Diffusion Foundations Vol. 13

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Authors: Sangeeta Santra
Abstract: The Ni-aluminides are integral constituents of thermal barrier coatings applied over Ni-based superalloys. These aluminides provide oxidation-resistance by forming a protective α–Al2O3 surface layer. The Pt-modified β–NiAl bond coat has been developed with an impetus to increase the service-life of Ni-based superalloys. The Pt-modified β–NiAl bond coat significantly improves the oxidation-resistance of superalloys. An interdiffusion zone containing topologically closed packed phases develops at the bond coat/superalloy interface. This eventually leads to Al-lean γ′–Ni3Al transformation, whose oxidation resistance is inferior to that of β–NiAl. The Pt-group metals Ir and Ru delay this transformation and impart creep-resistance to the bond coat. Recent investigations demonstrate that alloying with transition metals such as Cr, Mo and Fe enhance the mechanical strength. The functional stability of bond coat-superalloy assembly counts on the interfacial reaction and associated local structural variations which is a function of bond coat composition. This chapter elucidates the effect of various alloying elements on phase constitutions, crystallographic structural stability and thermodynamics of Ni-and Fe-aluminides to engineer a prospective bond coat.
Authors: Alexander Kodentsov
Abstract: A critical review of interdiffusion processes in the binary β-NiAl and γ'-Ni3Al intermetallic compounds is presented. The molar volume changes associated with interdiffusion and partial molar volumes of the reacting species, required for the determination of the diffusion parameters, are found using lattice parameter data and, in the case of NiAl, the available information about vacancy concentration within homogeneity range of the β-phase. The presented treatment is purely phenomenological, and its use is convenient since no exlicit assumption of the underlying mechanisms required. A critical analysis of diffusion data for β-NiAl and γ'-Ni3Al ordered phases is followed by discussion of error sources encountered in the interdiffusion experiments. From Kirkendall marker experiments with incremental diffusion couples, information about relative mobilities of species in the intermetallic phases can be obtained, and tracer diffusion coefficients can be deduced using pertinent thermodynamic data on the nickel aluminides. Contribution of the vacancy wind effect to the calculated tracer diffusivities can also be estimated. The Kirkendall plane bifurcation in the Ni41.7Al58.3/Ni72.24Al27.76 reaction couple, in which a single-phased layer of β-NiAl intermatallic is formed during interdiffusion from its adjacent phases, is directly related to the growth of grains of the reaction product at a location in between interfaces with starting materials. This diffusion phenomenon can be rationallised using a corresponding Kirkendall velocity diagram. Changes in magnitude and sign of the difference in intrinsic mobilities of the components inside the homogeneity range of the β-NiAl lead to a velocity curve that makes bifurcation of the Kirkendall marker plane possible.
Authors: Kaustubh N. Kulkarni
Abstract: Both FeAl and NiAl with B2 crystal structure are envisaged for their usage in high temperature applications and hence, availability of diffusion data in these intermetallics is crucial in designing their alloys and processes as well as deciding their in-service performance. A comprehensive overview of diffusion data available in B2 FeAl and NiAl and their alloys is provided in this article. Nearest neighbor vacancy jumps in B2 intermetallic lead to a local disorder in the lattice and hence it is not necessarily the unit step of diffusion in these structures. Several mechanisms of diffusion proposed in the literature are discussed including nearest neighbor jumps, next nearest neighbor jumps, six-jump vacancy cycle, triple defect and antisite bridge. Relevance of these mechanisms in FeAl and NiAl is discussed. An overview is given on the self-and solute diffusion and interdiffusion data available in both binary FeAl and NiAl. Due to wide solubility range of both FeAl and NiAl as well as their alloying requirements for improved properties, it becomes pertinent to study the multicomponent diffusion in the alloys based on these B2 itnermetallics. Hence, in the latter part of the article, various methods used for determining multicomponent diffusion data are reviewed. A detail overview is also provided on the diffusion studies available in literature on ternary alloys based on FeAl and NiAl with an emphasis on highlighting the diffusional interactions observed in these systems.
Authors: Li Jun Zhang, Juan Chen, Wei Min Chen, Na Ta, Qin Li
Abstract: Advanced modern gas-turbine engines strongly rely on high-temperature thermal barrier coatings (TBCs) for the improved efficiency and power. Interdiffusion between the bond coat and the underlying Ni-based superalloy is one key factor limiting the lifetime of TBCs. In order to assist the engineering-oriented lifetime assessment and even design new TBCs, reliable composition- and temperature-dependent interdiffusivity databanks for γ, γ′ and β phases in different types of bond coats and Ni-based superalloys are the prerequisite. This chapter starts from a very brief introduction of the state-of-art experimental techniques and calculation methods for interdiffusivity determination in ternary systems. After that, the status of the interdiffusion databanks of γ, γ′ and β phases in NiAl-based ternary systems is then summarized, with a special focus on the demonstration of interdiffusivity data measured by means of single-phase diffusion couple/multiple techniques in combination with Matano-Kirkaldy method or numerical inverse method. Several typical results for NiAl-based γ, γ′ and β phases are also given. Finally, two examples of successful applications of the available interdiffusion databanks of ternary NiAl-based γ, γ′ and β phases are presented. One lies in the Re-substitutional element searching in potential new-generation Ni-based superalloys, while the other is the phase-field modeling of interdiffusion microstructure in ternary mode NiAlCr-based TBCs without/with the effect of temperature gradient.
Authors: Aloke Paul
Abstract: The diffusion-controlled growth and microstructural evolution at the interface of aluminide coatings and different substrates such as Ni-base superalloys and steel are reviewed. Quantitative diffusion analysis indicates that the diffusion rates of components in the β-NiAl phase increases with the addition of Pt. This directly reflects on the growth rate of the interdiffusion zone. The thickness and formation of precipitates increase significantly with the Pt addition. Mainly Fe2Al5 phase grows during hot dip aluminization of steel along with few other phases with the very thin layer. Chemical vapor deposition process is being established for a better control of the composition of the Fe-aluminide coating on steel.

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