Design Of Ti-Zr-Ta Alloys With Low Modulus And High Strength Based On Spinodal Decomposition And Optimization Of Their Microstructures And Mechanical Properties

Novel nontoxic ?-type titanium alloys exhibit low elastic modulus,high specific strength,good corrosion resistance and good biocompatibility,which are considered to be one of the most promising metal materials for biomedical use.Genernally,the conventional methods to improve the strength of ? titanium alloys lead to the increase of their elastic modulus while increasing the strength of the alloys.The spinodal decomposition has the advantages of fine structure,uniform structure and the same crystal structure,which can greatly improve the strength of the alloy without increasing the elastic modulus of the alloy.It has been comfirmed that a wide solid solution gap exists Ti-Zr-Ta the phase diagram of the ternary alloy system,which provides the basis for the design of spinodal decomposition Ti-Zr-Ta alloys with low modulus and high strength.Thus,in the present thesis,Ti-Zr-Ta ternary alloy system was taken as the research object,and the spinodal decomposition Ti-Zr-Ta alloys with high strength were designed based on Ti-Zr-Ta thermodynamics and d-electron orbital theory.The microstructure evolution of the designed alloys during aging and its influence on the mechanical properties of the alloys were ivestigated,and the mechanisms of the spinodal decomposition strengthening in Ti-Zr-Ta alloys were analyzed.The main research contents and conclusions are summerized as follows:(1)On the basis of the ternary phase diagram of Ti-Zr-Ta alloy and the d-electron orbital theory,the spinodal decompsoition alloys with low elasticity modulus,Ti-40Zr-20Ta?Ti-35Zr-20 Ta,Ti-30Zr-20 Ta,were designed.The microstructure and mechanical properties of the three alloys were characterized.The results show that the spinodal decompostion could occure in all of the three alloys as they were aged at proper temperature.Among them,the alloy with higher Zr content,Ti-40Zr-20 Ta,exhibits higher strength and lower elastic modulus after solid solution treatment and solid solution treatment,and with the content of Zr increasing,the elastic modulus of the alloys decreases but their strength increases.(2)The three Ti-Zr-Ta alloys designed were aged at different temperatures and time,and the microstructure evolution during aging and its influence on their mechanical properties were studied.The results show that after aging treatment at 923 K,973 K and 1023 K,the spinodal decomposition occurs in all three alloys,and the alloys after aging at 973 K shows better comprehensive mechanical properties.As the aging time increases,the volume fraction of the spinodal decompsited products increases,while the yield strength and hardness of the alloy increase with the aging time,which increases first and then decreases slightly and finally remains stable.(3)The spinodal decomposition strengthening mechanisms in Ti-Zr-Ta alloy were analyzed.The results show that in the early stage of aging,the wavelength of spinodal decomposited structure changes very little,and the wavelength increases with extension of aging time.The quantitative analysis results show that the yield strength increment of the alloy is proportional to the wavelength in the early aging stage,while the yield strength is inversely proportional to the wavelength in the later aging stage.