【作者】 张长江；

【导师】 陈玉勇；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2013， 博士

【摘要】 原位自生钛基复合材料具有低密度、高比强度和比模量、优异的疲劳和蠕变性能，有望在航空航天、先进武器系统及汽车制造等领域获得广泛应用。然而，钛基复合材料室温塑性差，高温变形抗力大，限制了其大规模的工程化应用。本文采用真空感应熔炼技术制备了不同（TiC+TiB）含量的钛基复合材料，基体合金成分为Ti-6Al-2.5Sn-4Zr-0.7Mo-0.3Si。研究了增强相含量对铸态复合材料显微组织和力学性能的影响；阐明了（TiB+TiC）/Ti复合材料的热压缩变形行为及组织演变规律；探讨了热加工过程中的组织性能对应关系；开展了钛基复合材料板材的超塑性研究并揭示了其超塑性变形机理和失效机制。凝固析出的TiB和TiC相易于偏聚于原始β晶界处，TiB相主要呈晶须状，而TiC则为近等轴状，二者均与基体界面结合良好。TiB和TiC的引入细化了原始β晶粒和α片层，改变了α相的集束特征，并且使得α片层的取向更加随机。β晶粒的细化机制为C与B在先析出的β-Ti界面前沿富集引起成分过冷及抑制已析出的β-Ti生长。β晶粒的细化增多了β晶界，α相的形核位置增加，并且生长空间缩小，二者导致α片层发生细化。TiB和TiC的存在显著提高了铸态（TiB+TiC）/Ti复合材料的室温及高温强度。室温下，相比于基体合金，增强相体积分数分别为2.5%、5%、7.5%的复合材料的屈服强度分别提高了16.2%、20.2%、28.3%。室温屈服强度的提高主要是因为基体组织的细化。高温下，随测试温度升高，复合材料相对于基体合金的抗拉强度增幅呈先增加后降低趋势，在650℃时达到峰值。650℃以下复合材料强度提高主要归因于组织细化，增强相承载强化以及C的固溶强化，而700℃以上的原因是增强相承载强化和C的固溶强化。采用热物理模拟方法，研究了5vol.%（TiB+TiC）/Ti复合材料的热压缩变形行为，揭示了流变应力与变形温度和应变速率之间的关系，峰值应力和流变应力均随温度的升高和应变速率的减小而降低，且峰值应力σp与（1000/T）、ln之间都满足线性关系。该复合材料的热变形激活能为608.3kJ·mol-1，硬化因子为4.27，建立了该复合材料（α+β）相区热变形本构方程，为后续热变形参数的选择和设备吨位的确定提供了指导。阐明了5vol.%（TiB+TiC）/Ti复合材料热压缩过程中的组织演变规律和软化机制。该复合材料的变形组织是相变、动态回复和动态再结晶综合作用的结果，高温低应变速率有助于增强相与基体间的协调变形，且有利于动态再结晶过程的进行。（α+β）相区变形的软化机制主要是α相的动态再结晶，β相含量的提高有助于流变应力和热变形激活能的降低。TiB及TiC对基体热变形行为的影响依赖于α和β两相比例的变化。通过高温锻造及后续多道次轧制工艺，成功制备出高质量的TiB/Ti及（TiB+TiC）/Ti复合材料板材，最大尺寸可达2000mm×300mm×2mm。阐明了增强相含量、热加工温度、轧制变形量对钛基复合材料显微组织的影响规律，TiB及TiC增强相促进了（α+β）相区变形时α相的动态再结晶；提高轧制变形温度可明显减少增强相的折断比例；（α+β）相区轧制得到双态组织，β相区轧制得到片层组织；随轧制变形量的提高，增强相分布均匀性明显提高，基体组织得到了显著细化。多道次轧制的钛基复合材料板材具有优异的综合性能。对于7.5vol.%TiB/Ti复合材料板材，室温抗拉强度可达1342.4MPa，延伸率达5.73%，600℃时抗拉强度高达849.7MPa；对于β相区轧制的5vol.%（TiB+TiC）/Ti复合材料板材，室温抗拉强度达1298.6MPa、延伸率为4.94%；650℃下，抗拉强度仍可达660.5MPa。到700℃，各加工态复合材料强度差别不大。热变形引起的晶粒细化、位错增殖，增强相分布均匀性提高，有效地改善了钛基复合材料的强度和塑性。细晶强化效果随温度的升高而逐渐减弱，650℃及以上温度，细晶强化不再起作用；在界面结合良好的前提下，增强相的承载能力随温度的升高而逐渐提高，在700℃及以上温度，增强相与基体的脱粘降低了增强相的承载能力。研究了细晶5vol%（TiB+TiC）/Ti复合材料板材在900℃-1050℃，5×10^-3s^-1-10^-4s^-1条件下的超塑性变形行为及失效机制。发现该复合材料板材在1000℃、10^-3s^-1变形条件下获得最佳超塑性延伸率达328.8%；通过组织观察及超塑性变形激活能计算分析，超塑性变形机制主要是位错运动和动态再结晶共同协调的晶界滑动，高温低应变速率下，增强相与基体协调变形能力的提高有利于超塑性变形。对超塑性变形试样断裂前沿进行了组织观察，发现空洞主要形核于增强相与基体界面处、α晶/相界及三叉晶界处，空洞和空洞链的横向连接造成了复合材料的失效。更多还原

【Abstract】 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.更多还原