| In situ synthesized titanium matrix composites have potential applicationprospects in aerospace, advanced weapons systems and automotive manufacture dueto their low density, high specific strength and modulus, excellent fatigue and creepproperties. However, poor room-temperature ductility and high high-temperaturedeformation resistance of titanium matrix composites limit their large-scaleengineering applications. In this paper, titanium matrix composites reinforced withdifferent volume fractions of （TiC+TiB） were prepared by vacuum induction meltingtechnology. Matrix nominal composition is Ti-6Al-2.5Sn-4Zr-0.7Mo-0.3Si. Theinfluence of reinforcement content on microstructures and mechanical properties ofas-cast composites was investigated. Hot compressive deformation behavior andmicrostructure evolution of （TiB+TiC）/Ti composites were studied. Thecorresponding relationship between microstructure and properties during thermalprocessing was discussed. In addition, superplasticity of titanium matrix compositesheets was researched and superplastic deformation and failure mechanisms wereanalyzed.Solidified TiB and TiC are prone to be segregated in primary β grain. TiBmainly exhibits whisker morphology, where as TiC shows near-equiaxed shape. Theinterface between reinforcements and matrix is very clean. The presence of TiB andTiC refines primary β grain and α lath, changes the colony characteristic of α phaseand makes the orientation of α lath more random. The refinement mechanism of βgrain is attributed to constitutional supercooling resulted from solute enrichment ofB and C in the solid-liquid interface and its impeditive effect on the growth ofprecipitated β grain. The refinement of prior β grain results in more grain boundariesacting as heterogeneous nucleation sites and narrower growth space for α phase,which leads to the refinement of α phase together.The introduction of TiB and TiC significantly improves the ambient and hightemperature strengths of as-cast （TiB+TiC）/Ti composites. Compared with matrixalloy, the yield strengths of composites with （TiB+TiC） volume fraction of2.5%,5%and7.5%increase by16.2%,20.2%and28.3%, respectively. The enhancementof room-temperature yield strength is mainly attributed to the refinement of matrixmicrostructure. Composites show advantages in strength below750℃compared tomatrix alloy and the strengths of composites increase with the increase ofreinforcements. With the raise in tensile temperature, the increments of tensile strengths of composites increase first and then decrease and reach to their maximumvalue at650℃. This is because the refinement of matrix microstructure still canmakes obvious contribution on strengths of composites and load-bearing role ofreinforcements can be enhanced.Thermal physics simulation method was employed to research hot compressivedeformation behavior of5vol.%（TiB+TiC）/Ti composite. The relationship amongflow stress and deformation temperature as well as strain rate is revealed. The peakand flow stresses all decline with increasing temperature and decreasing strain rate.The variations of peak stress σpwith （1000/T） and ln all meet the linearrelationship. Hot deformation activation energy and hardening factor of thiscomposite are608.3kJ·mol-1and4.27. In addition, constitutive equation of thiscomposite hot-deformed in α+β phase field is established. These can guide theselection of thermal deformation parameters and equipment tonnage.Microstructure evolution and softening mechanism of5vol.%（TiB+TiC）/Ticomposite during thermal compressive process were clarified. The formation ofdeformation microstructures results from the comprehensive role of phasetransformation, dynamic recovery and recrystallization. High temperature and lowstrain rate are conducive to the coordinated deformation between reinforcements andmatrix and dynamic recrystallization process. The softening mechanism of thecomposite deformed in α+β phase field is mainly dynamic recrystallization. Theimprovement of β phase content is helpful for the decrease of flow stress and hotdeformation activation energy. The influence of TiB and TiC on thermal deformationbehavior of matrix mainly depends on the variation of the ratio of α and β phase.TiB/Ti and （TiB+TiC）/Ti composite sheets with high quality were producedsuccessfully through isothermal forging and subsequent multi-pass rolling process.The maximum size of sheet reaches to2000mm×300m×2mm. The influence ofreinforcement content, thermal processing temperature and rolling deformation onmicrostructures of titanium matrix composites were revealed. The results show thatthe presence of TiB and TiC promotes the dynamic recrystallization of α phase whenthe composites were deformed in α+β phase field. Improving rolling temperaturecan decrease the content of fractured reinforcements obviously. Bi-modalmicrostructure can be obtained as the composites were rolled in α+β phase field,whereas lamellar microstructure can be obtained as the composites were rolled in βphase field. In addition, with the increase in rolling deformation, the distribution ofreinforcements is more homogeneous and matrix microstructure can be refinedremarkably.Titanium matrix composite sheets with multi-pass rolling exhibit excellent comprehensive performance. Room-temperature tensile strength of7.5vol.%TiB/Ti composite sheet is up to1342.4MPa and its elongation can reach to5.73%.As tensile temperature is600℃, its tensile strength can be up to849.7MPa. For5vol.%（TiB+TiC）/Ti composite sheet rolled in β phase field, its tensile strength andelongation are1298.6MPa and4.94%, respectively, at room temperature. At650℃,its tensile strength can attain660.5MPa. It should be noted that when temperatureincreases to700℃, the discrepancy in tensile strength of the composite withdifferent processing state is small.The strength and ductility can be meliorated effectively due to grain refinement,dislocation multiplication and the enhanced uniform distribution of reinforcementscaused by thermal deformation. Fine grain strengthening effect gradually decreaseswith increasing temperature. As temperature is higher than650℃, its role is notpositive. The load-bearing effect of reinforcements is enhanced gradually withtemperature under the condition that interface bonding is well. This role is decreaseddue to interface decohesion between reinforcements and matrix as temperatureexceeds700℃.Superplastic deformation behavior and failure mechanism of5vol.%（TiB+TiC）/Ti composite sheet with fine grain in the temperature range of900℃to1050℃and in the strain rate range of5×10-3s-1to10-4s-1were investigated. It is foundthat optimal superplastic elongation of328.8%is obtained when the composite sheetwas tested at1000℃and strain rate of10-3s-1. Superplastic deformation mechanismis mainly grain boundary sliding coordinated by dislocation motion and dynamicrecrystallization together through microstructural observation and calculation ofsuperplastic deformation activation. The enhancement of coordinated deformationcapacity between reinforcements and matrix is in favor of superplastic deformationunder the condition of high temperature and low strain rate. The microstructuresahead the failed superplastic deformation samples were observed. It is found thatvoids mainly nucleate at the interface between reinforcements and matrix, α/βinterface and trigeminal grain boundary. Cross coalescence of adjacent voids leadsto the failure of the composite sheet. |
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