Microstructure And Mechanical Properties And Fracture Mechanism Research Of New ? Titanium Alloy BTi20

Beta titanium alloys are widely used in aerospace field because of its high strength-to-weight ratio,easy to heat treatment,good hardenability and processing performance,etc.As new aerospace craft direction towards high speed-large scale-complex-low fuel development,ordinary type high strength beta titanium alloy is difficult to meet strict requirements for using the environment aerospace materials,research and development of beta type titanium alloy with high strength and high tenacity have been paid close attention to by countries around the world.In this paper,the effect of forging process and heat treatments on microstructures and mechanical properties both of ?180 mm forgings and ?90 mm forgings was studied in BTi20 titanium alloy,analyzed fracture characteristics and fracture mechanism of the alloy,and used a model of fracture toughness to predict the fracture toughness values of the alloy.The main results were as follows:The original microstructures of ?180 mm forgings via ?+? forged of BTi20 titanium alloy exists inhomogeneity.When the alloy solution treated below ? transus temperature,the form of primary ? phase from edge to core of the alloy,showed a change rule as equiaxial to equiaxial+short rod-like+strip-like to short rod-like+ strip-like,and with the increase of solution temperature,the volume fraction of primary ? phase reduced gradually.When the alloy solution treated above ? transus temperature,there were only equiaxial ? phase exist,and the higher the solid solution temperature,the greater the size of ? grains.Microstructures with “? fleck” phenomenon were appeared in the alloy after solution and aging.When the alloy solution treated at 750? plus aging,needle-like secondary ? phase and equiaxial primary ? phase were precipitated on ? matrix,from the edge to the core of the alloy,the precipitation of secondary ? phase gradually became uniform;as the alloy solution treated at 800? plus aging,the microstructures include needle-like secondary ? phase and ? phase,the precipitation of secondary ? phase gradually became nonuniform from the edge to the core of the alloy.?180 mm forgings solution treated at 750? plus aging at 510? has good fracture toughness,but poor in strength and plasticity,from the edge to the core of the alloy,the strength gradually increased,the plasticity and fracture toughness decreased.The 1/2R of the alloy has a good strong toughness match: Rm=1337MPa,A=8.5 %,KIC=59.8MPa·m1/2;However,the alloy solution treated at 800? plus aging at 510? has high strength,Rm?1400MPa,plasticity and toughness are poorer,the strength gradually decreased from the edge to the core of the alloy,and the plasticity and fracture toughness increased.?90mm forgings via ? forging as well as ?+? forging have homogeneous original microstructures.There is no “? fleck” phenomenon in the alloy after solution and aging.When the alloy via ? forged solution treated at 800? plus aging,with the increase of aging temperature,the precipitation way of secondary ? phase form orthogonality to “delta” shape to paralleling to each other,and the higher the aging temperature,the greater the size of secondary ? phase;When the alloy via ?+? forged solution treated at 800? plus aging,the secondary ? phase characterized by three precipitation features:“delta” form precipitation,parallel or vertical precipitation and “fish-bone structure” precipitation.The tensile strength decreased,the elongation and fracture toughness increased with increasing aging temperature as the ?90mm forgings under the same solution condition.In addition,fracture toughness of the alloy via ? forged is better than the alloy via ?+? forged under the same heat treatment.The alloy solution treated at 800? plus aging at 510? has the good strength-toughness match: the mechanical properties of the alloy via ? are Rm=1400.8MPa,A=8%,KIC=65.3MPa·m1/2;and the alloy via ?+? forged are Rm=1484.8MPa,A=8%,KIC=58.6MPa·m1/2.For compact tension specimens of BTi20 titanium alloy,the bigger the shear lip area,and the more tortuosity the crack propagation path,the higher the fracture toughness of the alloy.The fracture mode of the crack propagation region of the alloy includes microvoid accumulation fracture,transgranular dissociation and intergranular fracture,which is mainly on microvoid accumulation fracture.Under different condition,the intrinsic fracture toughness of BTi20 titanium alloy are almost the same;the extrinsic fracture toughness is affected by forging process,solid solution temperature and aging temperature.A simulation formula could be used to predict the fracture toughness values of the alloy after aging treated,and the error is within 10%.The formula is as follows.