【作者】 戚继球；

【导师】 魏尊杰； 王宏伟；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2013， 博士

【摘要】 与钛合金相比，钛基复合材料具有更高的比强度、比模量以及高温强度等特点，在航空航天领域有着潜在的应用前景。了解钛基复合材料高温力学性能与组织和成分的关系对扩大其应用范围有重要的理论和实际意义。本文将原位合成技术与传统铸造相结合制备了多种成分的TiC增强钛基复合材料，主要选用三种成分的基体合金：Ti-6Al-3Sn-3.5Zr-0.4Mo (Matrix1)；Ti-6Al-2Zr-1.5Mo(Matrix2)；Ti-6Al-3Sn-9Zr-1.5Mo (Matrix3)。研究TiC含量，基体成分和凝固条件对复合材料凝固组织的影响，分析复合材料基体组织在热处理过程中的演变行为，探讨了复合材料的室温和高温拉伸性能以及断裂行为，讨论复合材料组织与力学性能的对应关系，强化机制以及温度对复合材料拉伸强度的影响。XRD分析表明凝固析出的TiC处于缺C状态，TiC含量越高，缺C越严重。β热处理后，TiC内的C含量明显增加。随着TiC含量增加，其形貌由长条状向等轴状再向枝晶特征过渡。壁厚的降低和Mo含量的提高均可使Ti-C共晶点向高C迁移，致使初生TiC减少，共晶TiC增多。C可显著细化复合材料的β晶粒，并且析出的TiC可细化复合材料中α片层和α集束，改变了α相的集束特征。TiC含量高于15vol.%时，α相演变为等轴或近等轴形貌，因为TiC阻碍了α片层的生长。改变凝固条件来提高复合材料的冷却速度可细化α相尺寸，且α相以集束方式生长。说明TiC只能在较低冷速下才能阻碍α片层的生长。复合材料经β处理后，基体均为具有网篮特征的片层组织。以Matrix1合金为基体时，β处理后复合材料的α片层尺寸稍微降低，不过以Matrix2为基体时，复合材料经类似工艺处理，α片层明显细化。研究表明β处理能否引起片层细化主要取决于β稳定元素含量，特别是Mo含量。经α+β处理，复合材料基体演变为双相组织，α相的形貌和数量主要取决于热处理温度。β-transus+(20℃-40℃)/FC处理后，基体合金的组织为粗大的魏氏组织，而10vol.%TiC/Matrix1复合材料基体演变为等轴组织。该差异主要是因为该复合材料中TiC相的存在。α-Ti析出相与TiC相之间存在以下位向关系：[11-20]α//[0-11]TiC，(000-1)_α//(111)TiC；[2-1-10]_α//[011]TiC，(01-10)_α//(1-11)TiC。晶格错配度计算进一步说明TiC可以作为α-Ti析出相的形核基底。考虑到α相与TiC之间不存在严格的晶体学位向关系，α相只能以等轴方式生长。TiC含量，基体成分和凝固条件均对复合材料铸态室温力学性能有较大的影响。少量TiC的引入可显著提高复合材料的室温强度，不过以牺牲塑性为代价，TiC含量超过15vol.%，复合材料脆化严重。壁厚的降低可显著提高10vol.%TiC/Matrix2复合材料的室温屈服强度，但降低了室温塑性。增加基体中Mo含量有利于提高复合材料室温塑性的。β处理后以Matrix1为基体的复合材料的室温塑性明显提高，但强度变化较小。由于β处理引起的片层细化，以Matrix2为基体的复合材料的室温强度显著提高，不过塑性大幅降低。采用经典屈服理论来评估复合材料的屈服强度，结果显示理论值与实测值比较吻合。600℃下TiC含量的增加几乎未引起复合材料抗拉强度的提高，但700℃以上高含量TiC的复合材料更有优势，相比于基体合金，700℃下15vol.%TiC/Matrix1和20vol.%TiC/Matrix1复合材料拉伸强度增幅分别达81.1MPa和152.4MPa。β和α+β热处理对以Matrix1为基体的复合材料高温拉伸性能的影响很小。壁厚不同引起的10vol.%TiC/Matrix2复合材料抗拉强度差异随拉伸温度的提高逐渐减少，到650℃时壁厚对复合材料强度几乎没有影响。热处理可提高650℃以下以Matrix2和Matrix3为基体的复合材料的强度，更高温度下热处理引起的组织强化被基体软化所抵消。显然，热处理强化存在很大的局限性。断裂研究表明，较低温度范围内复合材料发生解理或准解理断裂，TiC的开裂主导了复合材料的断裂过程；在更高温度下复合材料发生微孔聚集型断裂。复合材料发生解理或准解理断裂的上限温度主要受TiC含量和基体成分影响。热处理对复合材料的断裂方式影响较小。TiC含量低于10vol.%，固溶强化和细晶强化只能在650℃以下才可发挥其作用。TiC含量高于15vol.%，TiC的承载强化效果在高温下得以增强且TiC含量越高其承载效果越明显。研究指出在不同温度区间，提高复合材料拉伸强度的路径是有很大差异的。650℃以下通过提高基体中合金化程度来提高基体强度可有效提高复合材料拉伸强度；700℃以上，只有基体强度和TiC含量同时提高，才可显著复合材料的拉伸强度。总体上，复合材料以及基体合金的拉伸强度在不同温度区间呈不同的降低趋势。从室温到600℃，材料的拉伸强度缓慢降低；600℃以上拉伸强度降低速率加快。复合材料基体随温度的软化特征与拉伸强度的变化相似，而TiC的承载应力呈先增加后降低的规律。基体强度的变化主导了复合材料拉伸强度的演化行为，TiC承载能力的变化只改变了复合材料较低温拉伸强度的演变特征。更多还原

【Abstract】 Compared with titanium alloys, titanium matrix composites (TMCs) exhibitexcellent properties, such as higher specific strength, specific modulus, hightemperature strength. Hence, TMCs have a potential application prospects in theaerospace field. The understanding of the relationship between high-temperaturemechanical properties of TMCs and microstructure and composition for expandingtheir applications has an important theoretical and practical significance.In thispaper, in situ synthesis technology and traditional casting were combined to prepareTiC reinforced titanium matrix composites (TMCs). The main three types of matrixalloys selected were Ti-6Al-3Sn-3.5Zr-0.4Mo (Matrix1), Ti-6Al-2Zr-1.5Mo(Matrix2) and Ti-6Al-3Sn-9Zr-1.5Mo (Matrix3). The influence of TiC content,matrix composition and solidification condition on solidification microstructures ofTMCs was investigated. The evolution behavior of the microstructures of TMCs duringheat treatment was analyzed. The tensile properties and fracture behaviors of TMCs atroom and high temperatures were studied. Subsequently, the correspondingrelationship between microstructures and mechanical properties, high-temperaturestrengthening mechanisms and strengthening route were discussed.XRD analysis showed that the solidified TiC is of carbon deficiency and withthe increase in TiC content, carbon deficiency is more serious. After β heattreatment, C content in TiC increases obviously. The morphologies of TiC aretransformed from long strip-like to equiaxed shape and then to dendritic characteristicas TiC content increases. The decrease in wall thickness and the enhancement of Mocontent all can make Ti-C eutectic point shift to high C direction. This results in thedecrease of primary TiC volume fraction and the increase in eutectic TiC content. Caddition can lead to the refinement of β grain of TMCs significantly. Meanwhile, TiCprecipitation can refine α lamellae and α colony and change colony feature of α phasecolony feature. As TiC volume fraction is higher than15%, α phase evolutes intoequiaxed or near-equiaxed shape. The reason is that TiC can impede the growth of αphase. Increasing the cooling rate of TMCs through the change of solidification condition can decrease α phase size and the growth of α phase is in colony style. Thisshows that only when cooling rate is slow, TiC can impede the growth of α phase.After β heat treatment, matrix of TMCs exhibits lamellar structure withbasket-weave characteristic. When Matrix1alloy was selected as matrix, α lamellarsize is slightly reduced after β heat treatment. When Matrix2alloy was selected asthe matrix of composite, α lamellae is refined obviously. Research shows thatwhether β heat treatment can lead to the refinement of α lamellae or not mainlydepends on the content of β-stabilizing elements, particularly Mo content. Matrix ofTMCs exhibits duplex microstructure after α+β heat treatment and the morphologyand content of α phase is mainly dependent on heat treatment temperature.After β-transus+(20℃-40℃)/FC heat treatment, the microstructure of matrixalloy is typical widmanst tten microstructure, whereas matrix of10vol.%TiC/Matrix1composite evolves into equiaxed microstructure. Thisdifference is mainly because of the presence of TiC phase in this composite. Thefollowing orientation relationships between α-Ti precipitation and TiC phase exist:[11-20]Ti//[0-11]TiC,(000-1)Ti//(111)TiC;[2-1-10]α//[011]TiC,(01-10)α//(1-11)TiC. Thecalculation of lattice disregistry between the two parallel planes further confirmsthat TiC particles can act as substrates for α phase heterogeneous nucleation. Takinginto account that strict orientation relationship between α phase and TiC does notexist, the growth of α phase is in equiaxed way during furnace cooling from aboveβ-transus temperature.The influence of TiC content, matrix composition and solidification condition onroom-temperature mechanical properties of TMCs is significant. Introducing a smallamount of TiC into titanium alloys leads to the remarkable enhancement in roomtemperature strength at the expense of plasticity. Once TiC volume fraction exceeds15%, TMCs were embrittled seriously. The decrease of wall thickness cansignificantly improve room-temperature yield strength of10vol.%TiC/Matrix2composite, but reduces the room temperature ductility. Increasing Mo content inmatrix is benefit for the enhancement of room-temperature ductility of TMCs. Theroom-temperature elongations of the composites with the matrix of Matrix1alloyare increased obviously after β heat treatment. However, the variation of strength is small. With similar heat treatment, the strengths of the composites with the matricesof Matrix2and Matrix3alloys are enhanced significantly due to the refinement of αlamellae. However, the elongations of these composites are all reduced. Classicyield theory was employed to evaluate the yield strength of TMCs. The results showthat theoretical values are relatively close to the experimental values.As tensile temperature is600°C, increasing TiC volume fraction almos doesnot cause the improvement of TMCs. However, TMCs with high content of TiCexhibits advantage in strength above700°C. Compared to matrix alloy, theenhancements of tensile strengths of15vol.%TiC/Matrix1and20vol.%TiC/Matrix1composites reach to81.1MPa and152.4MPa, respectively, at700°C. The effect ofβ and α+β heat treatment on high-temperature tensile properties of the compositeswith the matrix of Matrix1alloy is very small. The discrepancy of tensile strengthof10vol.%TiC/Matrix2composite resulting from wall thickness differencebecomes small with increasing tensile temperature. As temperature reaches to650°C, wall thickness almost has no effect on the tensile strength. Heat treatment canimprove the strengths of the composites with the matrices of Matrix2and Matrix3alloys below650°C. At higher temperatures, microstructure strengthening resultingfrom heat treatment can be offset by the softening of matrix. It is obvious that heattreatment strengthening has significant limitations.Research on the fracture behavior shows that the fracture mechanism of TMCsat low temperatures is cleavage or quasi-cleavage fracture and TiC fracturedominates the fracture process of TMCs; at higher temperatures, nucleation, growthand coalescence of voids lead to the damage of TMCs. The ceiling temperature thatcleavage or quasi-cleavage fracture occurs is mainly influenced by the TiC contentand matrix composition. Heat treatment has little impact on the fracture mode ofTMCs.As TiC content is lower than10vol.%, solid-solution strengthening and finegrain strengthening can play their role on strength of TMCs below650°C. WhenTiC volume fraction exceeds15%, TiC load-bearing effect is enhanced at hightemperature and this effect becomes more obvious with the increase of TiC.Research shows that the paths to improve the tensile strength of TMCs in different temperature ranges are different. Enhancing matrix strength through increasingalloying extent of matrix can improve the tensile strengths of TMCs moreeffectively below650°C; only when matrix strength and TiC content are increasedsimultaneously, the tensile strengths of TMCs can be improved above700°C.Overall, the tensile strengths of TMCs and matrix alloy exhibit differentdecreasing trend in the different temperature ranges. The tensile strengths decreaseslowly from room temperature to600°C; decreasing rate of tensile strengths is fastabove600°C. The softening trend of matrix is similar to the variation of tensilestrengths, whereas bearing-load of TiC increases first and then decreases withincreasing temperature. The variation of matrix strength dominates the evolutionbehavior, whereas the variation of bearing stress of TiC only changes the evolutioncharacteristics of tensile strength of TMCs in low temperature range.更多还原