Microstructure And Properties Of TiAl/Ti₂AlNb Laminated Composites Prepared By Foil Metallurgy

Structural materials with lighter weight and better high-temperature performances are becoming in urgent need with the development of a new generation hypersonic aircraft.TiAl alloys possessing low density,high specific modulus and specific strength,and good high-temperature strength retention and oxidation resistance,are regarded as potential materials used in hyper sonic aircraft.However,TiAl alloys are intrinsic brittle,have poor room-temperature ductility and toughness,and are also difficult to process and forge.These problems extremely hinder the further commercial applications of TiAl alloys.In this thesis,TiAl/Ti₂AlNb laminated composites are successfully synthesized.Effects from the ductile Ti₂AlNb alloy component and laminated structure can not only improve the room-temperature mechanical properties of TiAl alloys but also ratain their advantages in high-temperature performances.Relationships among the processing parameters-microstructure-laminated structure-mechanical properties are built up,and the strengthening and toughening mechanisms are clarified.Taking TiAl and Ti₂AlNb alloy foil as raw materials,TiAl/Ti₂AlNb laminated composites are fabricated under different processing parameters by vacuum hot pressing combined with foil metallurgy.Subsequently,the optimized processing parameters are achieved.It found that processing parameters make effects on the interface bonding,interface microstructure,interface thickness and microstructure of base alloys of composites,which finally decides the coordinated-deformation ability of composites.Composites achieve a good interface bonding,perform the well-coordinated deformation ability,and have the best comprehensive properties under the processing parameter of 800℃/2h/0MPa+1050℃/2h/65MPa,which has an interface composed of region I withα₂,O and B2/βphase,and region II with a singleα₂phase.Based upon toughness measurements,the toughness of composites is significantly improved with a maximum value of 32.6 MPa.m^1/2and shows anisotropy.By the analyses of crack propagation behaviors of composites,it concluded that the major crack is effectively deflected and fracture resistance of TiAl alloy is enhanced because of the combined effects of Ti₂AlNb alloy and laminated structure.Meanwhile,the formation of microcracks,crack bridging and crack blunting in crack propagation,etc.can also toughen composites.Through studies of component layer thickness parameters and interface,influences of component layer thickness and thickness ratio on properties,and the interface structure of TiAl/Ti₂AlNb laminated composites are both clarified.Results show that the increasing of component layer thickness under the thickness ration of1:1 between TiAl alloy and Ti₂AlNb alloy has little effects on toughness but decreases the tensile performances of composites.Increasement of thickness ratio of TiAl alloy deteriorates ductile and toughness of composites.Correspondingly,raising the thickness ratio of Ti₂AlNb alloy enhances ductile and toughness at the expenses of increasing the density and worsening high-temperature properties of composites.For the interface structure of composites with component layer thickness of 100μm and component layer thickness ratio of 1:1,we can conclude that both O and B2/βphase in Ti₂AlNb alloy achieve coherent crystallographic orientation with the interfacialα₂-Ti₃Al phase,and the stress at the interface is at low level.Meanwhile,the interfacialα₂-Ti₃Al phase has an incoherent crystallographic orientation with theγ-TiAl phase in TiAl alloy,and the stress concentration formed at the incoherent interface is effectively released by dislocation glide on the（111）plane ofγ-TiAl phase from the Frank-read dislocation source and Ledge dislocation source.Researches on the bending deformation and tensile deformation of TiAl/Ti₂AlNb laminated composites at room temperature are used to construct the relationships among the laminated structure-deformation behaviors-fracture characteristics-mechanical properties.Composites behave disparities in bending properties with different load directions.Specifically,composites have a higher bending strength of 964.06MPa with the load direction parallel to the component layer plane and better bending strain of 1.75%with the load direction perpendicular to the component layer plane in the laminated structure.The major crack proceeds with a zig-zag path under the load direction perpendicular to the component layer plane,and lots of deformation bands and microcracks are formed in composites.The formation of many microcracks can not only relieve the local stress concentration caused by bending deformation but also improve the macroscopic bending strain of composites.Through the in-situ tensile test,it can be found that the laminated structure will change the stress state and the uniaxial stress state is transformed into biaxial stress state in composites.Under the effects of laminated structure,the appearance of strain partitioning can relieve the stress concentration of TiAl alloy formed by deformation and improve the deformation ability of composites.The existence of interface affected zone（IAZ）and heterostructure deformation induced hardening（HDI）make their contributions to the improvement over the tensile performances of composites.The mechanical properties,deformation behaviors,strengthening-toughening mechanisms,oxidation behaviors and oxidation mechanisms of TiAl/Ti₂AlNb laminated composites at high-temperature are revealed by the uniaxial tension and oxidation experiments.Through high-temperature tensions,we find that tensile strength and ductility of composites are simultaneously enhanced under the effect of laminated structure,which embodys a temperature-dependent characteristic.From750℃to 850℃of tensile temperature,deformation degree of microstructure in composites is increasing.At 900℃,the microstructure turns into recovery.At 850℃,the composite presents the best synergy between tensile strength of 365.41MPa and tensile strain of 22.25%.Influences of interface affected zone,stress redistribution by laminated structure and heterostructure deformation induced hardening are composed of main mechanisms of strengthening and toughening of composites in the process of high-temperature tensile deformation.Moreover,the deformation-induced solid-state transformation ofα₂-Ti₃Al toγ-TiAl significantly improve the deformation ability of interfacialα₂-Ti₃Al layer.High-temperature cyclic-oxidation experiments indicate that composites display a good oxidation resistance at 800℃.With oxidation temperature up to 850℃,oxidation resistance of composites begins to decease.During cyclic oxidation,TiAl alloy presents a better oxidation resistance than that of interfacialα₂-Ti₃Al layer which is superior to Ti₂AlNb alloy.Cyclic-oxidation resistance of composites depends on the integrity and compactness of oxide layers.Under the cyclic oxidation,the mismatch of thermal stress among oxides is the main driving force to form the spall ing off,cracking and delamination of oxide layers,which breaks the compactness of oxide layers and finally weakens the cyclic-oxidation resistance of composites.