reduce the grain size of high-temperatureαphase and promote the heterogeneous nucleation ofγphase,significantly reducing the grain size and the lamellar spacing.Moreover,Y₂O₃addition can improve the room-temperature strength and ductility of T4822 and T486alloys,which plays a better strengthening and toughening effect in the former.For T4822-Y₂O₃ alloy,the ultimate tensile strength is increased from 466MPa to 561MPa,and the elongation is increased from 0.60%to 1.49%.For T486-Y₂O₃ alloy,the ultimate tensile strength is increased from 537MPa to 611MPa,and the elongation is increased from 0.51%to 0.79%.Owing to the stronger grain refinement strengthening and second phase strengthening as well as more massiveγphases,T4822-Y₂O₃ alloy shows higher increments of strength and elongation than T486-Y₂O₃ alloy.Creep behavior and microstructure evolution mechanism of the Y₂O₃ particles reinforced Ti-48Al-2Cr-2Nb alloy were studied.The addition of Y₂O₃ significantly improves the creep properties of T4822 alloy.The creep life at 800℃under different stresses is increased by more than 200%.With the increase of temperature,the strengthening effect induced by Y₂O₃ addition decreases slightly,and the creep life at850℃is increased by about 149%.The improvement of creep resistance could be attributed to dispersed Y₂O₃ particles and thin lamellar structures.During creep,severe microstructure degradation occurs in T4822-Y₂O₃ alloy,including dissolution ofα₂ laths,coarsening ofγlaths,and precipitation of B2 and C14-Laves phases.Increasing creep stress or time can facilitate the microstructure evolution,and the former plays a major role.Moreover,Y₂O₃ addition promotes the formation of C14-Laves phase.On the one hand,Y₂O₃ acts as the nucleus of heterogeneous nucleation,resulting in the precipitation of C14-Laves phase on the second phase.On the other hand,there are many lattice defects and high strain energy around Y₂O₃ particles,which can accelerate the nucleation of C14-Laves phase in theγphase.Creep behavior and brittle-ductile transition mechanism of the Y₂O₃ particles reinforced Ti-48Al-6Nb alloy were studied.For T486-Y₂O₃ alloy,the average creep life at 800℃under different stresses is only increased by about 40%.With the increase of temperature,the strengthening effect induced by Y₂O₃ particles increases gradually,and the creep life at 850℃goes up by about 79%.The contribution of Y₂O₃addition to creep properties of T486 alloy is lower than that of T4822 alloy,due to the weak effect of second phase strengthening after high-Nb alloying.The brittle-ductile transition temperatures（BDTTs）of T486 and T486-Y₂O₃ alloys were determined to be 800～825℃by creep tests,which reduce about 25℃than those measured by tensile tests.Obviously,the BDTTs of TiAl alloys drop with the decrease of strain rate.High-temperature plasticity of the lamellar structure is limited by the brittleα₂ phase.When creep below the BDTT,deformation ofα₂ laths is mainly provided by （?）type dislocations,which results in the poor plastic deformation ability.When creep above the BDTT,2（?）+（?）type dislocations are activated,which can adjust the deformation along the （?）axis,thus increasing the plastic deformation ability ofα₂ phase and contributing to the cooperative deformation of TiAl alloys.The effect of heat treatment on the comprehensive mechanical properties and microstructure evolution of the Y₂O₃ particles reinforced TiAl alloys was studied.Firstly,T4822-Y₂O₃ alloy was used to investigate the microstructure evolution of Y₂O₃ particles during heat treatment.After long-term heat treatment,micron Y₂O₃particles can fragmentate and spheroidize.With the increase of temperature or time,there is a morphological transformation of dendrite/long rod→short rod→nodular for Y₂O₃ particles.After short-term heat treatment,some nano Y₂O₃ particles precipitate in the matrix,and then will gradually grow with the increase of time.To obtain nano Y₂O₃ particles,short-term heat treatment was adopted to regulate the microstructure and mechanical properties of both TiAl alloys.After heat treatment at 1350℃/1h/AC,T4822-Y₂O₃ alloy transforms into fully lamellar from near lamellar.T486-Y₂O₃ alloy is still near lamellar,but the content ofγphase decreases significantly.At this time,the mechanical properties of both TiAl alloys,especially high-temperature creep properties,were improved.The ultimate tensile strength at room-temperature is increased by about 30MPa,and that at 800℃is increased by about 50MPa.At800℃/325MPa,the creep life of T4822-Y₂O₃ alloy is increased by about 65%,and that of T486-Y₂O₃ alloy is increased by about 99%.The excellent high-temperature creep resistance can be attributed to the lamellar microstructure,thin lamellar spacing and dispersed nano Y₂O₃ precipitates.Moreover,T486-Y₂O₃ alloy shows a higher increment of creep life due to the decrease of segregation.