【作者】 崔喜平；

【导师】 耿林； Hua-Xin Peng；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2012， 博士

【摘要】 为满足超音速飞行器和未来涡轮发动机以及壳体热防护系统对800¹000oC使用的轻质高强合金薄板的迫切需求，TiAl基合金板材的开发与制备至关重要。然而由于本质脆性，直接轧制脆性的TiAl基合金锭制备板材十分困难，因而本文通过热轧塑性变形良好的多层Ti-（TiB₂/Al）复合板及后续的多步热处理成功制备出微叠层TiB₂-TiAl复合材料板材并采用真空热压烧结致密化处理显著提高了其致密度。探索并优化了TiAl基复合材料板材的制备工艺，利用扫描电镜（SEM）、X射线衍射（XRD）和透射电镜（TEM）系统研究了反应退火过程中反应层的相组成及反应动力学并揭示了其反应机理，探讨了TiB₂含量对TiAl基复合材料组织成分与力学性能的影响，并合理表征和评价了微叠层TiB₂-TiAl复合材料板的组织结构及力学性能。利用热压及热轧制成功制备出界面结合良好的多层Ti-（TiB₂/Al）复合板。为保证多层Ti-（TiB₂/Al）复合板层厚均匀，轧制变形量不宜超过65%。TiB₂/Al复合材料板取代多层Ti-Al复合板中的纯Al板，显著改善多层复合板的变形协调性。变形协调性的改善有利于设计和控制最终TiAl基复合材料的组织成分。多层Ti-（TiB₂/Al）复合板在520⁷00oC低温反应退火时，TiAl₃相为首要产物，TiB₂不参与反应与扩散。TiAl₃层形成和生长是反应扩散过程，且伴随其有序-无序转变。TiAl₃层向TiB₂/Al层的生长速率高于向Ti层的生长速率。铝熔点（660oC）以下反应退火时，TiAl₃层生长遵循抛物线生长动力学规律；铝熔点及以上反应退火时，TiAl₃层生长遵循直线生长动力学模式。基于对低温退火反应动力学研究，再综合考虑退火过程中不能发生Al大量熔化流出及尽量减少TiAl₃层中孔洞产生，确定660oC为最佳低温反应退火温度。低温反应退火过程中存在Kirkendall效应，Al消耗完全后，TiAl₃层中间位置形成疏松多孔的TiB₂堆积层，严重降低材料的致密度与力学性能。1225oC/2h/60MPa条件下的真空热压烧结能显著提高多层复合材料板材的致密度，且致密化过程不引入二次裂纹。随后在950¹200oC高温反应退火时，反应扩散继续进行，TiB₂亦不参与反应，最终得到层状TiB₂-TiAl-Ti3Al复合板。继续在1400oC片层化热处理22min，制备出由少量γ-TiAl连接着大量TiB₂颗粒的TiB₂-rich层和近全片层组织的γ-TiAl层交替排列的微叠层TiB₂-TiAl复合材料板材。TiB₂-rich层存在显著地限制α2γ片层组织粗化。且TiB₂-rich层中TiB₂与γ-TiAl间界面结合良好、局部区域存在位向关系：[1100]TiB₂//[112]TiAl和（001）TiB₂//（111）TiAl。通过研究TiB₂含量对TiAl基复合材料板材组织成分与性能的影响，发现微叠层2.6vol.%TiB₂-TiAl具有最佳的成分、组织结构、最小的密度和最高的弹性模量，其值达到172.15GPa。断裂韧性试验表明，微叠层TiB₂-TiAl复合材料板表现出良好的断裂韧性，随着TiB₂-rich层增厚，断裂韧性增加。且断裂韧性强烈地依赖于加载方向，2.6vol.%TiB₂-TiAl复合材料板平行于法向的断裂韧性值达到15.12MPa·m1/2，比平行于横向的断裂韧性值提高27.6%。TiB₂-rich层对裂纹扩展的阻碍和令裂纹发生的偏转及TiB₂-rich层内产生的大量微裂纹即微裂纹增韧均使裂纹在扩展中消耗能量增加，是平行于法向的断裂韧性显著高于平行于横向的主要原因。随拉伸测试温度升高，微叠层TiB₂-TiAl复合材料板强度先增加后减小，延伸率增加，弹性模量减小。750oC时屈服强度达到最大值为339.92MPa。可见，微叠层TiB₂-TiAl复合材料板材是具有一定发展潜力的高温轻质结构材料。TiB₂的强化作用和TiB₂-rich层对α2γ片层组织的细化作用是导致其高温性能提高的主要原因。断裂研究表明，裂纹首先在TiB₂-rich层中萌生，当继续加载，裂纹穿过TiB₂-rich层到达TiAl层，产生沿片层微裂纹或穿片层微裂纹，沿片层微裂纹通过主裂纹与沿片层微裂纹的连接及剪切而扩展；穿片层微裂纹依靠对沿片层和穿片层两种形式微裂纹的连接进行扩展。更多还原

【Abstract】 In order o satisfy the requirements of supersonic aircrafts, future gas turbineengines and thermal protection systems for high-strength light-density alloy sheetsapplied in the range of800¹000oC, it is of crucial importance for development andmanufacture of TiAl-based alloy sheets. However, owing to intrinsic brittleness, it israther difficult to to fabricate TiAl-based alloy sheets by means of direct rolling ofTiAl-based alloy ingots. Therefore, in this paper micro-laminated TiB₂-TiAlcomposite sheets have been successfully produced by the processing route of hotrolling multi-layered Ti-（TiB₂/Al） composite sheets which have excellent plasticdeformation capacity and subsequent multistep heat treatment. The relative densityof micro-laminated TiB₂-TiAl composite sheets has been significantly improved byhot-pressing sintering densification process. Preparation processes of TiAl matrixcomposite sheets were investigated and optimized. Phase compositions of thereaction layer and reaction kinetics in the process of reaction annealing weresystemically studied by scan electron microscopy （SEM）, X-ray diffraction （XRD）and transmission electron microscopy （TEM） and the reaction mechanism wasrevealed. In addition, the effect of volume fractions of TiB₂on microstructures andmechanical properties of TiAl matrix composite sheets were investigated. Moreover,microstructures and mechanical properties of micro-laminated TiB₂-TiAl compositesheets were reasonably characterized and evaluated.Multi-layered Ti-（TiB₂/Al） composite sheets with good interface bonding canbe fabricated by hot pressing and hot rolling. However, rolling deformationreduction in thickness should be not more than65%to ensure the thicknessuniformity of Ti and TiB₂/Al composite layers. Deformation compatibility ofmulti-layered Ti-Al composite sheets have been pronouncedly improved by thereplacement of pure Al sheets by TiB₂/Al composite sheets, which is benefit fordesigning and controlling compositions and microstructures of resulting TiAl matrixcomposite sheets.For multi-layered Ti-（TiB₂/Al） composite sheets, after the initial reactionannealing between520oC and700oC, TiAl₃was primary phase and the introductionof TiB₂has little influence on reaction and diffusion between Ti and Al. Formationand growth of TiAl₃layer is a reaction-diffusion process accompanied withorder-disorder transition and Growth velocity of TiAl₃towards TiB₂/Al layer ismuch faster than that towards Ti layer. When annealing temperature is below themelting point of pure Al （660oC）, growth of TiAl₃layer is in consistent with theparabolic growth law, while annealing temperature is the melting point of pure Al or above, its growth accords with linear growth law. Based on reaction kinetics,controlling leakage of liquid Al and reducing formation of pores within TiAl₃layer,optimum initial reaction annealing is660oC.Because of Kirkendall effect during initial raction annealing, when Al has beenexhausted, porous TiB₂layer forms in the middle of TiAl₃layer, which severelyreduces the laminate’s relative density and mechanical properties. Relative densityof micro-laminated TiB₂-TiAl composite sheets can be significantly improved byhot-press sintering in vacuum under60MPa at1225oC for2h, which avoidsformation of secondary cracks. When the laminate is subsequently annealed at950¹200oC, the reaction diffusion proceeds and TiB₂still keep stable. Finally,multi-layered TiB₂-TiAl-Ti3Al composite sheets are obtained. Finally，after lamellartreatment at1400oC for22min, micro-laminated TiB₂-TiAl composite sheetsconsisting of TiB₂-rich layer containing numerous TiB₂particles connected by asmall quantity of γ-TiAl and fully lamellar TiAl layer were obtained. Moreover, theinterface between TiB₂and TiAl phases exhibits sound bonding and follows theorientation relationship:[1100]TiB₂//[112]TiAland （0001）TiB₂//（111）TiAlwthin the localTiB₂-rich layer. TiB₂-rich layer significantly hinders growth of α2γ lamellar colony.Results indicate that2.6vol.%TiB₂-TiAl have optimum composition, microstructure,the lowest density and the highest elastic modulus value of172.15GPa.Fracture toughness testing shows that micro-laminated TiB₂-TiAl compositesheets exhibit excellent fracture toughness and with increasing thickness ofTiB₂-rich layer, fracture toughness increases. Furthermore, fracture toughnessdepends on loading direction, fracture toughness of2.6vol.%TiB₂-TiAl compositesheets parallel to normal direction （ND） is15.12MPa·m1/2, which is27.6%higherthan the value parallel to transverse direction （TD）. TiB₂-rich layer hinderspropagation of cracks and induces crack deflection and crack branching, andmicrocrack toughening occurs. All above factors make crack propagation consumeamount of energy, which contributes to higher fracture toughness of themicro-laminated composite sheets in parallel to normal direction.With raising tensile testing temperature, strength of micro-laminated TiB₂-TiAlcomposite sheets increases firstly and then decreases, elongation increases andelastic modulus reduces. Yield strength at750oC reaches the highest value of339.92MPa. Therefore, miro-laminated TiB₂-TiAl composite sheets have potentialfor high temperature structural applications. Improvement of properties at hightemperature is because of strenghening of TiB₂and refinement of α2γ lamellarcolony due to existing of TiB₂-rich layer.Fracture research results show that initial cracks nucleate within TiB₂-richlayers. With increasing loading, cracks propagate into TiAl layer and interlamellar crack and translamellar cracks are produced. Interlamellar cracks propagate bynucleation, growth and linkage with interfacial microcracks, while translamellarcracks propagate by nucleation, growth and linkage with two different types ofmicrocracks, e. g. interface delamination and translamellar microcracks.更多还原