【作者】 艾桃桃；

【导师】 王芬；

【作者基本信息】 陕西科技大学 ， 材料物理与化学， 2015， 博士

【摘要】 由于具有密度低、熔点高、比强度和比模量高、抗高温氧化能力出众和蠕变速率小,Ti-Al系金属间化合物可用于制造超高速飞行器的翼、壳体以及喷气发动机和涡轮等航空航天、汽车工业的耐高温部件等,而且可替代钛、镍基高温合金,因此被认为是新一代轻质耐高温结构材料的典型代表。但是,Ti-Al系金属间化合物存在脆性大、高温强度差及超过800℃高温氧化抗力不足等问题,严重阻碍了其应用化进程。复合化技术是改善Ti-Al系金属间化合物性能缺陷的一种行之有效的手段,其中增强相的选择尤为关键。目前,MAX层状三元化合物的出现受到了研究者的广泛关注,它们同时兼具金属和陶瓷的优良性能,被认为是Ti-Al系金属间化合物理想的增强相。本文采用压力辅助原位反应技术制备高纯度Ti3Al C2和内生MAX(Ti2Al C/Ti3Al C2/Ti3Al C2-Ti2Al C)陶瓷颗粒增强Ti-Al系金属间化合物,探索不同种类、数量及组合方式的内生陶瓷颗粒(Ti2Al C/Ti3Al C2/Ti3Al C2-Ti2Al C)对Ti-Al系金属间化合物微观结构和力学性能的影响规律,建立结构与性能之间的关系,探讨强韧化机制,为发展高强高韧原位内生陶瓷颗粒增强Ti-Al系金属间化合物提供必要的技术支持。本文主要研究结果如下:(1)利用Ti-1.2Al-2Ti C、2Ti C-Ti-1.2Al-0.1Sn、2Ti C-Ti-1.2Al-0.1Si、2Ti C-Ti-1.2Al-0.05Sn-0.1Si和2Ti C-Ti-1.2Al-0.1Sn-0.1Si体系的原位反应经1350℃烧结后获得了高纯度Ti3Al C2。2Ti C-Ti-1.2Al体系与Ti-Al-2Ti C体系相比,Ti3Al C2合成纯度较高(90.62wt.%)。通过Si或Sn掺杂,大大提高了Ti3Al C2的合成纯度,尤其是Si和Sn协同掺杂后,Ti3Al C2的合成纯度最高达98.94wt.%。2Ti C-Ti-1.2Al-0.05Sn-0.1Si体系合成产物的综合性能最佳,其Vickers硬度、抗弯强度和断裂韧性分别为3.68GPa、530.23MPa和7.06MPa·m1/2。(2)利用Ti-Al-Ti--3Al C2体系900℃/2h低温原位反应成功制备了双相Ti3Al C2-Ti2Al C协同增强Ti Al基复合材料。Ti3Al C2和Ti2Al C增强相颗粒主要分布在基体晶界处。随着Ti3Al C2掺杂量的增大,结构变得疏松,增强相团聚严重。当Ti3Al C2掺杂量为5wt.%时,Ti3Al C2-Ti2Al C/Ti Al基复合材料的性能最佳,Vickers硬度、抗弯强度和断裂韧性分别为2.7GPa、316MPa和7.3MPa·m1/2。合适的界面结合强度、双相Ti3Al C2-Ti2Al C协同配合是赋予高韧性的主要原因。(3)利用Ti-Al-Ti3-Al C2体系的低温原位反应(1000℃/2h)成功制备了Ti2Al C/Ti Al基复合材料。利用Ti3Al C2分解反应原位形成的Ti2Al C陶瓷颗粒尺寸细小,主要分布在Ti Al基体的晶界处,并构成了网络状结构。随着Ti2Al C生成量的增加,团聚现象加剧,结构疏松。当Ti3Al C2掺杂量为5wt.%时,Ti2Al C/Ti Al基复合材料的性能最佳,Vickers硬度、抗弯强度和断裂韧性分别为3.7GPa、651.5MPa和10.89MPa·m1/2。提出了Ti2Al C/Ti Al基复合材料的韧化机制:基体晶粒的细化和均匀分布的Ti2Al C陶瓷颗粒;原位Ti2Al C的存在引发的裂纹偏转,棒状Ti2Al C的架桥效应以及穿晶断裂。(4)利用TixAly-Ti3Al C2体系1150℃/2h原位反应成功制备了高强高韧Ti2Al C增强的Ti Al基复合材料。Ti2Al C颗粒主要呈2种形态:弥散和大颗粒状,结构致密。随着Ti3Al C2掺杂量的增大,基体晶粒明显减小。当Ti3Al C2掺杂量为5wt.%时,获得了最佳综合性能,Vickers硬度、抗弯强度和断裂韧性分别为5.14GPa、921.8MPa和7.2MPa·m1/2,与Ti Al合金相比,抗弯强度提高了232.6%,断裂韧性提高了23.6%。弥散分布的第二相Ti2Al C的形成,细化了Ti Al基体晶粒,同时穿晶断裂、裂纹的偏转和桥联、增强相颗粒的拔出等混合断裂特征使裂纹扩展需要更多的能量,裂纹扩展受阻,韧性得以改善。(5)利用Ti-Al-Ti C体系原位反应(1300℃)成功制备了原位内生Ti2Al C/Ti Al基复合材料。原位形成的Ti2Al C陶瓷颗粒尺寸约5~10μm;Ti2Al C颗粒大部分分布在Ti Al基体的晶界处;随Ti2Al C生成量的逐渐增大,Ti2Al C呈团簇状分布。当Ti2Al C含量为15wt.%时,Ti Al/Ti2Al C基复合材料的抗弯强度和断裂韧性达到最大,分别为486±16MPa和7.78±0.13MPa·m1/2,较Ti Al合金提高了5.65%和8.2%。Ti2Al C陶瓷颗粒的生成,使得Ti Al合金的强度和塑性同时得以提高,但Ti2Al C含量高时,反而不利于强度和塑性的提高。提出了强韧化机制:柔性第二相Ti2Al C强化和细晶强化;混合断裂模式(穿晶断裂、层间撕裂、层状剥离以及桥联、裂纹分叉和裂纹偏转等)。(6)利用Ti-Al-Ti C-CNTs体系的原位反应成功制备了Ti3Al C2/Ti Al3基复合材料。增强相Ti3Al C2主要分布在Ti Al3晶界处,形成网络状结构,Ti3Al C2和Ti Al3结合紧密。Ti3Al C2呈2种不同形态分布:粒子和针状。Ti3Al C2/Ti Al3基复合材料的弯曲强度和断裂韧性分别达487.2MPa和5.5MPa·m1/2,远高于Ti Al3合金的弯曲强度(162MPa)和断裂韧性(2MPa·m1/2),分别提高了约200.7%和175%。Ti3Al C2/Ti Al3基复合材料存在多种增韧机制,包括裂纹的偏转、分支和桥联,颗粒的拔出以及穿晶断裂等。(7)利用Ti-Al-Ti C-CNTs体系的原位反应成功制备了Ti2Al C Ti2Al C/Ti Al3复合材料,并形成了搭接层状结构,样品的弯曲强度和断裂韧性最高可达343.21MPa和6.5MPa·m1/2,远高于Ti Al金的弯曲强度(162MPa)和断裂韧性(2MPa·m1/2),分别提高了225%和111.86%。更多还原

【Abstract】 As new light-weight high-temperature structural materials, titanium aluminides have attracted much attention due to their low density, high melting point, high specific strength and specific modulus, low creep rate and excellent high-temperature oxidation resistance. Therefore, titanium aluminides have become front-runners in replacing Ti alloys and Ni-based superalloys, and have the potential to enable high temperature automobile and aerospace applications. A foremost application under consideration for titanium aluminides is high performance gas turbine engines. However, the bottleneck problems including low ductility, poor high-temperature stength and oxidation resistance above 800 °C limit their practical applications.The composite technology is an effective approach to improve the properties of Ti-Al intermetallics, mainly through the methods of composition optimization and microstructure control. The selection of reinforcements is very important. Recently, layered ternary compounds, Mn+1AXn(where M is an early transition metal, A is an A group element, X is C or N, and n = 1-3) have been identified as compatible and thermochemically stable reinforcing phases for Ti-Al intermetallics, due to their ternary layered structure and their properties with both metallic and ceramic characteristics.In the present work, high purity Ti3 Al C2 and in situ MAX(Ti2Al C/Ti3 Al C2/Ti3 Al C2-Ti2 Al C) reinforced Ti-Al based intermetallics were fabricated by in situ hot-pressing process. The effect law of different kinds, contents and combination ways of in situ ceramic particles(Ti2Al C/Ti3 Al C2/Ti3 Al C2-Ti2 Al C) on the microstructure and mechanical properties of Ti-Al based intermetallics were investigated in detail. Meanwhile, we established the relationship between microstructure and properties, and explored the strengthening and toughening mechanisms. Our researches are to provide a new preparation technology of in situ ceramic particles reinforced Ti-Al based intermetallics. The results are listed as follows:High-purity Ti3 Al C2 was prepared by in situ reaction process of Ti-1.2Al-2Ti C, 2Ti C-Ti-1.2Al-0.1Sn, 2Ti C-Ti-1.2Al-0.1Si, 2Ti C-Ti-1.2Al-0.05Sn-0.1Si and 2Ti C-Ti-1.2Al-0.1Sn-0.1Si systems. The Ti3 Al C2 purity of the product corresponding to 2Ti C-Ti-1.2Al system is 90.62 wt.%, which is higher than the purity of the product corresponding to 2Ti C-Ti-Al system. Al-rich can offset the volatility of Al at high temperature. By means of doping Si or Sn into Ti C-Ti-Al system, the synthesis purity of Ti3 Al C2 can be improved observably. Especially doping Si and Sn into Ti C-Ti-Al system simultaneously, the purity of Ti3 Al C2 corresponding to 2Ti C-Ti-1.2Al-0.05Sn-0.1Si system can reach to 98.94 wt.%. The properties such as Vickers hardness, flexural strength and fracture toughness of the sample corresponding to 2Ti C-Ti-1.2Al-0.05Sn-0.1Si system are the best, which are 3.68 GPa, 530.23 MPa and 7.06 MPa·m1/2, respectively.A new Ti3 Al C2-Ti2 Al C/Ti Al composite was successfully fabricated by in-situ reaction hot-pressing process in the Ti-Al-Ti3 Al C2 system at 900 °C for 2 h. During the process, Ti3 Al C2 partially decomposed into Ti2 Al C and Ti C, and then Ti C reacted with Ti Al intermetallic to form Ti2 Al C further. The reinforcements mainly distribute in the grain boundaries. Ti3 Al C2 additive significantly restrains the growth of Ti Al grain. The dispersion of Ti3 Al C2 phase becomes worse with the increasing Ti3 Al C2 content. The composite with 5 wt.% Ti3 Al C2 additive presents better properties, the Vickers hardness, three-point bending strength and fracture toughness are 2.7 GPa, 316 MPa and 7.3 MPa·m1/2, respectively. The toughening of the composite is primarily attributed to zigzag crack deflection, transgranular and/or translamellar cracking, pull-out of the reinforcements and cleavage phenomenon, especially, suitable interface bonding strength, and the combination toughening of Ti3 Al C2 and Ti2 Al C.Ti2Al C/Ti Al in situ composites were successfully fabricated by reaction hot-pressing process using Ti3 Al C2, Ti, and Al powders as initial materials. The products are mainly composed of Ti2 Al C, γ-Ti Al and α2-Ti3 Al as major phases. In situ Ti2 Al C reinforcements are mainly distributed in the grain boundaries resulting in an obvious γ+α2 grains refinement. With increasing the Ti3 Al C2 content from 0 to 15 wt.%, the content of Ti2 Al C increases from 0 to 20.77 wt.% and the γ+α2 grain size decreases from 55 to 19 μm. With increasing Ti3 Al C2 content up to 5 wt.%, the Vickers hardness, flexural strength and fracture toughness of the as-sintered composite reach to the maximum values of 3.7 GPa, 651.5 MPa, and 10.89 MPa·m1/2, respectively. Analysis of fracture surface and crack propagation paths indicates that the grain refinement, crack deflection, crack bridging, in situ precipitated Ti2 Al C phases obtained by decomposing of Ti3 Al C2 are the main reasons responsible for the toughening of the composites.Ti Al/Ti2 Al C composites are successfully fabricated by in-situ reactive hot pressing method in the TixAly-Ti3 Al C2 system at 1150 °C. With increasing Ti3 Al C2 content, the grain size decreases gradually. With increasing Ti3 Al C2 content up to 5 wt.%, the Vickers hardness, flexural strength and fracture toughness of the as-sintered composite reach to the maximum values of 5.14 GPa, 921.8 MPa and 7.2 MPa·m1/2, respectively, which is 232.6% and 23.6% higher than Ti Al alloy. The action mechanism of in situ Ti2 Al C/Ti Al matrix composites is mainly attributed to the grain refinement, the uniform distribution of Ti2 Al C, the second-phase strengthening, grain refinement, transgranular cracking, crack deflection, crack bridging and pullout of Ti2 Al C. On account of the above reasons, the crack propagation needs more energy, which will hinder gradually. As a result, the toughness is improved.Ti Al/Ti2 Al C composites are successfully fabricated by in-situ reactive hot pressing method in the Ti-Al-Ti C system at 1300 °C. The size of the in situ Ti2 Al C particles is 5-10 μm. Ti2 Al C are mainly distributed in the Ti Al grain boundaries. With increasing Ti2 Al C content, Ti2 Al C are distributed over cluster status. Ti Al/15 wt.% Ti2 Al C composite possesses the highest bending strength of 486 MPa and the highest fracture toughness of 7.78 MPa·m1/2, due to the uniformly distributed Ti2 Al C grains with the moderate content. With increasing Ti2 Al C content, the ceramic particles distribute more densely, which lead to the decrease of mechanical properties and the increase of density. The action mechanism of in situ Ti2 Al C/Ti Al matrix composite is mainly attributed to the second phase strengthening, grain refinement and mixture crack modes such as transgranular cracking, interlaminar tearing, layered stripping, crack bridging, crack branching and crack deflection.Dense Ti3 Al C2/Ti Al3 composite has been successfully synthesized by in situ hot pressing in the Ti-Al-Ti C-CNTs system at 1350 °C for 2 h. As reinforcing agent, Ti3 Al C2 are mainly distributed in the Ti Al3 grain boundaries, and exhibits significant strengthening and toughening effect to the Ti Al3 matrix. The bonding between Ti3 Al C2 and Ti Al3 is very tight. There are two different morphologies of reinforcements, particle-like and needle-like precipitates. The flexural strength and fracture toughness of the Ti3 Al C2/Ti Al3 composite can reach to 487.2 MPa and 5.5 MPa·m1/2, respectively, which is higher than the flexural strength(162 MPa) and fracture toughness(2 MPa·m1/2) of Ti Al3. Toughening is mainly attributed to crack deflection, crack bridging, crack branching and pull-out of Ti3 Al C2 particles as well as transgranular cracking. The strong interface combination strength and the dispersed Ti3 Al C2 particulates are responsible for the enhancement of the strength.Ti2Al C/Ti Al3 composites are successfully fabricated by in situ hot pressing in the Ti-Al-Ti C-CNTs system. Overlap joint lamellas can be obtained. The flexural strength and fracture toughness of the Ti2 Al C/Ti Al3 composites reach to 343.21 MPa and 6.5 MPa·m1/2, respectively, which is 225% and 11.86% higher than Ti Al3 alloys.更多还原