Effects Of Alloying Elements And Semi-solid Treatment On The Structure And Properties Of Ti-Fe Alloys

Titanium and its alloys have excellent mechanical properties such as high strength,low density and good plasticity,which are widely used in industrial fields.In recent years,Ti-Fe alloys have received widespread attention due to their room temperature compressive strength exceeding 2 GPa and better plastic deformation.Taking Ti-Fe-Y alloy as the research object,in order to further improve its strength and plasticity as the goal,this article prepared Ti-Fe-Y,Ti-Fe-M-Y（M=Co,Mo,Ni）,Ti-Fe-Nb-Y series alloys.The microstructure and mechanical properties of the alloy were studied.In this paper,the effects of alloying elements and semi-solid isothermal heat treatment on the microstructure and mechanical properties of alloys ware studied,and the microstructure evolution law of semi-solid isothermal heat treatment and the mechanism of plasticity improvement ware discussed in depth.The main results are as follows:（1）By preparing(Ti_100-xFe_x)₉₈Y₂（x=10,13,18,22,29.5,35,40）series alloys,the effect of Fe element content on the structure and properties of Ti-Fe-Y alloys was studied.It was found that when the Fe element content is near the Ti-Fe eutectic point（x=22,29.5,35,40）,the microstructure follows the equilibrium solidification law in the eutectic phase diagram.Hypoeutectic is composed ofβ-Ti first precipitated dendrite phase and eutectic structure,and hypereutectic is composed of Ti Fe first precipitated dendrite phase and eutectic structure.During the alloy solidification process,the dendritic pre-precipitate phase is first precipitated,and the remaining liquid phase undergoes a eutectic reaction L→β-Ti+Ti Fe,forming a nano-scale lamellar eutectic structure composed ofβ-Ti and Ti Fe.The eutectic structure has an orientation relationship ofβ-Ti[110]∥Ti Fe[011].When the Fe element content is far from the eutectic point（x=10,13,18）,the alloy is composed ofβ-Ti matrix and precipitated Ti（Fe,Y）compound phase.The room temperature compressibility of Ti-Fe-Y alloy varied with the Fe content.With the increase of Fe content,the yield strength and compressive strength show a trend of first increasing and then decreasing,and the plasticity shows an overall decreasing trend.When the Fe element content is near the Ti-Fe eutectic point（x=22,29.5,35,40）,the yield strength of the alloy and the volume fraction of the eutectic structure conform to the linear relationship ofσ_YS=5.646V_E（%）+1384.25MPa.(Ti₇₈Fe₂₂)₉₈Y₂ has the best comprehensive properties of strong plasticity,with a compressive strength of 2382MPa and a plastic deformation of 33.4%.（2）In order to further improve the strength and plasticity of the alloy,(Ti₇₈Fe₂₂)₉₅M₃Y₂（M=Co,Ni,Nb,Mo）series alloys were prepared on the basis of(Ti₇₈Fe₂₂)₉₈Y₂ alloy.It was found that Nb element can be well dissolved intoβ-Ti,neither changing the phase composition of the alloy nor changing the eutectic structure of nano-scale ultra-fine lamellae.(Ti₇₈Fe₂₂)₉₅Nb₃Y₂ alloy shows the best overall performance,with a compressive strength of2317MPa and a plastic deformation of 35.8%.（3）After selecting the Nb element as the alloying element,the(Ti₇₈Fe₂₂)_98-xNb_xY₂（x=3,6,9,12）series alloys were prepared to study the effect of the Nb element content on the structure and properties.It was found that the change of Nb element content mainly affects the phase volume fraction of the alloy.With the increase of the Nb element content,the volume fraction of the eutectic structure in the alloy gradually decreases,and the content of the first precipitated dendrites increases accordingly,which makes the plasticity of the alloy improved.Among them,(Ti₇₈Fe₂₂)₈₉Nb₉Y₂ has the most excellent comprehensive performance,its compressive strength is 2538MPa,and its plastic deformation is 45.6%.（4）Semi-solid isothermal heat treatment transformed the pre-precipitated phase of(Ti₇₈Fe₂₂)₈₉Nb₉Y₂ alloy from dendrites to spherical dendrites.And with the increase of holding temperature and holding time,the spheroidization degree and size of dendrites gradually increase.After the alloy was kept at 1100°C for 60 minutes,the shape factor SF and average size D of the dendrites reached 0.805 and 73μm,respectively.The change of dendrites from dendrites to spherical maked the plasticity of the alloy significantly improved.Compression experiments at room temperature show that the plastic deformation of(Ti₇₈Fe₂₂)₈₉Nb₉Y₂ alloy gradually increases with the increase of holding time.The compressive strength after 60minutes of heat preservation was 2551 MPa,and the plastic deformation reached 50.5%.The formation ofβ-Ti micron dendrite/ultra-fine lamellar eutectic structure is an important reason for the improvement of the plasticity of(Ti₇₈Fe₂₂)₈₉Nb₉Y₂ alloy.（5）In this paper,a new titanium alloy(Ti₇₈Fe₂₂)₈₉Nb₉Y₂ with micron-scale dendrite/ultrafine lamellar eutectic structure was obtained by adding alloying elements and semi-solid treatment.The alloy has excellent mechanical properties of high strength and high plasticity.On the one hand,it overcomes the complex process required for preparing nanocrystalline or ultra-fine-grained metal materials.On the other hand,it solves the shortcomings of high strength and poor plasticity of nano-crystalline or ultra-fine-grained metal materials.