Interdiffusion Studies In Metal Silicon Systems
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Metal silicon systems have a wide range of applications, ranging from the use in electronic industry, as superconductors, protective coatings and as high temperature structural materials. Mo- and Nb-based silicides have emerged as suitable high temperature materials and extensive studies are being conducted make it suitable for various applications. Because of very good strength to density ratio, Nb-based silicides have attracted maximum attention. This is basically a mixture of Nb solid solution and Nb5Si3 intermetallic compound. A very small amount of NbCr2 Laves phase could also be present because of Cr addition. Incorporation of other alloying elements, which are mainly partitioned to these phases, helps to achieve a property balance like, high temperature strength, high fracture toughness, high creep and oxidation resistance. The knowledge on diffusion parameters is useful to understand many physical and mechanical properties. In this thesis, diffusion couple technique is used in different temperature ranges to study the growth kinetics and diffusion of the phases in an interdiffusion zone in binary silicides, Nb/Si, Mo/Si and V/Si, binary solid solutions, Nb/Mo, Nb/Ti, Nb/Zr and ternary silicides, Nb-Mo/Si, Nb-Ti/Si, Nb-Zr/Si. The parabolic growth constant, the integrated diffusion coefficients and the tracer diffusion coefficients are calculated from the experimental results obtained in this study and also from the results already available in the literature on the binary silicides. The activation energy for growth kinetics and the diffusion coefficients are also calculated to gain knowledge on the diffusion mechanism. The atomic mechanism of the diffusing species in all the phases of Nb and Mo silicide are discussed with the help of crystal structure and possible defects present. Also, a detailed analysis is done on the growth mechanism of the phases in Nb/Si and Mo/Si systems. In the Nb/Si system, Si is found to have higher diffusion rate in both the NbSi2 and Nb5Si3 phases. The number of nearest neighbour Si bonds is higher than nearest neighbour Nb bonds and hence one may predict high concentration of Nb antisites to be present in the NbSi2 phase. The growth mechanism analysis following the physico chemical approach explains the absence of the Kirkendall plane in the Nb5Si3 phase and duplex morphology in the NbSi2 phase in the Nb/Si couple. In the Mo/Si system, Si diffusion is faster than Mo in all the three phases. In the MoSi2 phase, Mo is practically immobile due to the absence of vacancies on the Mo sublattice. Similar defect structure is expected in the Mo5Si3 and Mo3Si phases also with additional Si antisite defects to assist Si diffusion. The growth mechanism analysis explains the absence of the Kirkendall plane in the Mo5Si3 and Mo3Si phases and continuous columnar grains in the MoSi2 phase in the Mo/Si couple. In the V/Si system, the activation energy for integrated diffusion coefficient of the VSi2 phase is found to be reasonably lower than the other phases which could happen because of very high concentration of defects, and/or because of contribution from the grain boundary diffusion as it shows the presence of columnar grains. Problems associated with the analysis done in literature are also discussed. A diffusion study is performed in different temperature ranges for the three binary metallic solid solution systems to determine the interdiffusion coefficients over the entire composition range using the relation developed by Wagner. The change in activation energy for interdiffusion with composition is also determined. It is found that activation energy for interdiffusion in Nb/Mo system is much higher than that for Nb/Ti and Nb/Zr system. Further the impurity diffusion coefficients of the species are determined and compared with the available data in literature. It is found that the activation energy for the impurity diffusion of Nb in Ti, Zr and Mo is higher than that of Ti, Zr and Mo in Nb. Interdiffusion study is done in the ternary silicides with the aim to examine the role of alloying additions, such as, Ti, Mo and Zr on the growth kinetics and diffusion behaviour of the phases in the Nb/Si system. The average interdiffusion (or integrated) coefficients are calculated when possible. The reaction and dissociation of the species at the interfaces are considered to understand the growth mechanism of the phases. An attempt is made to understand the change in diffusion mechanism because of the presence of third element. It is found that none of the alloying elements participate in the diffusion process although they do alter the growth kinetics and diffusion rate in both the phases, NbSi2 and Nb5Si3. It is also found that Nb becomes immobile in the NbSi2 phase in the presence of the alloying elements. Mo reduces the growth of both the phases while Ti addition does not cause any change in the growth but affects the diffusivity. Zr addition also reduces growth of the Nb5Si3 phase. It however complicates the interdiffusion zone in the Nb(Zr)/Si couple, which limits to qualitative study only. The Growth and consumption rate of the end members become very significant in many practical applications. Hence, relations for the growth and consumption rate in systems with finite end member thickness is developed considering single and double phase layer in the interdiffusion zone. Two different methodologies are used, the diffusion based and the physico-chemical approach to develop the same relations. We have shown that the diffusion based approach is rather straightforward; however, the physico-chemical approach is much more versatile than the other method. It is found that the position of the marker plane becomes vague in the second stage of the interdiffusion process in such a system, where two phases grow simultaneously.
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