The Synthesis Mechanism And High-temperature Mechanical And Oxidation Behavior Of （Ti₅Si₃-TiAl）/TiAl Laminated Composite Sheets

Intermetallicγ-TiAl based alloys are considered as a new generation of high temperature structural materials in aerospace,energy and automotive industries because of their remarkable properties,such as low density,high specific modulus and strength,good oxidation resistance and creep properties at high temperatures.However,with the fast development of modern industry,the supersonic aircrafts,future gas turbine engines and thermal protection systems propose higher demands on high-strength light-density alloy sheets.However,it is extremely difficult for TiAl alloys to prepare the sheet and complex components due to the poor hot deformability and low room-temperature ductility.In addition,the insufficient high temperature strength and oxidation resistance limit the application of TiAl alloys to a certain extent.Therefore,it is a challenge for the preparation and processing of high performance large-size TiAl alloy sheet.（Ti₅Si₃-TiAl）/TiAl laminated composite sheets were successfully fabricated by hot pressing and reaction annealing based on Al-Si alloy and Ti foils.This method here avoids the direct rolling deformation of brittle TiAl intermetallic and couples in situ technology and laminated structure to improve mechanical properties.Microstructure evolution and synthesis mechanism during reaction annealing were systemically studied by scanning electron microscope（SEM）,X-ray diffraction（XRD）and transmission electronic microscope（TEM）.The mechanical properties at room and high temperatures were investigated,and then establish the relationship among microstructure and mechanical properties.Moreover,the oxidation behavior was reasonably characterized at high temperatures.The initial reaction annealing of Ti/（Al-Si）laminated composite sheet at 700^oC mainly relied on solid-liquid reaction.Firstly,solid Ti chemically reacts with liquid Al to form TiAl₃ phase,and then Si gradually dissolved in TiAl₃ to generate Ti（Al,Si）₃ solid solution.After low temperature annealing,Ti/Ti（Al,Si）₃ laminated composite sheets are obtained.There is a chemical composition gradient of Si in Ti（Al,Si）₃ layer,that is the trend of gradual decline from the center to both sides.And it can be tailored by prolonged low temperature annealing time until the chemical composition of Si is uniform.The in situ precipitation behavior of Si in TiAl₃ was observed in Ti/Ti（Al,Si）₃ laminated composite sheet.The formation of Ti₅Si₃ precipitates is by transformation of metastable Ti₇Al₅Si₁₂from Si-rich nanoclusters during the early-stage precipitation.Microstructure observation shows that（Ti₅Si₃-TiAl）/TiAl laminated composite sheet comprising alternating layers ofγ-TiAl equiaxed grain layers and fully lamellar layers were prepared by further high temperature annealing（1250^oC/3h/40MPa+1300^oC/10min）.The generation of in situ Ti₅Si₃ particles also plays a role in impeding the migration of grain boundary,thus refining equiaxed grains and lamellar colonies.The large majority of in situ Ti₅Si₃ particles are detected in equiaxed grain layer with clean interfaces.The fully lamellar contains the rest of Ti₅Si₃ particles.The morphologies of Ti₅Si₃ mainly appear granular-like and some Ti₅Si₃ particles with needle-like structure are observed.The ultimate strength of 7.2vol.%（Ti₅Si₃-TiAl）/TiAl laminated composite sheet increases firstly and then decreases,and the elongation gradually increases with raising temperature.The brittle-to-ductile transition temperature lies between 750^oC and 800^oC.The ultimate strength at 800^oC reaches the highest value of 542 MPa with an elongation of 6.1%.The improvements in strength and elongation are attributed to the refinement of in situ Ti₅Si₃ particles and laminated structure.In addition,the content of reinforcement has a great effect on the tensile properties of laminated composites.The ultimate strength and elongation of laminated composite sheet increase first and then decrease with Ti₅Si₃ content increasing.The oxidation resistance research indicates that the oxidation kinetics of（Ti₅Si₃-TiAl）/TiAl laminated composite sheet shows parabola law in the temperature range of 750^oC⁹00^oC.The oxidation resistance of laminated composite sheet is superior to pure TiAl laminated sheet.Although the oxidation products and oxide scale structure of laminated composite sheet at 800^oC after 100 h are consistent with pure TiAl laminated sheet,the overall thickness of oxide scale decreased significantly and the surface formed a more compact Ti O₂ layer.With the increase of Ti₅Si₃ content,high temperature oxidation resistance of laminated composite sheet gradually increased.