吕维洁

 

 

论文题目：原位合成钛基复合材料的制备、微结构及力学性能

 

作者简介：吕维洁，男，1973年02月出生，1997年04月师从于上海交通大学张荻教授，于2000年11月获博士学位。

 

 

摘  要

 

面对高技术时代对高性能钛合金材料日益紧迫的要求，非连续增强钛基复合材料因其具有的高比强、高比刚度、耐高温和耐蚀性能已成为研究的热点。人们对其制备工艺、微结构、力学性能等进行了一系列的研究，而这些研究的主要目标为外加法制备的钛基复合材料。而本研究则采用原位合成工艺制备非连续增强的钛基复合材料。与外加法比较，原位合成法因其工艺简单、材料性能优异，在技术和经济上更为可行。增强体的原位合成，避免了增强体的污染问题，也避免了熔铸过程中存在的润湿性问题，有利于制备性能更好的复合材料。然而，为了低成本高效制备高性能的钛基复合材料尚有许多问题需要解决。因此，从理论和实验上研究这些问题，对低成本高效制备高性能的钛基复合材料具有非常重要的理论和实际意义。

针对金属基复合材料发展应用中的关键问题¾¾成本和性能，本文开发设计了新型的钛基复合材料的制备工艺，可以低成本高效制备性能优异的钛基复合材料。即可利用钛与碳化硼、硼及石墨之间的自蔓燃高温合成反应，采用普通的钛合金冶炼工艺制备出单纯TiB晶须、单纯TiC颗粒增强或TiB晶须和TiC粒子混杂增强的钛基复合材料。为了拓展钛基复合材料的应用领域，为制备高性能的钛基复合材料打下坚实的基础，本文的研究主要包括以下几个方面工作：

1、研究了利用钛与石墨、硼及碳化硼之间的反应制备TiB和TiC增强钛基复合材料的原位合成机理。利用热力学理论计算了钛与石墨、硼、碳化硼反应的Gibbs 自由能DG和反应生成焓DH，结果表明：各个反应的Gibbs 自由能DG值都为负值，说明在热力学上上述反应是可行的。虽然在热力学上可以利用钛与碳化硼之间的化学反应合成TiB2和TiC增强体，但从化学平衡考虑，TiB2不能稳定存在于过量钛中，因此能够稳定存在于普通钛合金中的增强体为TiB和TiC。上述反应都为高放热的反应，从理论上讲绝热温度都大于自蔓燃高温合成的判据，表明反应能自发维持。

2、利用非自耗电弧炉和自耗电弧炉经普通的钛合金铸造工艺制备出单纯TiB晶须、单纯TiC颗粒增强或TiB晶须和TiC粒子混杂增强的钛基复合材料。X射线衍射分析结果表明：原位合成的增强体为TiB、TiC。这些增强体分布非常均匀，主要呈现为短纤维状、树枝晶状和等轴或近似等轴状。电子探针和带能谱的扫描电镜分析结果表明：短纤维状增强体为TiB，而树枝晶状和等轴或近似等轴状增强体为TiC。实验结果与理论分析一致，这为原位自生钛基复合材料的工业化生产提供了依据。

3、研究了原位合成钛基复合材料增强体的生长机制，结果表明：增强体的生长机制与凝固过程及增强体的晶体结构密切相关。原位合成的增强体以形核与长大的方式从熔体中析出而长大。对于原位合成TiB和TiC混杂增强的钛基复合材料，经历了析出初晶、二元共晶和三元共晶三个阶段。由于不同的晶体结构，增强体TiB与TiC形成不同的生长形态。TiB具有B27晶体结构，易于沿[010]方向生长长成短纤维状，而且TiB横截面的形状呈多边形，其晶面主要由(100)、(10)和(101)组成。同时，在TiB的(100)面上容易形成层错。而TiC具有NaCl型对称结构，容易长成树枝晶状、等轴状和近似等轴状。发现原位合成的增强体TiB容易在（100）面上形成高密度的层错，层错的形成与增强体的晶体结构、生长机制有关，同时也有利于降低增强体与基体合金界面的晶格畸变。而原位合成增强体TiC的晶格比较完整，偶尔在(111)面上形成孪晶，该孪晶结构在增强体形核与长大的过程中形成。

4、研究了合金元素铝的加入对原位合成钛基复合材料微结构及力学性能的影响。合金元素铝的加入，并不改变复合材料的物相组成，也不改变复合材料的凝固过程，但由于合金元素的存在，阻碍了增强体的形核与长大过程，导致形成的TiB和TiC初晶更为细小，尤其是使TiC增强体易于形成等轴状。合金元素铝不仅固溶强化了基体合金，而且细化增强体也有利于提高复合材料的力学性能。

5、利用透射电镜、高分辨透射电镜对原位合成(TiB+TiC)/Ti复合材料界面微结构进行研究和分析，发现两种增强体与基体的界面均为清洁界面，为直接的原子结合、界面结合状况良好。TiC增强体与基体合金没有确定的位相关系，而TiB增强体与基体合金存在以下位相关系：、、(0002)Ti//(001)TiB和以及、(0002)Ti//(200)TiB和。该位相关系在凝固过程中形成，与增强体的晶体结构及基体合金的晶体结构密切相关，形成该位相关系有利于降低增强体与基体合金界面的晶格畸变能。

6、研究了铸态钛基复合材料和热锻后高温钛基复合材料的力学性能。由于原位合成增强体的加入，钛基复合材料的力学性能与相应基体合金比较有了明显的提高，在增强体含量为8％时，其弹性模量E、屈服强度s0.2和抗拉强度sb分别达到131.2GPa，1243.7MPa和1329.8MPa，与基体合金Ti6242比较分别提高了19.3%，47.4%和45.5%。其强化机理主要来源于增强体承载、晶粒细化及高密度位错的形成。石墨的加入，形成更多等轴状、近似等轴状TiC粒子有利于提高复合材料的室温性能，这与短纤维状TiB的存在导致复合材料低应力断裂有关。

7、研究了原位合成钛基复合材料的高温瞬时拉伸性能。在600oC、650oC和700oC的抗拉强度分别超过800MPa，750MPa和650MPa，与高温性能较好的IMI834合金比较，在600oC的抗拉强度提高幅度超过25%。随着温度的提高，其屈服强度、抗拉强度降低，塑性提高，但与基体合金比较高温强度有了明显的提高。断口分析表明：低温时，裂纹由增强体断裂引起，而在高温时裂纹最先在短纤维晶须TiB的端面上形核，然后裂纹扩展到基体合金中，最后导致材料失效。说明低温时，增强体承载对提高复合材料的力学性能非常有利，而在高温时，其强化作用主要由增强体与位错的交互作用引起。位错容易在短纤维状晶须TiB的端面处塞积，形成裂纹源导致材料失效。因此与等轴状及近似等轴状增强体TiC比较，短纤维状增强体TiB对复合材料高温力学性能的强化效果要低一些。这也是石墨的加入形成等摩尔的TiB和TiC增强体有利于提高复合材料高温性能的主要原因。

8、研究了原位合成钛基复合材料的高温蠕变性能和持久断裂性能。原位合成钛基复合材料的高温蠕变经历了典型的蠕变变形的三个阶段。蠕变持久强度与基体合金比较有了明显的提高。持久强度与温度及载荷密切相关，温度和载荷的提高都降低复合材料的高温蠕变和高温持久性能。石墨的加入形成更多的TiC粒子，同样有利于提高钛基材料的持久强度。在高温、持久载荷作用下，材料的失效仍然主要由短纤维端面处形成裂纹而导致材料失效引起。

本研究首先从理论上分析了原位合成TiB、TiC及TiB和TiC混杂增强钛基复合材料的原位合成机制，并以此为基础开发出了一种新型钛基复合材料加工工艺。利用该工艺钛合金生产厂家可以在不改变设备和工艺的条件下，低成本高效制备高性能的钛基复合材料。而采用该原位合成工艺制备复合材料的性能是可设计和可控制的，针对不同的应用条件，可以设计不同成分的基体合金及不同含量、不同配比增强体的复合材料以满足不同的需求。从合金相图、增强体晶体结构及凝固理论相结合分析了原位合成增强体的生长机制、生长形态、分布状态以及界面微区特征。研究了钛基复合材料的微观组织对钛基复合材料力学性能的影响规律。这些研究为以后制备高性能的钛基复合材料和拓展钛基复合材料的应用领域打下了坚实的理论基础和为批量生成提供了实用途径。近两年来，研究成果引起了国家航空航天部门的关注，国家“十五”军工重点课题和航天支撑基金、航天创新基金课题获得了批准。并将用于我国的先进战略导弹XX－2改，战术导弹XX－19及新一代洲际导弹和潜地导弹的构件。鉴于该技术在国防军工方面具有的战略意义以及在民用领域的潜在应用前景，与国内大型钛合金加工企业—宝钢集团五钢有限公司开展产业化研究，完成了该材料的中试过程，实现了新型钛基复合材料的工业化生产。研制开发的材料近期将在国家战略、战术武器、宇宙飞船等方面得到验证和应用。并将逐渐推广应用于民用领域，为国民经济的发展作出贡献。

 

关键词  非连续增强钛基复合材料，原位合成，生长机制，凝固，晶体结构，微观结构，力学性能，位向关系，界面结构

 

Fabrication, Microstructure and Mechanical Properties

of in situ Synthesized Titanium Matrix Composites

 

ABSTRACT

 

Due to increasing requirement for titanium alloy with high properties in high technology era, discontinuously reinforced titanium matrix composites own the following advantages: high specific strength, high specific modulus, high elevated temperature property, wear resistance and low fabricating cost, so they have become the research hotspot. The processing technique, microstructure and mechanical properties have been extensively studied. However, the main aim is discontinuously reinforced titanium matrix composite prepared by traditional technique such as powder technology and liquid metallurgy, where the ceramic particles are directly incorporated into solid or liquid matrices respectively. In this paper, a new in situ technique has been used to fabricate discontinuously reinforced titanium matrix composites. Compared with traditional technique, in situ technique own the following advantages: the technique is very simple and the properties are excellent, so it is easier to fabricate titanium matrix composites in technology and economic. The in situ synthesis of ceramic particle avoids the pollution of reinforcements and wettability existing in casting technique, so it is valuable to fabricate titanium matrix composites with better properties. However, there are still quite a lot of problems to be resolved in order to fabricate titanium matrix composites with high properties simply and at low fabrication cost. Therefore, the research on these problems in theory and experiment is very important.

It is well known that the key problem in development and application of metal matrix composites is cost and property. A new technique has been designed to produce titanium matrix composites, in which it is possible to fabricate titanium matrix composites with high properties simply and at low fabrication cost. Titanium matrix composites reinforced with TiB whisker, TiC particle or TiB whisker and TiC particle, were produced by common titanium alloy casting technique utilizing the self-propagation high-temperature synthesis reactions between titanium and boron, graphite, B4C. In order to develop the utilization area of titanium matrix composites and make basis for producing titanium matrix composites with high properties, the following works have been developed.

1.         In situ synthesis mechanism of titanium matrix composites reinforced with TiB, TiC or TiB and TiC utilizing the reactions between titanium and boron, graphite, B4C have been investigated. Gibbs free energy DG and formation enthalpy DH of reactions between titanium and boron, graphite, B4C were calculated by thermodynamic theory. The Gibbs free energy DG of above reactions is negative, which indicates that the above reactions all can take place. It is possible to synthesize TiB2 and TiC utilizing the reaction between titanium and B4C. However, considering from chemical balance, TiB2 can not exist in titanium matrix alloy stably. The above reactions release quite a lot of heat. Moreover, the adiabatic temperature is greater than the theoretical criterion, which indicates that the reaction can be sustained by itself, namely self-propagation high-temperature synthesis reaction can occur.

2.         Titanium matrix composites reinforced with TiB whisker, TiC particle or TiB whisker and TiC particle have been produced by non-consumable vacuum arc remelting furnace and consumable vacuum arc remelting furnace. The results of X-ray diffraction show that the in situ synthesized reinforcements are TiB and TiC. The reinforcements were distributed uniformly in matrix alloy. The shapes of reinforcements are short-fibre shape, dendritic shape and equiaxed shape or near-equiaxed shape. The reinforcement with short-fibre shape is TiB, the reinforcement with dendritic shape and equiaxed shape or near-equiaxed shape is TiC. The experimental result is in good agreement with theoretical result, which provides gist for commercial production of in situ synthesized titanium matrix composites.

3.         The growth mechanisms of reinforcements in in situ synthesized titanium matrix composites have been investigated. The growth mechanisms are closely related to solidification paths and crystal structures. The reinforcements disperse from melt and grow in the way of nucleation and growth. For the in situ synthesized TiB whisker and TiC particle reinforced titanium matrix composites, the reinforcements undertake the following three stages: primary crystal, binary eutectic and ternary eutectic. Due to the different crystal structures, TiB and TiC grow in different shapes. TiB is liable to grow along [010] direction and form short-fibre shape due to it’s B27 crystal structure. The shape of TiB at cross section is polygon, the crystal faces are composed with (100), (101) and (10). Moreover, there is stacking fault in TiB and the stacking fault is likely to form at (100) crystal face. TiC with NaCl crystal structure grows in dendritic, equiaxed or near-equiaxed shape.

4.         The effects of aluminum addition on microstructure and mechanical properties of in situ synthesized titanium matrix composites have been investigated. The addition of alloying element aluminum doesn’t change phases and adjust the solidification path. However, the alloying element hinders the nucleation and growth of reinforcements that result in more fine TiB and TiC reinforcements and make TiC reinforcements grow with equiaxed particles easily. Aluminum not only strengthens the matrix alloy by solid solution strengthening, but also improves the mechanical properties by refining the reinforcements.

5.         The interfacial microstructures of in situ synthesized TiB whisker and TiC particles reinforced titanium matrix composites have been observed by means of transmission electronic microscopy and high- resolution transmission electronic microscopy. The results show that the interfaces are very clean. They are bonded well. There is no consistent crystallographic relationship between TiC and titanium. However, there are following consistent crystallographic relationships between TiB and titanium: , , (0002)Ti//(001)TiB,  and , , (0002)Ti//(200)TiB. Moreover, it is closely related to the crystal structures of reinforcement and matrix alloy. The formation of above crystallographic relationships is valuable to decrease the energy of lattice strain between reinforcement and matrix alloy.

6.         The mechanical properties of cast-titanium matrix composites and high-temperature titanium matrix composites after hot-forging have been investigated. Due to the incorporation of in situ synthesized reinforcements, the mechanical properties improve obviously compared with matrix alloy. When the volume of reinforcements is 8%, the Young’s modulus E, yield strength s0.2 and tensile strength sb are 131.2GPa, 1243.7MPa and 1329.8MPa, respectively. They improve 19.3%, 47.4% and 45.5%, respectively. The strengthening mechanisms include the following factors: undertaking load of reinforcements, refinement of grain size and formation of high-density dislocations. The addition of graphite forms more TiC particles with equiaxed or near-equiaxed shape that is valuable to improve the mechanical properties of titanium matrix composites at room temperature. This is related to existing of TiB that result fracture of composites at low level of applied strain.

7.         The ultimate tensile mechanical properties of titanium matrix composites at elevated temperature have been investigated. The ultimate tensile strengths of in situ synthesized titanium matrix composites at 600oC, 650oC and 700oC are 786.1MPa, 657.4MPa and 564.3MPa, respectively. Compared with IMI834 alloy, the ultimate tensile strength at 600oC improves 23.8%. As temperature increases, the yield strength and ultimate strength decrease, plasticity increases. Compared with matrix alloy, the mechanical properties at high temperature of in situ synthesized titanium matrix composites improve obviously. The analysis of fracture surfaces show that crack are formed due to the fracture of reinforcements at low temperature, while the cracks are likely to nucleate at the ends of short-fibre TiB and propagate to matrix alloy at high temperature so that composites failure. They indicate that undertaking load of reinforcements is valuable to improve the mechanical properties at low temperature. At high temperature, the strengthening effect results from the interaction between reinforcements and dislocations. Dislocations are liable to accumulate and entangle at the ends of TiB whiskers, which becomes crack resource and makes titanium matrix composites failure. Compared with equiaxed or near-equiaxed TiC particle, the strengthening effect of TiB whisker on titanium matrix composites is lower than that of TiC. This is also the main reason that the addition of graphite to form more TiC is valuable to improve the mechanical properties at high temperature.

8.         The creep deformation and creep rupture lives have been investigated. The creep deformation undertakes the following three stages: primary creep, steady creep and tertiary creep. The creep rupture lives of in situ synthesized titanium matrix composites improves obviously compared with matrix alloy. The creep rupture lives are closely related to temperature and applied stress. As the temperature and applied load increase, the creep and creep rupture life decrease. The addition of graphite to form more TiC particle is valuable to improve the creep rupture life. Under the condition of high temperature and applied stress, the failure of titanium matrix composites results from the crack that forms at the ends of TiB whisker.

    In this paper, the synthesis mechanisms of in situ synthesized TiB, TiC or TiB and TiC reinforced titanium matrix composites are firstly discussed in theory. Basis on the result, a new technique has been developed to fabricate titanium matrix composites. The titanium matrix composites with high mechanical properties can be produced simply and at low fabrication cost under the condition that processing factory can not change their equipment and technique. Moreover, it must be pointed that the mechanical properties of in situ synthesized titanium matrix composites fabricated by the technique can be designed and adjusted. The titanium matrix composites with different matrix alloy, different percent of reinforcements and different moles ratio between reinforcements can be designed to meet different requirement. The growth mechanisms of in situ synthesized reinforcements were analyzed by means of alloy phase diagram, crystal structures of reinforcements and solidification theory. The interface microstructures are also discussed. The effects of microstructures on mechanical properties of titanium matrix composites are investigated. Those results make basis for fabricating discontinuously reinforced titanium matrix composites with high properties and broadening the application area of discontinuously reinforced titanium matrix composites. They also provide practical way for commercial production of titanium matrix composites with high properties. These research fruits have attracted attention of aviation and aerospace department. The 10th five war industry key project, aerospace support foundation project and aerospace innovation foundation project have been approved. The innovation titanium matrix composites will be utilized in the following war system: advanced strategic missile XX-2 revised type, tactical missile XX-19 and new generation ICBM. Due to strategic signification of the technology on war industry and potential utilization foreground in civil field, the large enterprise of titanium alloy processing----Shanghai the fifth steel corporation of Baogang Group has been making industrial research with us. The middle test research has been finished and the industrial processing will be achieved. The processing production will be validated and utilized in strategic, tactical arms and aerospace etc in the near future. Moreover, they also will be extended in civil field and make contribution to the development of economy.

 

KEY WORDS  discontinuously reinforced titanium matrix composites, in situ formation, growth mechanism, solidification, crystal structures, microstructure, mechanical properties, crystallographic relationships, interfacial structure, creep rupture life

 

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