| In order to meet the requirements of aerospace lightweight structural materials from 600 oC to 800 oC, Ti Bw/Ti60 composite with network structure were fabricated by low energy milling and reactive hot pressing technique based on the system of Ti60(~110um)-Ti B2(3~5um)in the present work.The volume fraction of Ti Bw reinforcement is 1.7%, 3.4%, 5.1% and 6.8%, respectively.The hot deformation behaviors of Ti B whiskers reinforced Ti60 alloy matrix composites with network microstructurewere investigated by hot compression tests in the temperature range of 900~1050 oC and strain rate range of 0.001~1 s-1.The microstructure of as-sintered and as-extruded Ti Bw/Ti60 composite, the morphology and distribution of reinforcement, were investigated using optical microscopy(OM), scanning electron microscopy(SEM) and electron back-scattering analysis(EBSD). The interface between reinforcement and matrix, the precipitate were investigated using transmission electron microscope(TEM).The mechanical properties at room and high temperatures of the as-sintered and the as-extruded composites were studied. The influence of extrusion deformation on the microstructure and properties of the composites was analyzed.The microstructure analysis shows that the Ti Bw were in situ by the reaction between Ti and Ti B2. They were distributed around the Ti60 particles in the form of a networkstructure. With increasing the volume fraction of Ti Bw, the size of networkstructure have no changed and the thickness of networkstructure boundary are increased.The TEM results of the as-sintered Ti Bw/Ti60 composites indicated that the ordered a2 precipitation(Ti3Al) were formed due to the slowly cooling rate.Comparing with Ti60 alloy, the strength at room temperature of the as-sintered 1.7vol.%,3.4vol.%and 5.1vol.%Ti Bw/Ti60 composites with network structure have been increased by 9%, 11%and 15% respectively.Theelongations of Ti Bw/Ti60 compositeshave a remarkable reduction compared with those of Ti60 alloy.Base on the hot compression test, the hot deformation behaviors of 3.4 vol.%Ti B/Ti60 composites with network microstructurewere investigated.The flow stress curves exhibited obviousflow softening phenomenon at low strain rates in the a+b phase region. With increasing the deformation temperatures, the degree of flowsoftening were decreased. Above the deformation temperature of 950 o C, the degre e of flow softening were increased with decreasing the strain rates. In a+b phase region, flow softening is mainly attributed to globularization of ap phase with the CDRX mechanism. In b phase region, recrystallization and the growth of primary b grains occur with the DDRX. With increasing the temperatures and decreasing the strain rates, the fraction of LAGBs is decreased. Ti Bw played an important role on the microstructure evolution of matrix. Globularization of elongated ap and DRX of primary b grains occur primary near Ti Bw region due to providing high density dislocation and nucleation for DRX. Ti Bw also restrict the grain size of as colony The apparent activation energyis calculated to be 765.07 k J/mol in the temperature range from 950 oC to 1050 oC,The processing maps show two regions:the safe domain of 1000-1100oC/0.001-0.5s-1and the instable deformation region of 900-1000oC/>0.5s-1.Superior tensile properties(at room and high temperatures) and microstructure evolutionafter extrusion(1200oC) have been systematically investigated in Ti Bw/Ti60 composites with network architecture. The results showed that ordered a2phase formed in sintering process was re-dissolved after extrusion.The Ti Bw were rotated to alignment the extrusion direction and discretely distributed in matrix of the composites with the low volume fractions(1.7% and 3.4%). The as-extruded composite exhibit strong <0001>a // ED fiber texturewhich is transformed from <110>b //ED fiber texture based on Burgers’ relationship.EBSD results showed that grain size of Ti60 matrix was significantly refined from ~30um to ~10um.The superior combinationof tensile strength(1364MPa) and elongation(6%) of the asextruded 3.4 vol.% Ti Bw/Ti60 composites have been obtained at room temperature. In particular, the as-extruded composites also exhibited super-high tensile strength at elevated temperatures(close to 1000 MPa at 600 oC and 780 MPa at 700 oC).Multisection fractureof Ti Bwin the longitudinal sections fracture indicated superior strengthening effect of Ti Bw and good ductility of the composites.The effects of different heat treatments on the microstructure and tensile properties of the as-extruded Ti B whisker reinforced near-a Ti(Ti60)(Ti Bw/Ti60) composites have been investigated. Without solution treatment, the a2(Ti3Al) phase are precipitated from a phase and mounts of elliptical S2 silicide((Ti Zr)6Si3) are formed at the interface between a phase and b phase during the aging at 700 o C for5hrs. After the solution and aging treatment(1000 oC/2hrs/AC+700 oC/5hrs/AC), more a2 phase is precipitated from primarya(ap) phase and less spherical S2 silicide is precipitated from the body of both ap and transformed b phase microstructure.The orientations relationships betweenthe a2phase and the amatrix are[ 1?21?0 ]a// [11?02?]a2 and(0002)a//(202?1)a2. With increasing the aging temperature, the size of the a2(Ti3Al) phase are increased. Moreover, they are prefer to form at dislocation and near the Ti Bw region. At the test temperature of 600 oC, the tensile strength of the composites are increased by 98 MPa by solution and aging treatment and 40 MPa by one step aging treatment. |
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