Regulation Of Strong Plastic Deformation And Heat Treatment On Microstructure And Properties Of Ti-555 Titanium Alloy

Titanium alloy is widely used in aerospace and other high-end industries because of its excellent comprehensive performance.In order to meet more stringent requirements,the research on more advanced heat treatment technology is an unabated research hotspot.Based on the existing research on the mechanism of heat treatment and strong plastic processing of titanium alloy at home and abroad,a new idea of coupling heat treatment（solution and aging）with strong plastic deformation（ECAP:Equal Channel Angular Pressing）was put forward.In this study,Ti-5.4Al-4.1Mo-5V-3Cr-1Zr（Ti-555）was taken as the research object,the effects of 820℃/880℃Solution-ECAP-550℃/580℃/610℃Aging and ECAP-820℃/840℃/860℃/880℃/900℃Solution-580℃Aging on microstructure and mechanical properties were studied.At the same time,the high temperature oxidation performance of titanium alloy oxidized at800℃for 50h by different coupling schemes and the hot corrosion performance of titanium alloy corroded at 800℃for 40h by 75 wt.%NaCl+25 wt.%Na₂SO₄ as the medium were studied.The main work and research results of this paper are as follows:（1）The microstructure and properties of titanium alloy after 820℃/880℃Solution-ECAP-550℃/580℃/610℃Aging were investigated.The results show:the refined grains are elongated and broken after Solution-ECAP-Aging.The dislocation density and dislocation contribution of 880℃Solution-ECAP were 1.18×10¹⁴·m^-2 and59.96MPa respectively,indicating that ECAP played a strengthening role and was better coupled with the single phase region.With the increase of aging temperature,both hardness and tensile strength firstly increased and then decreased.Under the Solution-ECAP-Aging process route,the material strength is optimal at 580℃for both820℃and 880℃.The hardness after 820℃Solution-ECAP-580℃Aging and880℃Solution-ECAP-580℃Aging are 457.9HV and 490.0HV respectively.The tensile strength are 1151MPa and 1252MPa respectively.The elongation are 2.89%and2.79%respectively.Therefore,the properties of Ti-555 after 880℃Solution-ECAP-580℃Aging are better.（2）The microstructure and properties of ECAP-820℃/840℃/860℃/880℃/900℃Solution-580℃Aging process were investigated.The results show:With the increase of solution temperature,the contribution of ECAP mainly changes from grain strengthening to enhanced dispersion strengthening.With the increase of solution temperature,the dislocation contribution firstly increases and then decreases.The coupling between the solution at 840℃and 880℃is better,the dislocation contribution is 44.73 MPa and 44.58 MPa respectively.The hardness and strength of Ti-555 after ECAP-Solution-Aging are significantly improved and the plasticity is similar to that of the sample after heat treatment only.The hardness of Ti-555 after ECAP-860℃Solution-580℃Aging is 485.1HV,which is increased by 9.3%compared with that of the sample without ECAP.The hardness of Ti-555 after ECAP-900℃Solution-580℃Aging is503.3HV,but only increased by 3.1%compared with that of the sample without ECAP.The tensile strength of ECAP-880℃Solution-580℃Aging is 1177MPa,which is the best and increased by 8.0%compared with that of the sample without ECAP.Meanwhile,the elongation decreases with the increase of tensile strength.Therefore,ECAP has a good coupling with the Solution temperature of 840℃-880℃.（3）The high temperature oxidation properties of titanium alloy after 820℃/880℃Solution-580℃Aging,820℃/880℃Solution-ECAP-580℃Aging,ECAP-820℃/880℃Solution-580℃Aging at 800℃for 50h were investigated.The results show:The oxidation weight gain curve tends to be stable after 40h and the aging oxidation weight gain curve of 820℃Solution-ECAP-580℃Aging is 0.75(mg·cm^-2),which is the most gentle.The oxidation surface is mainly composed of TiO₂ and Al₂O₃,also contains ZrO₂,V₂O₃,Cr₂O₃,MoO₃ and other oxides.During the transition from the matrix to the oxide layer,Ti（%）decreases and Al（%）,O（%）increases gradually.With the action of ECAP,the grain size of the material is refined,the internal dislocation increases and the porosity decreases.The oxidation resistance is improved and the oxidized surface is more complete and compact.Among them,Ti/Al ratio of the surface of 820℃/880℃Solution-ECAP-580℃Aging surface is 1:1.5,the transition morphology is smooth,uniform and compact,which prevents oxygen atom infiltration.In ECAP-Solution-Aging process,the grain’s growth during solution reduces the contribution of dislocation provided by ECAP.At 820℃/880℃Solution-ECAP-580℃Aging,the thickness of oxidation cross section is the thinnest,which is 250μm and 255μm respectively.On the whole,the antioxidant performance showed as Solution-ECAP-Aging>ECAP-Solution-Aging>Solution-Aging.（4）The high temperature corrosion properties of titanium alloy after 820℃/880℃Solution-580℃Aging,820℃/880℃Solution-ECAP-580℃Aging,ECAP-820℃/880℃Solution-580℃Aging at 800℃on the medium of 75wt.%NaCl+25wt.%Na₂SO₄ for40h were investigated.The results show:The corrosion weight gain curve tends to be stable after 35h and the corrosion weight gain at 880℃Solution-ECAP-580℃Aging is0.88(mg·cm^-2),which is the slowest.The corrosion surface is mainly composed of TiO₂and Al₂O₃ and Na⁺,Cl^-,S²⁺,O^2-diffuse to form TiSO₄,Na₂TiS₃,NaAlO₂ and other phases.During the transition from the substrate to the corrosion layer,Ti（%）decreases and O（%）gradually increases.Al（%）,Na（%）,S（%）,Cl（%）appears a small amount of aggregation in the corrosion layer.The microstructure after Solution-ECAP-Aging has good interfacial adhesion and compactness and the surface of Solution-ECAP-580℃Aging has compact lamellar structure,which indicates that lots of Na⁺has been adsorbed to prevent the invasion of corrosive ions.The corrosion section thickness of820℃/880℃Solution-ECAP-580℃Aging is 70μm,50μm respectively.On the whole,the corrosion resistance showed as Solution-ECAP-Aging>ECAP-Solution-Aging>Solution-Aging.In summary,strong plastic deformation（ECAP）and heat treatment mechanism（Solution and Aging）were coupled in this paper.Ti-555 titanium alloy was used as the research object,exploring the effects of a variety of coupling processes on Ti-555titanium alloy and the properties were improved,which provided more references for the subsequent research on Ti-555 titanium alloy.