【作者】 朱艳；

【导师】 杨延清；

【作者基本信息】 西北工业大学 ， 材料学， 2003， 博士

【摘要】 SiC纤维增强Ti基复合材料在航空航天领域有重要的应用前景，但由于Ti的化学活性大，在复合材料的SiC-Ti界面处存在严重的界面化学反应，会极大的损害SiC／Ti基复合材料的力学性能。然而迄今为止，对SiC／Ti基复合材料的界面反应的研究特别是理论研究很不深入。因此本文运用量子化学、热力学、动力学、扩散等理论，并采用实验观察，对SiC／Ti基复合材料界面反应进行了多方面的研究，揭示了界面反应的本质。 首次将量子化学计算理论运用于金属基复合材料界面反应的研究中，运用Gaussian 98量子化学计算程序，找到了适合于研究过渡族金属Ti的碳化物和硅化物的计算方法，获得了SiC／Ti基复合材料界面反应的热力学和动力学数据。 根据Korler和Miedema模型，推导了三元合金各组元活度系数的对称计算公式，确定了经验常数a的值。对Ti-5Al-2.5Sn、Ti-6Al-4V、Ti₃Al、TiAl的计算表明计算值与试验值吻合较好，而且比非对称计算公式获得的结果更精确。结合量子化学研究结果，计算了SCS-6 SiC纤维与Ti-Al金属间化合物和典型的近α、α+β、近β及β钛合金组成的复合材料体系可能发生的16个界面反应的Gibbs函数变Δ_rG。计算表明，在所研究的温度范围内（600-1600K），16个界面反应均可正向进行。即TiSi、TiSi₂、Ti₅Si₃、Ti₃Si、TiC、Ti₅Si₄、Ti₃AlC、Ti₂AlC、Ti₃SiC₂、Ti₃SiC几种产物都可能形成。从热力学上预测了不同复合材料体系的界面反应性，表明SiC／TiAl、SiC／Ti₂AlNb、SiC／Ti40等体系的界面反应较轻。为复合材料体系的选材指明了方向，可大大减少试验工作量。 在量子化学研究的基础上，建立了SCS-6 SiC／Ti基复合材料界面反应的反应动力学模型，即反应是通过原子态的中间态分二个步骤进行的。量子化学计算与经验估算相结合，求出了可能发生的界面反应的活化能，表明第一步骤的活化能远大于第二步骤的活化能，因而第一步骤是反应的动力学控制因素，在这一步骤中，原子态的Ti、Si、C分别从基体钛合金和纤维中分解出来。量子化学计算表明其离解能分别为108KJ／mol、499.7KJ／mol和626.1KJ／mol，与文献报道的试验值吻合。离解能数据及速率常数的计算均证明Ti的离解较为容易。活化能数据表明界面反应的第二步骤属于动力学快速反应，其中反应Ti+SiC=TiC+Si将优先进行，形成界面反应区中细小晶粒的第一反应层，该反应不受扩散所控制。尽管随后进行的界面反应仍西北工业大学博士学位论文属动力学快速反应，但受Ti、C、Si等元素的扩散所控制。 根据活度系数研究结果，推导了对称的元素互扩散系数计算公式，比非对称的互扩散系数计算公式的计算结果更接近实验值。计算了个界面反应层中C和Si的互扩散系数，获得了SCS一6 SIC/TiZAINb复合材料界面反应过程中C和Si的浓度分布具体关系式，并计算了SCS一6 SIC/TiZAINb基复合材料-界面各产物层C和Si的浓度分布，与实验测定吻合较好。由于受到元素扩散的控制，界面反应产物的形成应当遵循相图上浓度与各相区之间的关系，因此，SCS一6 SIC/TiZAINb基复合材料中，可形成的界面反应产物为Tissi:、T 1351、TIC和Ti3AIC。 采用透射电镜对SCS一6 SIC/TiZAINb复合材料和SCS一6 SIC/Ti3AI复合材料的界面反应进行了观察研究，发现界面反应产物呈层状分布，表现出明显的反应扩散特征。SCS一6 SIC/TiZAINb复合材料界面反应产物依次为:紧挨着纤维的晶粒细小的TIC层、晶粒细小的Ti5Si3层、等轴晶粒的TIC层及Ti3Si层。试样经过900oC长期热暴露后，各反应层的厚度增加，在TiZAINb基体中还发现一些个别的Ti3AIC颗粒。SCS一6 SIC/Ti3AI复合材料的界面反应产物可多达6层，依次为:紧挨着纤维的晶粒细小的TIC层、晶粒细小的Tissi;层、等轴晶粒的TIC层、Ti3Si层、Ti3AIC层及Ti5Si3层。t匕较而台，二种复合材料中SCS一6 siC/TiZAINb的界面反应程度较轻，与热力学预测结果一致。 根据实验研究结果，从热力学、扩散动力学及晶体学上分析讨论了界而反应产物Ti3AIC的形成机理，表明是由于C的扩散进入Ti3AI晶粒，发生了Ti3A卜C分Ti3AIC的反应所致。在SCS一6 SIC/TiZ川Nb复合材料「「，，由，-基体TiZAINb中只有个别的Ti3AI晶粒，因此会形成个别的Ti3AIC颗粒，而在 SCS一6 SIC/Ti3AI复合材料中，则形成层状的Ti3AIC。 研究了界面障碍涂层TIB:的作用，证明TIB:可以明显的阻止元素扩一散，但会与Ti反应生成TIB相。在SCS一6 SIC/ TIBZ/TiZAINb复合材料体系中，只有当TIB:消耗殆尽后，才‘会形成诸如TIC、Ti3si、r’15513、Ti3AIC等界由l反应产物。热力学和动力学分析表明，TIB:与Ti的反应过程为:所需活化能较小的Ti+TIBZ= 2 TIB反应优先进行并逐渐减弱，最终停止。随后所需活化能较大TIBZ令TIB+B反应开始进行并与Ti+B斗TIB反应构成循环，相互促进。更多还原

【Abstract】 Continuous SiC fiber reinforced titanium matrix composites hold immense potential for advanced aircraft and aerospace structural applications. However, due to the high reactivity of titanium, brittle reactive products at fiber/matrix interface are easy to form and may lead to a degradation of the mechanical properties of SiC/Ti composites. In contrast, very little is reported on theory study of interfacial reaction in SiC/Ti composites. Therefore, in this paper, the interfacial reactions of SiC/Ti composites were studied by means of quantum chemistry, thermodynamics, kinetics, diffusion dynamics and experiment.It is the first time to study the mechanism of interfacial reaction in SiC/Ti composites by quantum chemistry computation methods. A suitable method to calculate titanium carbide and silicide was found and the thermodynamic and dynamic data involved in interfacial reaction of SiC/Ti composites have been obtained.Based on Kohler’s ternary solution model and Miedema’s model for calculating the heat of formation of binary solution, the integral equation has been established for calculating activity coefficients in ternary alloys and intermetallics. The constant, a, in the formula was determined. The activity coefficients for components in alloys Ti-5Al-2.5Sn, Ti-6Al-4V and intermetallics TiAl, TiaAl have been calculated with the equation. The calculated data coincide well with the experimental ones found in literatures, which are more accuracy than ones calculated by dissymmetry formula. After getting thermo-chemical data of some compounds by using Gaussian 98 software of quantum chemistry and obtaining activities of elements in Ti matrix, the Gibbs function increments, rG, for 16 chemical reactions at the interface of 12 SCS-6 SiC/ Ti composites were calculated. Results predicted by thermodynamic calculation indicate that 16 interfacial reactions may take place spontaneously from the left to the right, namely reaction products TiSi, TiSi2, Ti5Si3, Ti3Si, TiC, Ti5Si4, TiaAlC, Ti2AlC, Ti3SiC2 and TisSiC may form. It is shown that the interfacial reactions of SCS-6 SiC/TiAl, SCS-6 SiC/Ti2AlNb and SCS-6 SiGTi40 are not so severe as in other SCS-6 SiC/Ti composites. The result is useful for us to choose the Ti matrices.Based on quantum chemistry, a two-step dynamic model of interfacial reaction in SCS-6 SiC/ Ti composites was built up. Combining quantum chemistry calculation with experimental evaluation, the activation energies of possible interfacial reaction were calculated. It is shown that the first step in which the atomic Ti, C and Si were decomposed from Ti matrices and fiber, respectively, is a rate-controlling step because the activation energy of the step is larger than second one. It is indicated by quantum chemistry calculation that dissociation energy of Ti, C and Si is 108KJ/mol, 499.7 KJ/mol and 626.1 KJ/mol, respectively, coincided well with the experimental ones found in literatures. It was shown from both dissociation energy and rate constant that dissociation of Ti is much easier than others. It was confirmed that the first reaction layer of TiC with very fine crystal grains is due to the reaction Ti+SiC-TiC+Si. This reaction is a reaction-controlled one. However, the other reactions in the second step are controlled by elements diffusion.Based on the thermodynamics for getting activity coefficient, a symmetry equation wasestablished for calculating inter-diffusion coefficient. The calculated data coincide well with the experimental ones found in literatures. It means that the data from the symmetry equation are more accurate than the ones from dissymmetry formula By calculation, the distribution of C and Si in the interfacial layers of SCS-6 SiC/Ti2AlNb composite were gained, which is consistent well with the experimental ones. It is obviously that the interfacial reaction is diffusion controlled. Therefore, the form of interfacial reaction products should follow the phase rule in phase diagram, So interfacial reaction products existed in SCS-6 SiC/Ti2AlNb composite sh更多还原