Study On Thermo-process And Microstructure Evolution And Mechanical Properties Of Bimodal Sized Particles Reinforced Titanium Matrix Composites

Particle reinforced titanium matrix composites have the advantages of low density,high specific strength and excellent high temperature resistance,and have great development prospects in aerospace and automotive manufacturing.However,as cast titanium matrix composite has poor plasticity and large deformation resistance,it limits its application.Therefore,in-situ synthesis technology and traditional casting method were combined to prepare two-scale particles?micron TiB and submicron Y₂O₃?reinforced titanium matrix composites with different volume fractions?its matrix composition is a new near-alpha superalloy Ti-6Al-3.5Sn-4.5Zr-2.0Ta-0.5Mo-0.7Nb-0.4Si?.The effect of dual-scale particle content on Microstructure and properties of as-cast near-alpha superalloy was studied.The deformation behavior and microstructure evolution of dual-scale particle reinforced titanium matrix composites under hot compression were elucidated,and their hot working diagrams were constructed.On this basis,the double-scale particle reinforced titanium matrix composites with fine grains and excellent mechanical properties were prepared by severe plastic deformation?multi-directional forging?.The main findings of this paper are as follows:?1?The?TiB+Y₂O₃?/Ti composites with different volume fractions were prepared by vacuum melting and in-situ synthesis.During solidification,Y₂O₃ and TiB were mainly distributed on the primary beta grain boundaries,in which Y₂O₃ was equiaxed and TiB was short fiber,and both of them were well bonded to the matrix interface.The in situ formation of Y₂O₃ and TiB significantly refined the primary primary beta grain size and the width of the alpha lamellae.The average grain size of primary beta phase of 5vol.%?TiB+Y₂O₃?/Ti composite is 65.3 micron,which is one order of magnitude smaller than that of matrix alloy,and the width of alpha lamella is refined by 9.6%.The refinement of primary beta crystals is mainly due to the undercooling of Y₂O₃ and TiB precipitated at the front of solid-liquid interface during solidification,which promotes the nucleation of primary beta crystals.The appearance of Y₂O₃ and TiB further pins the growth of primary beta crystals.The refinement of alpha lamellae is mainly due to the refinement of primary beta grain and the fact that Y₂O₃and TiB can be used as nucleation particles of alpha-Ti.?2?The mechanical properties of?TiB+Y₂O₃?/Ti composites with different volume fraction were tested at room temperature.It was found that the compressive strength increased with the increase of the volume fraction of the reinforced phase,while the compressive plasticity and fracture toughness decreased gradually.The compressive strength,compressive plasticity and fracture toughness of 5vol.%?TiB+Y₂O₃?/Ti composites are 2030.8MPa,9.5%and 60.1 MPam^-1/2,respectively.The increase of strength of the composites is mainly due to the grain refinement caused by Y₂O₃ and TiB.The decrease of fracture toughness is mainly due to the crack growth and propagation caused by the fracture of Y₂O₃ and TiB.?3?The hot compressive deformation behavior of 5vol.%?TiB+Y₂O₃?/Ti composites at900¹020?,0.01s^-110s^-11 was studied.The relationship between the rheological stress and deformation temperature and strain rate was revealed.The rheological stress and peak stress decreased with the increase of temperature and the decrease of strain rate.The activation energy of thermal deformation of the composites in the?alpha+beta?two-phase region is614.1kj/mol,and the constitutive equation of the composites is established.The microstructure evolution of the composites under different deformation,temperature and strain rate was revealed.Based on the dynamic material model,the hot working diagram of5vol.%?TiB+Y₂O₃?/Ti composites was constructed.It was found that the composites were prone to instability at low temperature and high strain rate,to instability and dynamic recovery at low temperature and low strain rate,to dynamic recovery and recrystallization at high temperature and high strain rate.Dynamic recrystallization is prone to occur at low temperature and strain rate.?4?Based on the experimental results of hot compression deformation,5vol.%?TiB+Y₂O₃?/Ti was forged in the two-phase region?1010??and the beta phase region?1070??,respectively.It was found that isothermal multidirectional forging significantly improved the microstructure of the composites.Compared with the as-cast titanium matrix composites,the lamellar width of the as-cast titanium matrix composites is 8.54 micron,the average grain size of the composites forged at 1010?is 4.54 micron,and the average grain size of the composites forged at 1070?is 6.52 micron.EBSD analysis shows that the grain refinement is mainly due to the dynamic recrystallization of the composites after multi-directional forging,and the dynamic recrystallization of the composites is mainly due to the full deformation of the composites and the presence of reinforcements.?5?Isothermal multi-directional forging significantly improves the mechanical properties of 5vol.%?TiB+Y₂O₃?/Ti composites at room and high temperature,and has excellent Strength-plasticity matching.The tensile strength and elongation of the composites were1178.5MPa and 11.2%respectively at room temperature and 1081.4MPa and 9.1%respectively after multi-directional forging at 1010?and 1070?.The composites still exhibit excellent tensile properties at 650?after multidirectional forging at 1010?and1070?.The tensile strength can reach 709.3 MPa and 677.4 MPa,respectively.Based on the analysis of microstructure and fracture morphology,the microscopic nature of isothermal multidirectional forging for improving the strength and plasticity of 5vol.%?TiB+Y₂O₃?/Ti composites was revealed.