Mechanical Properties of TB17 Titanium Alloy with Different Microstructures Types
In recent ten years,with the rapid development of the new generation of weapons and equipment such as hypersonic air-craft,unmanned combat aircraft,high stealth and high mobility aircraft in the aerospace industry and other military fields,metastableβ-type titanium alloy has many advantages such as high heat treatment strengthening effect,deep hardenability,good cold and hot pro-cessing performance and excellent fracture toughness.Through solution and aging heat treatment,high strength can be obtained and good plasticity and toughness can be obtained,which has become the key direction of the development and application of new titanium alloys.It is also a new type of military advanced material that is focused on and prioritized at home and abroad.In particular,the ideal candidate materials for important load-bearing components such as aircraft joint parts,frame beam parts and fasteners are more and more favored by designers in the design of new aerospace vehicles.The metastable β-type titanium alloy with light weight and high spe-cific strength is used to replace the high-strength structural steel material.The structural weight can be reduced by 20％～40％.Its ap-plication proportion and application field are increasing,and the application prospect is broad.The solid-state phase transformation of titanium and titanium alloys has the characteristics of diversity and complexity.In the relationship among the four elements of titanium alloy material composition,process,microstructure and properties,the chemical composition determines the type of alloy,the pro-cess determines the type of microstructure,and the type of microstructure determines the mechanical properties of the alloy.When the composition of titanium alloy is determined,by adjusting the microstructure type,the microstructure parameters of titanium alloy parts can be adjusted within a certain range,and the comprehensive matching of mechanical properties such as strength,plasticity,tough-ness and fatigue can be realized.Therefore,it is of great practical significance to study the relationship between the microstructure type and mechanical properties of titanium alloy materials.Therefore,in this paper,the effects of different microstructure types(bi-modal microstructure,basketweave microstructure and lamellar microstructure)of TB17 titanium alloy on the phase composition,room temperature tensile properties and fracture toughness of TB17 titanium alloy in solid solution and solid solution aging were stud-ied by means of X-ray diffraction(XRD),scanning electron microscope(SEM),transmission electron microscopy(TEM).In order to provide data support for its engineering application and material selection.The results showed that,TB17 titanium alloy was forged in two-phase region and solution treated in two-phase region,its microstructure was composed of discontinuous primary α phase distribut-ed on β transformation matrix,and there was no obvious grain boundary.The morphology of the primary α phase was mainly equiaxed,and there was a small amount of coarse flake,and the content was about 6.08％.After isothermal aging,the dispersed fine lamellar ag-ing α phase precipitates on the metastable β matrix with the thickness of 20～50 nm.After solution treatment in two-phase region forg-ing and β phase region,the microstructure of the solid solution state was a single β phase structure,and no martensite α"phase or sec-ondary α phase was produced.After isothermal aging,the microstructure was composed of grain boundary α phase,fine lamellar α cluster and intragranular α phase distributed on β transformation matrix,and the thickness of the lamella was not much different(30～80 nm).When the quasi-β forging and two-phase heat treatment was adopted,the microstructure was composed of residual β phase and coarse lamellar α phase with different sizes and interlaced on it,and the lamellar thickness was 0.2～0.3 μm.After isothermal ag-ing,the fine lamellar α phase a thickness of 50～100 nm was precipitated in β transformation matrix.In the state of solid solution treat-ment,the morphology and content of α phase were main factors affecting tensile properties of TB17 titanium alloy under solution treat-ment,with the increase of α phase content,the resistance of dislocations in slip system increased obviously.And the hindering effect increased with the increase of the volume fraction of α phase,so the tensile strength of the alloy was improved.In terms of tensile plas-ticity,the highly dense crystal plane and crystal orientation were most conducive to the dislocation movement in plastic deformation,and the existence of β phase was beneficial to the improvement of plasticity macroscopically.Therefore,TB17 titanium alloy with sin-gle β phase structure had the highest tensile plasticity.In the state of solid solution aging,the thickness of fine lamellar α phase pre-cipitated during isothermal aging was an important factor affecting the tensile strength of the alloy.The thicker of the lamellar α phase,the lower of the tensile strength.The increase of tensile strength after aging was positively correlated with the volume fraction of fine la-mellar α phase precipitated after isothermal aging.The thickness of lamellar α phase in the microstructure of TB17 titanium alloy was positively correlated with its fracture toughness,and the effect of coarse lamellar α phase after solution treatment was greater.
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