Codeposition of Al and Si on nickel base superalloys by pack cementation process

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Abstract

The vapour pressure of halide species generated at high temperatures in pack powder mixtures containing Al, Si, halide salt and Al2O3 were analysed by means of thermochemical calculations in an attempt to identify suitable halide activators and pack compositions for codepositing Al and Si to form diffusion coatings on nickel-base superalloys by the pack cementation process. The halide salts investigated were AlF3, AlCl3, NH4Cl and CrCl3·6H2O. The results of thermochemical calculations suggested that compositions of pack powder mixtures activated by NH4Cl and CrCl3·6H2O may be adjusted to create deposition conditions favourable for codepositing Al and Si, but the pack powder mixtures activated by AlF3 or AlCl3 may only deposit Al. Guided by the thermochemical calculations, a series of pack powder mixtures activated by CrCl3·6H2O were formulated and coating deposition experiments were carried out at 1000 and 1100 °C. With adequate control of pack compositions and deposition conditions, it was found that codeposition of Al and Si could indeed be achieved at these temperatures. A mixture of elemental Al and Si powders may be used as a depositing source instead of using Al–Si master alloy powders as conventionally recommended. Depending on the deposition temperature used, the coatings could be formed either through the inward diffusion of Al and Si or through the outward diffusion of Ni together with other substrate elements such as Cr and Co with the respective coating structures associated with the two coating formation mechanisms. The pack compositions and deposition conditions may be adjusted to control the microstructure of the coatings formed by the codeposition process.

Introduction

Diffusion coatings for protecting the nickel-base superalloy components at high temperatures are mostly based on the simple NiAl or Pt modified NiAl compounds [1], [2], [3]. They are usually produced by the pack cementation process or, in the case of Pt modified NiAl coatings, by electroplating Pt followed by pack aluminising. These coatings are highly effective in protecting the components against high temperature oxidation. But, they cannot provide adequate resistance to hot corrosion caused by fused salts such as Na2SO4, which are often formed on the component surface as a result of reactions between impurities in fusel fuels and the intake atmosphere. Degradation of these coatings caused by hot corrosion can be particularly severe and rapid in reducing atmospheres of low oxygen potential [4]. To substantially increase the hot corrosion resistance, sufficient quantities of Cr or Si must be introduced into the coatings. Therefore, there is a strong technical incentive to further develop the pack cementation process so that multiple elements such as Al, Cr, Si, Y, and Hf can be codeposited to form coatings resistant to high temperature oxidation and corrosion.

In the past decade, considerable research efforts have been made to identify suitable conditions for codepositing Al and Cr to form diffusion coatings on nickel base superalloys and on low alloy steels [5], [6], [7], [8], [9], [10]. More recently, significant progress has also been made in understanding the mechanisms of the codeposition process [10], [11]. However, detailed studies on the feasibility of codepositing Al and Si to form Si modified aluminide diffusion coatings on nickel base superalloys have been very limited. This paper reports the results of an investigation aimed to assess, by means of thermochemical analysis in combination with experimentation, the feasibility of codepositing Al and Si to form multiple element diffusion coatings on nickel base superalloys.

Section snippets

Thermochemical considerations

In a normal pack cementation process, the substrates are placed in a sealed or semi-sealed container together with a well-mixed pack powder mixture containing the depositing elements, halide salt activator and inert filler (usually alumina). The substrates may be buried in (in-pack process) or suspended above (out-pack process) the pack powder mixture. At high temperatures, the halide salt would react with depositing elements to form a series of halide vapour species containing the depositing

Experimental procedures

The substrate used for this study is a commercial alloy CMSX-4 with a nominal composition of 61.7Ni–5.6Al–6.5Cr–9.0Co–6.0W–6.5Ta–3.0Re–1.0Ti–0.6Mo–0.1Hf wt.%. The alloy rod of about 16 mm in diameter was sliced into buttons with a thickness between 2 and 3 mm. The surfaces of the buttons were ground and polished to a 1200 grit finish. The specimens were then degreased and weighed before placing them in pack powders.

Pack powder mixtures were prepared by accurately weighing out and thoroughly

Results and discussions

Guided by the results of thermochemical calculations, a series of pack powder mixtures were formulated and prepared using powders of Al, Si, CrCl3·6H2O and Al2O3 with Al varying from 1.1 to 3 wt.%, Si 3 to 10 wt.%, CrCl3·6H2O 3 to 6 wt.% with the balance being Al2O3. Coating deposition experiments were carried out at 1000 and 1100 °C. After a few trials, it was found that, with careful adjustment of pack compositions and precise control of coating deposition conditions, codeposition of Al and Si

Conclusions

The equilibrium partial pressures of vapour species generated in halide activated pack powder mixtures at high temperatures can be calculated and applied to identify pack compositions and deposition conditions suitable for codepositing Al and Si to form diffusion coatings on metal alloy substrates by the pack cementation process. These calculations suggested that compositions of pack powder mixtures activated by NH4Cl and CrCl3·6H2O can be adjusted to create deposition conditions favourable for

Acknowledgements

This research programme was funded by the EPSRC and QinetiQ and supported by ALSTOM and C-UK. Authors wish to thank B. Best for his skills in assisting the EDS/SEM measurements.

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