Forging Forming And Microstructure And Properties Control Of TiB_w Reinforced High Temperature Titanium Matrix Composites

Particle reinforced titanium matrix composites(PRTMCs)have the advantages of high specific strength,high creep resistance and high temperature resistance,which are the key candidate materials to realize the lightweight of aircraft launch system.The traditional casting method is not suitable for large-scale application because of its coarse grains and high deformation resistance.In recent years,hot-working technology has become an effective means to improve the micro structure and properties of titanium matrix composites(TMCs).In this paper,TMCs reinforced with low volume fraction TiB short fiber(TiBw)were prepared by in-situ method.The effect of deformation temperature on the micro structure and mechanical properties of a near a high temperature titanium alloy and its composites was studied.The effect of deformation temperature on dynamic recrystallization mechanism(DRX)was clarified.The dynamic precipitation behavior and precipitation mechanism of silicide were concluded.Combined with the finite element numerical simulation method,the forging process of TMCs disk was determined.The main results are as follows:IMDF can significantly change the micro structure of the composites,promote the dynamic recrystallization of the matrix grains and improve the distribution of TiB.The DRX mechanism dominates the refinement of high temperature titanium alloy during IMDF deformation.With the decrease of forging temperature,the recrystallization mechanism of the alloy changes from discontinuous dynamic recrystallization mechanism(DDRX)to continuous dynamic recrystallization mechanism(CDRX).The average grain size decreases from 4.02 μm at 1010℃ to 1.88 μm after forging at 920℃.The result shows that the low deformation temperature of IMDF can promote the synergistic effect of DDRX and CDRX.The precipitation of silicides at grain boundaries and dislocation is observed in the micro structure after forging.The composites obtained uniform ultrafine grains after two pass IMDF.The matrix was completely DRX,and after 870℃ deformation,the matrix was composed of DRXed grains and dislocation cell,while a large number of DRXed grains and ultrafine nano grains were formed after forging at 800℃.The average grain size of matrix decreased from 0.98 μm to 0.59 μm.In addition,IMDF also reduced the aspect ratio of TiBw from 7.3 to 4.2,and TiBw also promoted the refinement of the matrix grains during deformation.IMDF promotes the precipitation of S2 type silicides in high temperature titanium alloy and its TMCs,and the precipitation behavior of silicides is closely related to the deformation temperature.With the decrease of forging temperature,the precipitation position of silicide extends from α and β phase interface and grain boundary to phase boundary,grain boundary and grain interior.The average size of silicide decreases from 200～400 nm after forging at 950℃ to 100 nm after deformation at 800℃.Low temperature IMDF deformation can promote the uniform precipitation of nano silicide,and its nucleation and growth depend on the diffusion of Zr and Si.In addition,there is interaction between DRX and silicide in the matrix,and silicide disperses and precipitates due to dislocation.Silicide can promote DRX behavior of matrix grains,pin grain boundaries and inhibit grain growth.After IMDF process,the strength and plasticity of high temperature titanium alloy and its composites are improved simultaneously.After forging at 920℃,the tensile strength at room temperature can reach 1148.7 MPa,and the elongation is increased to 10.6%;the tensile strength at 650℃ can reach 719.5 MPa,and the elongation is 19.3%.The improvement of strength and plasticity of as-forged alloy is attributed to fine grain strengthening and dislocation strengthening.The maximum tensile strength of TMCs after IMDF is 1320.3MPa and the elongation is 5.8%,while the tensile strength after forging at 950℃ is 1126.0 MPa and the elongation is 9.3%.The strength and plasticity of TMC show the characteristics of mismatch,which is related to the fracture of reinforcements and the precipitation of coarse silicide at grain boundary.The tensile strength of the composite at 650℃ is 758.3 MPa and the elongation is 19.7%.The improvement of the strength of the composites after forging is mainly caused by fine grain strengthening,second phase strengthening and TiB short-staple load transfer.In addition,the room temperature properties of TMCs are sensitive to the tensile strain rate.With the increase of strain rate,the tensile strength increases and the elongation decreases.The forming law of TMCs disk was studied by numerical simulation technology,and the best forming process was established.TMCs discs with excellent micro structure and properties were obtained by open die forging.The tensile strength at room temperature is 1281 MPa and the elongation is 6.14%;the tensile strength at 650℃ is 778 MPa and the elongation is 14.15%.The relationship between the heat treatment and the microstructure and properties of the forging was explored.The micro structure with excellent high temperature properties at 700℃ can be obtained by controlling the equiaxed grain content.