Precipitation Behaviors Of O Phase And Mechanical Properties Of Mo-modified Ti₂AlNb Alloys Prepared By Powder Metallurgy

The light-weight intermetallic Ti₂AlNb alloys,based on the orthorhombic O phase,have attracted great attention in the aeronautics and aerospace fields,owing to the high elevated-temperature strength,excellent corrosion resistance,and favorable oxidation resistance.However,the low room-temperature plasticity,as well as the resultant forming difficulties,restrict the application of the Ti₂AlNb alloys.The net-shape powder metallurgy technique is commonly used to form components with irregular shape,which reduced the cutting and shaping schedules.Therefore,it is essential to figure out the relationship between the morphology of the O phase and the mechanical properties of the powder metallurgic Ti₂AlNb alloys.It will be beneficial to expand the application of the Ti₂AlNb alloys.In this thesis,the Ti₂AlNb alloys are prepared by powder metallurgy from pre-alloyed Ti-22Al-25Nb powder.Ball milling and adding B2 phase stabilizing element(Mo)were used to improve the plasticity of the alloy.After illustrating the effects of Mo on the phase transformation and microstructure evolution of the Ti₂AlNb alloys upon cooling,Mo-modified Ti₂AlNb alloys are prepared by spark plasma sintering(SPS)and hot isostatic pressing(HIP).Combined with the heat treatment and hot rolling,the content and morphology of the O phase are tuned to improve the mechanical properties,and the deformation mechanism is investigated.Firstly,the Ti₂AlNb alloys are prepared from ball-milled Ti₂AlNb powder by spark plasma sintering,followed by ageing in the(B2+O)phase region.The pre-milling facilitates the precipitation of the O phase,as the primary O phase changes from equiaxed shape to rod-like shape,and the second O phase follows the precipitation sequences:amorphous O?coherent precipitation of O phase with(110)_B2//(002)_O?semi-coherent interface?incoherent interface.Pre-milling also improves the precipitation strengthening effect;as a result,the hardness,strength and plasticity are enhanced.However,the pre-milling inevitably accelerates the formation of the?₂ phase.Although the plasticity is improved,the strength of these alloys is relatively low,by using the techniques of ball milling and SPS.In order to further improve the plasticity of the Ti₂AlNb alloys,the B2-phase stabilized element,Mo,is added to the Ti₂AlNb alloys for increasing the content of the body-centered-cubic B2 phase.The phase transformation and microstructure evolution are first investigated by cooling the Mo-modified alloys from different phase region.When the alloy is sintered in the(B2+O)phase region,equiaxed B2 transformed into equiaxed O phase during cooling,i.e.B2?O.When the alloys are sintered in the(?₂+B2+O),(?₂+B2),and B2 phase regions,phase transformation of B2?B2+O occurs,forming(B2+O)Widmanstatten structure.Kinetics analysis indicates that the phase transformation of?₂?B2 is retarded by Mo addition.The Widmanstatten structure becomes curvy in the alloy sintered in the single B2 phase region,due to the segregation of Mo.Considering the microstructure and hardness,the spark plasma sintering is performed in the B2 phase region,and the ageing temperature lies in the(B2+O)and(?₂+B2+O)phase region.After figuring out the phase transformation of the Mo-modified Ti₂AlNb alloys upon cooling,the microstructure of the spark plasma sintered Mo-modified Ti₂AlNb alloys are regulated by different heat treatment processes,and the effects of the morphology and content of the O phase on the hardness are investigated.The homogenized and aged alloys exhibit higher hardness than the directly aged alloys.Moreover,due to the fine O laths and the well-arranged(B2+O)structure,the hardness of the alloys aged in the(B2+O)phase region is higher than that of the ones aged in the three-phase region.The Mo substitution for Nb induces severe lattice distortion along the(200)and(211)crystal planes of the B2 phase,resulting in the well-arranged structure.In addition to the common relationship of(110)_B2//(001)_O,the distortion also results in the orientation relationship of(211)_B2//(261)_O in the solution treated and aged alloys.To avoid the brittleness of the spark plasma sintered Mo-modified Ti₂AlNb alloys,hot isostatic pressing is carried out to prepare the Mo-modified Ti₂AlNb alloys,and the room-temperature mechanical properties are tested,as well as the high-temperature properties.The addition of Mo on the one hand refines both the B2 grains and the O-phase laths,on the other hand,it increases the density of dislocations in the B2-phase grains.Therefore,the high-temperature strength and plasticity are improved in the Mo-modified alloy.The deformation mechanism of the Ti₂AlNb alloy is twinning,and that of the Mo-modified alloy is dislocation glide.By ageing in the(B2+O)phase region,complete Widmanstatten microstructure is obtained in Ti₂AlNb and Mo-modified Ti₂AlNb alloys.With the increasing ageing time,the room-temperature strength and plasticity are suppressed in the Mo-modified alloy;Nevertheless,the high-temperature strength and plasticity are improved by Mo addition.Although the Mo-modified Ti₂AlNb alloys exhibit decreased tensile strength and elongation at room temperature,they show a better high-temperature ductility than that at room temperature.In the end,the influence of hot rolling on the mechanical performance of Ti₂AlNb and Mo-modified Ti₂AlNb alloys is studied.High-throughput experiments are used to investigate the effects of rolling reduction on the microstructure and texture of the Ti₂AlNb alloys.On this basis,the Ti₂AlNb alloys with a progressive increase in rolling reduction are fabricated by hot rolling.As the rolling reduction increased,the phase transformation of B2?O and O?B2 occurred by sequence,and the orientation of the(110)faces of O phase and the B2 phase changes orderly.Both the strength and the ductility are enhanced in the heavily deformed gradient alloy,and the fracture surfaces show the features of quasi-cleavage fracture.