Research On Design,preparation,microstructure And Properties Of Low Cost And High Strength Titanium Alloys Containing Cr Based On Diffusion Couple Technique

Titanium alloy is an important strategic metal because of its balanced physical and mechanical properties such as,high specific strength,non-magnetic,excellent corrosion resistance and good bio-compatibility,which make it an attractive material in aviation aerospace,sport and leisure,weapon,chemical industry,automobile,marine engineering and biomedicine fields.However,the high cost of titanium alloy severely limits its further application.The development of low-cost titanium alloy has becoming the hotspot problem in the major world powers.In this work,the target alloys composition were designed and quickly screened based on high-throughput diffusion couple technique using cheap alloy elements(Al,Cr,Fe)as alloying raw materials,which have significant strengthening effect on titanium alloy,The evolutions of the microstructure and properties of the target alloys under different heat treatment systems were systematically investigated.The relationship between the microstructure and tensile properties was established.The strengthening and plasticizing mechanism of the alloys was revealed.That provides theoretical and technical basis for the design and preparation of low-cost and high-strength titanium alloys.The main contents and conclusions of this work are as follows:(1)Three diffusion couples(Ti-6Al)-(Ti-6Al-20Cr),(Ti-6Al-1Mo-1Fe)-(Ti-6Al-1Mo-1Fe-20Cr)and(Ti-6Al-1.7Fe-0.1Si)-(Ti-6Al-1.7Fe-0.1Si-20Cr)were prepared by high-throughput diffusion couple technique,using Ti-6Al,Ti8LC(Ti-6Al-1Mo-1Fe)and Timetal 62S(Ti-6Al-1.7Fe-0.1Si)as matrix alloys,respectively.The corresponding relationships among composition,microstructure and hardness of Ti-6Al-x Cr,Ti-6Al-1Mo-1Fe-x Cr and Ti-6Al-1.7Fe-0.1Si-x Cr alloy systems were obtained through microstructure characterization,composition analysis and microhardness test.Three novel Cr-containing alloys in the respective alloy systems were selected,which had the maximum hardness and superfine α phase.They were Ti-6Al-10.9Cr,Ti-6Al-1Mo-1Fe-6.9Cr and Ti-6Al-1.7Fe-0.1Si-7.3Cr alloys.(2)The phase compositions and morphology of the above three alloys after quenching at β single-phase region and aging were characterized.The results showed that the three alloys were all composed of a single β phase,when quenched at β single-phase region,while a large amount of nanoscale α phase was precipitated in the aged alloys,which was distributed in the β matrix.It was confirmed that the strengthening mechanism for the alloys was precipitation strengthening.(3)The microstructure evolutions and the influences on the tensile properties of the alloys heat treated along different regimes were studied.It was found that the morphology and content of the primary α phase in the alloys were affected by the solution temperature.When solution treated at a low temperature of(α+β)dual-phase region(770℃,800℃and 800℃),the primary α phase in the three alloys had small size and high volume fraction,which mainly consisted of coarse flake α phase for Ti-6Al-10.9Cr and Ti-6Al-1.7Fe-0.1Si-7.3Cr alloys,and spherical αphase for Ti-6Al-1Mo-1Fe-6.9Cr alloy.Therefore,Ti-6Al-10.9Cr and Ti-6Al-1.7Fe-0.1Si-7.3Cr alloys had higher strength and lower ductility,while Ti-6Al-1Mo-1Fe-6.9Cr alloys had lower strength and better ductility.When the alloys were solution treat at a high temperatures of(α+β)dual-phase region(810℃,840℃ and 840℃),the primary α phase became coarse,the aspect ratio and volume fraction significantly decreased,and a sharp increase in the content of metastable β phase,which precipitated much of secondary α phase during aging.The excellent strength-ductility synergies for the three alloys were realized by incorporating multiple precipitation of α phase into the alloy,which was composed of spherical α phase,coarse flake α phase and acicular α phase.When the solution temperature was above(α+β)/β phase transition temperature,the absence of α phase and growth of β grains resulted in the poor ductility of the three alloys(≤2.1%).Furthermore,the aging temperature had an important effect on the secondary α phase in titanium alloy.When the aging temperature increased from 400℃ to 470℃,the amount of the secondary α phase increased,resulting in the increase of strength and reduction of elongation for the alloys.When the aging temperature was above 540℃,the ductility of the alloy exhibited a precipitous decline because of the precipitation of the Laves phase(Ti Cr2).(4)The microstructure of titanium alloy was controlled by adjusting the heat treatment process to obtain low cost titanium alloy with excellent strength-ductility combinations.The results revealed that two optimal strength-ductility synergies were achieved for Ti-6Al-10.9Cr alloy after heat treatments at 810℃/0.5h/AC+400℃/6h/AC and 810℃/0.5h/AC+470℃/6h/AC with the ultimate tensile strength and tensile elongation values of 1324MPa/16.3% and 1656MPa/9.2%,respectively.Two superior strength-ductility balances were successfully obtained for Ti-6Al-1Mo-1Fe-6.9Cr alloy with values of 1395MPa/16.0% and1614MPa/9.7%,respectively,when heat treated at 800℃/0.5h/AC+470℃/6h/AC and 840℃/0.5h/AC+470℃/6h/AC,Two excellent strength-ductility combinations were successfully achieved for Ti-6Al-1.7Fe-0.1Si-7.3Cr alloy after heat treatments at 800℃/0.5h/AC+400℃/6h/AC and 840℃/0.5h/AC+470℃/6h/AC with values of1395MPa/12.7% and 1753MPa/5.1%,respectively.The values of the maximum tensile strength all exceed 1600 MPa,and the elongation were higher than 5% for the three low-cost and Cr-containing titanium alloys designed in this work.These strength-ductility balances for the alloy were superior to many ultra-high strength titanium alloys.