Effect Of Multistage Heat Treatment On Microstructure And Mechanical Properties Of TB8 High Strength Titanium Alloy

Metastableβtitanium alloy not only has high strength and excellent fatigue resistance,but also has good toughness,so it is widely used in aeroengine,aircraft parts and other aerospace fields.Titanium alloys with excellent comprehensive properties matching gradually become the focus of attention,it is particularly important to explore the relationship between its microstructure and mechanical properties.This paper takes TB8 titanium alloy as the research object.SEM,XRD,EBSD and TEM are used to characterize the evolution of the microstructure.Tensile test and fracture toughness test are used to characterize the effect of the microstructure on the mechanical properties.The changes of microstructure state and properties after solution treatment,single aging treatment and descending duplex aging treatment are studied respectively.The relationship between preparation technology,microstructure and mechanical properties is investigated.After solution treatment,the microstructures are composed of primaryαphase andβgrains during solution atα/βregion,while the microstructure is composed of singleβgrain during solution atβregion.The amount of primaryαphase and the size ofβgrain are affected by solution temperature.With the increase of solution temperature,the amount of primaryαphase gradually decreases,andβgrain gradually grows.When the temperature reaches 830℃（βzone）,the primaryαphase disappears completely,and the size ofβgrain reaches the maximum.With the amount of primaryαphase gradually decreases,the strength of the alloy decreases and the plasticity increases gradually.The tensile fracture mode belongs to microporous polymerization ductile fracture,and the fracture morphology is dominated by dimples.With the increase of plasticity,the amount of dimples gradually decreases and the density becomes sparse.After single aging treatment,a large amount of secondaryαphase precipitates out of the microstructure,the size of secondaryαphase is affected by aging temperature.With the increase of aging temperature,the secondaryαphase gradually aggregates,grows and coarsenes,and the distribution density decreases gradually.With the growth of the secondaryαphase,the strength of the alloy decreases,while the plasticity and fracture toughness increase.The tensile fracture shows a mixed fracture mode of transgranular fracture and intergranular fracture,and the transgranular fracture characterized by dimple gradually becomes dominant.The fracture toughness fracture morphology is typical ductile fracture characteristics.After duplex aging in descending order,the effects of duplex aging temperature for solution treated alloy and solution temperature for duplex aged alloy on the microstructure and mechanical properties are respectively studied.On the one hand,when the solution processes are the same,on the basis of the precipitation of secondaryαphase during first-step aging at 520℃,and finer secondaryαphases are precipitated during second-step aging at 200°C-400°C.With the increase of second-step aging temperature,the distribution density of secondaryαphase gradually increases,the strength of the alloy continues to increase,while the plasticity and fracture toughness of the alloy continue to decrease.The tensile fracture shows mixed intergranular fracture and transgranular fracture.With the increase of distribution density of secondaryαphase,the brittle fracture becomes more and more prominent.The fracture toughness fracture morphology is mainly ductile fracture features with dimples.On the other hand,when first-step aging and second-step aging are the same,with the increase of solution temperature,the content and width of secondaryαphase increase,the corresponding strength increases first and then decreases,and the plasticity and fracture toughness decrease first and then increase.The zigzag degree of crack propagation path decreases first and then increases,which corresponds to the change of fracture toughness.The rougher the fracture morphology is,the higher the zigzag degree of crack propagation path is,which corresponds to the high fracture toughness.After solution at 750°C and duplex aging,it can be found that strength,plasticity and toughness of alloy can be achieved the best match,the highest fracture toughness is103 MPa·m^1/2,the corresponding tensile strength is 1334 MPa,and the elongation is8.25%.After solution at 790°C and duplex aging,strength of alloy can be achieved the highest,the tensile strength is 1456 MPa,corresponding elongation is 4.25%and fracture toughness is 78.4 MPa·m^1/2.After solution at 830°C and duplex aging,strength and plasticity of alloy can be achieved the best combination,the tensile strength is 1409 MPa,elongation is 6.25%,fracture toughness is 80.2 MPa·m^1/2.