The Microstructure And Properties Control Of The TB8 Alloy By Strong Plastic Deformation And Heat Treatment

Titanium alloy is widely used in transportation and military industry due to its low density,good fracture toughness and high strength.It has been a hot research material at home and abroad,so as to meet the demand of high-performance materials gradually.In this paper,the metastableβtitanium alloy TB8（Ti-12.76Mo-2.13Nb-2.73Al-0.16Si）was investigated through equal channel angular pressing（ECAP）and heat treatment（solution-ECAP-aging,ECAP-solution-aging）.The effects of different processes on the microstructure and properties of the material were studied.The main results are as follows:（1）The microstructure and properties of titanium alloy after 800℃/850℃solution ECAP-520℃/540℃/560℃aging treatment were studied.The results show that after solution-ECAP-aging treatment,the grains ofβphase are elongated and fracture defects appear;the dislocation strength of titanium alloy with 850℃solution-ECAP reaches 82.15 MPa,and after 520℃aging treatment,the dislocation strength increases to 85.55 MPa.The order of hardness of titanium alloy by different secondary treatment is as follows:solution-ECAP-aging,solution-aging and solution;the hardness of 850℃solution-ECAP-520℃aging is better,up to 531.2 HV.At room temperature,the tensile strength of ECAP treated titanium alloy is higher than that of non ECAP treated titanium alloy,and the tensile strength decreases with the increase of aging temperature.The tensile strength of titanium alloy treats at 850℃solution-ECAP-520℃aging is better,up to1176.25 MPa.（2）The microstructure and mechanical properties of ECAP-775℃/800℃/825℃/850℃/875℃solution-520℃aging titanium alloy were studied.The results show that the grains are more compact in the cross-section of ECAP extrusion,and the broken and elongated grains can be observed in the longitudinal section.The dislocation strength of ECAP-850℃solution titanium alloy reaches 77.70 MPa,while the dislocation strength of 520℃aging titanium alloy increases by 41.80%to 110.18 MPa.In ECAP-solution process,the hardness of the alloy decreases continuously due to the rising of solution temperature;while in ECAP-solution-aging process,the solution temperature continues to rise,which leads to the tendency of hardness rising first and then decreasing.The hardness of titanium alloy after ECAP-850℃solution-520℃is better,reaching 440.1 HV.In the room-temperature tensile test,with the increase of solution temperature,the tensile properties first increase and then decrease.When the secondary treatment processing is 850℃solution-ECAP-520℃,the elongation after fracture and tensile strength are 2.76%and1166.67 MPa,respectively,which has a better strength plasticity matching.（3）The high-temperature oxidation resistance of titanium alloy after 800℃/850℃solution-520℃aging,ECAP-800℃/850℃solution-520℃aging and 800℃/850℃solution-ECAP-520℃aging was investigated.The results show that the oxidation products of the six materials are similar after 50h cyclic oxidation at 850℃,mainly composed of Ti O₂,Al₂O₃,SiO₂,Mo O₃ and Nb₂O₅.In the oxidation layer of alloy by 850℃solution-ECAP-520℃aging,the structure of Ti O₂+Al₂O₃/Ti O₂ is formed from the inside to the outside.The content of Si in the oxide layer is negatively related to Ti and positively related to Al,which can inhibit the activity of Ti ions and promote the activity of Al ions,forming a protective Al₂O₃ film.The high-temperature oxidation resistance of the six samples is as follows:850℃/800℃solution-ECAP-520℃aging,ECAP-850℃/800℃solution-520℃aging and 850℃/800℃solution-520℃aging.800℃solution-520℃aging is reactive oxidation,850℃solution-520℃aging and ECAP-800℃/850℃solution-520℃aging have both reactive oxidation and diffusion oxidation,while 800℃/850℃solution-ECAP-520℃aging is diffusion oxidation.In the process of high-temperature oxidation,the oxidation depth,average oxidation rate and parabolic rate constant of titanium alloy by 850℃solution-ECAP-520℃aging are the smallest,which are 55.0μm,0.0036 mg·cm^-2·h^-1 and 45.30 mg²·cm^-4·h^-1,respectively,showing good high-temperature oxidation resistance.（4）The hot corrosion resistance of titanium alloy after 800℃/850℃solution-520℃aging,ECAP-800℃/850℃solution-520℃aging and 800℃/850℃solution-ECAP-520℃aging were investigated.The results show that the phases of six samples are mainly composed of oxidation products（Ti O₂,Al₂O₃,SiO₂,Nb₂O₅）and corrosion products（Na₂Si₂O₅,Mo S₂,Ti Cl₂,Ti（SO₄）₂,Ti S）after being corroded at 850℃in 75 wt.%Na₂SO₄+25 wt.%Na Cl corrosion medium for40h.The cross-section without ECAP treatment has pits and a small amount of porosity defects,while the cross-section with ECAP treatment has less defects.Al element is enriched on the surface of corrosion layer of titanium alloy by 850℃solution-ECAP-520℃aging to form a protective Al₂O₃ film,which can inhibit the penetration of corrosion atoms to make the corrosion thickness and maximum corrosion increment smaller,which are 102.3μm and 0.7507 mg·mm^-2,respectively.Therefore,it shows the best corrosion resistance.To sum up,this paper effectively combines the strong plastic deformation and heat treatment mechanism,uses TB8 titanium alloy as the research object,explores the influence of secondary processing on TB8 titanium alloy,and many properties are improved,which provides more reference for the follow-up research of TB8 titanium alloy.