Hot Deformation Of (TiB_w+TiC_p) Reinforced Titanium Matrix Composites And Microstructure And Mechanical Properties Of The Sheets

Due to the demand of hypersonic aircraft for ultra-high flight speed,there are also strict requirements for high-temperature structural materials used in aircraft engines,shells,skins and other structures.The"thermal barrier"temperature of traditional high-temperature titanium alloy is limited to 600℃.Increasing the service temperature will lead to a sharp decline in the creep performance and high-temperature oxidation resistance of the titanium alloys,which can’t meet the requirements of new generation of aircraft for lightweight,high-temperature resistant structural materials.The in-situ reinforced titanium matrix composites have the advantages of low density,high specific strength and good thermal strength,and become the preferred material to break through the temperature limit of existing titanium alloys.In this paper,in-situ（Ti B_w+Ti C_p）reinforced titanium matrix composites（TMCs）with excellent mechanical properties were prepared by vacuum self-destructive arc melting and hot deformation technology.The deformation behavior,microstructure evolution and mechanical properties of hot deformation composites were studied,and the texture weakness and high-temperature strengthening mechanism of TMCs during heat treatment were also discussed.The recrystallization mechanism and high-temperature tensile deformation behavior of TMCs by multi-directional forging were studied.After multi-directional forging,the composite forms uniform equiaxed microstructure and precipitates a large amount of silicides.The formation of subgrain boundary inαphase is caused by the strong recovery during hot deformation and subsequent annealing.The pile-up of dislocations against reinforcements can promote primaryαphase（α_p）recrystallization.The refinement of TMCs is affected by the by continuous dynamic recrystallization（c DRX）,dynamic recrystallization（DRX）and reinforcement promoted recrystallization behavior.The tensile properties of（Ti B_w+Ti C_p）reinforced TMCs treated by multi-directional forging and annealing at 650℃increases by 7%-14%compared with the as-cast TMCs,and the plasticity has been improved significantly.The high temperature tensile fracture behavior is related to the barrier of the phase boundaries to dislocation,the interaction of dislocations and the high-temperature sliding of boundaries,which will lead to the nucleation of pores,crack propagation and bridging during tensile deformation,and ultimately accelerate the failure of TMCs.The static softening mechanism of the as-forged TMCs under double-hits compression and the microstructure evolution and mechanical properties of the as-rolled sheets with different deformation amount were studied.After the study of double-hits compression behavior,it was found that the interrupted compression of as-forged composite in（α+β）region is controlled by c DRX caused by the transformation from low-angle boundaries（LABs）to high-angle boundaries（HABs）through lattice rotation,while inβregion,controlled by the discontinuous dynamic recrystallization（d DRX）involving nucleation and growth.After（α+β）region multi-pass rolling,initial preheating,inter pass heating and increased deformation amount during hot rolling make theα_p phase significantly refined,and the precipitation of multi-scale silicides is affected by initial preheating and multiple heating.The cooling rate of the sheets after rolling is fast,and the micro-silicides and reinforcements can provide nucleation particles and restrict the migration of interface to prevent its growth,improving the precipitation of nano-α_s fromβtransformation（β_t）.Refinedα_p reduces the effective slip distance of dislocations,and the multi-scale silicide precipitated insideαphase can strengthen theα_p,thus improvingα_p strengthen.Otherwise,precipitation of nano-α_s can strengthenβ_t.Due to the difference strength of two phases,the nonuniform deformation induced strengthening effect is formed.At the same time,due to the accumulation of dislocations againstα_p/β_t boundary,the fracture of TMC sheets will be caused by theα_p/β_t boundary cracks.The effect of rolling temperature on microstructure evolution and mechanical properties of TMC sheets was studied.Based on the rolling process optimization,the sheets with excellent mechanical properties were obtained.It is found that theβgrain is obviously refined and the silicide is completely dissolved.The formation of mixed microstructure with equiaxed and short lamellarαphase.The crystallography orientation in equiaxedαphase is richer than that in lamellarαphase,which weakens the texture of sheets.The formation of equiaxed microstructure is associated with the reinforcements promote dynamic spheroidization caused by the accumulation of dislocations and theα/βlamellar deformation promotes spheroidization caused by the temperature dropping during rolling.The sheet forms a T-shaped texture which is approximately parallel to the TD direction.Because the activation of<c+a>dislocation can produce a<c>direction slip,the texture isn’t strictly towards TD.The texture intensity of as-rolled sheet inβsingle region is much greater than that in（α+β）region,which is the comprehensive effect of deformation and phase transformation.The UTS and EL of high-performance sheet at room temperature are 1268 MPa and 7.2%,respectively,and the UTS at 650℃and 700℃were 798 MPa and 597 MPa,respectively,which can meet the high-temperature requirements for TMC sheets in the aerospace field.The effects of heat treatment（HT）on the microstructure and mechanical properties of the TMC sheets with different rolling processes were studied,and the mechanism of texture weakening and high temperature strengthening of the sheets was also discussed.The reinforcements can improveβgrain refinement through static recrystallization and affect variation selection byα→β→αtransformation during HT,which weakens the texture and reduces the generation of"macrotexture".The source of equiaxedαphase in various sheets is different,but theα_p orientation of（α+β）region HT is inherited from the original sheet,and the subsequentα_s doesn’t keep the same/similar crystallography orientation withα_p.The precipitation ofα_s with non-Burges orientation relationship（BOR）against reinforcements is related to the non-OR of reinforcements and originalβgrains during rolling deformation,which weakens the texture of sheets.The UTS of heat-treated sheet at 700℃can reach 686 MPa,the YS can reach 591 MPa,the EL is 22.9%,and the UTS increases by 30%.The strengthening ofα/βboundary by nano-silicides and the simultaneous strengthening of intragranularαphase and grain boundaryαphase of nano-α₂ play an important role on the high-temperature mechanical properties,which makes the interfacial strength between reinforcement phase and matrix became the control factor of tensile fracture of heat-treated sheet at 700℃.