Oxidation protection of γ-TiAl-based alloys – A review

Relationship between pack chemistry and aluminide coating formation for low-temperature aluminisation of alloy steels

Synthesis and characterisation of pack cemented aluminide coatings on metals

This study aims to investigate the feasibility of aluminising low alloy steels at temperatures below 700 °C by pack cementation process to increase their high temperature durability in oxidative and corrosive environments without adversely affecting their mechanical strength and creep resistance at elevated temperatures. Packs activated by AlCl 3 , AlF 3 , NaCl and NaF were used to carry out coating deposition experiments at 650 °C in an attempt to identify the most suitable activator for the intended pack aluminising process. Once this was achieved, a series of further experiments were undertaken to investigate the effects of pack composition, deposition temperature and time on the kinetics of coating growth process. The pack Al content was varied from 1 to 6 wt.%, deposition temperature from 600 to 750 °C and deposition time from 1 to 16 h. Within the ranges of these parameters, it was observed that these parameters only affected the coating thickness, but not the surface Al concentration and with the packs activated by AlCl 3 , the coatings formed via an inward reactive Al diffusion mechanism, which led to the formation of a surface Fe 14 Al 86 layer and an inner FeAl 3 layer. Thermochemical calculations were also undertaken to analyse the equilibrium vapour pressures of depositing halide species (AlF or AlCl) generated at deposition temperatures in packs activated by different halide salts. The results obtained were discussed in relation to the observed deposition tendencies of these packs and their influence on the kinetics of coating growth.

Pack aluminisation of low alloy steels at temperatures below 700 °C

Hot corrosion behavior of Co modified NiAl coating on nickel base superalloys

Codeposition of Al and Si to form oxidation-resistant coatings on γ-TiAl by the pack cementation process

Low-temperature formation and oxidation resistance of nickel aluminide/nickel hybrid coatings on alloy steels

Both theoretical and experimental studies were carried out to identify pack powder compositions and coating conditions suitable for codeposition of Al and Cr by the pack cementation process. It has been demonstrated that for pack powders containing chloride salts as activators and elemental Al and Cr as source materials, codeposition of Al and Cr occurs when the partial pressures PAlCl and PCrCl are comparable or when PAlCl /PCrCl is close to one. But, only Al or Cr may be deposited when PAlCl /PCrCl is much larger or smaller than one. Pack powder compositions suitable for codeposition of Al and Cr depend very much on the type of activators used. For pack powders containing 25 wt-% of source elements of Al and Cr, codeposition of Al and Cr may occur when the Al content is within the range 1·25–1·90 wt-% for the packs activated by 3 wt-%NH4 Cl, and 1·85–1·95 wt-% for the pack activated by 4 wt-%CrCl3 . 6H2O. However, codeposition of Al and Cr may be not achievable with pack powders activated by Na...

Conditions for Codeposition of Al and Cr on Ni Base Superalloys by Pack Cementation Process

Thermochemical analyses were carried out for a series of pack powder mixtures for deposition of aluminide and for co-deposition of aluminide and silicide coatings on γ-TiAl by the pack cementation process. Based on the results obtained, experimental studies were undertaken to identify optimum pack powder mixtures for depositing adherent and coherent aluminide and silicide coatings. Pack powder mixtures activated by 2 wt% AlCl3 was used to aluminise γ-TiAl at 1000°C. With proper control of pack compositions and coating conditions, an aluminide coating of TiAl3 with a coherent structure free from microcracking was deposited on the substrate surface via inward diffusion of aluminium. The results of thermochemical calculations indicated that co-deposition of Al and Si is possible with CrCl3 · 6H2O and AlCl3 activated pack powders containing elemental Al and Si as depositing sources. Experimental results obtained at 1100°C revealed that CrCl3 · 6H2O is not suitable for use as an activator for co-depositing aluminide and silicide coatings on γ-TiAl. It caused a significant degree of degradation instead of coating deposition to the substrate. However, adherent coatings with excellent structural integrity consisting of an outer TiSi4 layer and an inner TiAl3 layer were successfully co-deposited at 1100°C and 1000°C using pack powder mixtures activated by AlCl3. IT is suggested that such coatings were formed via a sequential deposition mechanism through inward diffusion of aluminium and silicon. Discussion is presented on the issues that need to be considered to ensure the deposition of aluminide and silicide coatings with coherent structure free from microcracking on γ-TiAl by the pack cementation process.

Co-deposition of aluminide and silicide coatings on γ-TiAl by pack cementation process

Thermochemical analyses were undertaken for a series of pack powder mixtures containing elemental Al and Hf or W powders as depositing elements and CrCl3·6H2O or AlF3 or CrF3 as an activator and Al2O3 as the inert filler for codepositing Al with Hf or W to form diffusion coatings on nickel–base superalloys by the pack cementation process. The results indicated that Al could be codeposited with Hf, but not with W, from the vapour phase. Compared with both AlF3 and CrF3, CrCl3·6H2O has been shown to be a more suitable activator for codepositing Al and Hf. The optimum coating temperature has been identified to be in the range of 1050–1150 °C. Based on the thermochemical analysis, a series of coating deposition studies were carried out, which confirmed that codeposition of Al and Hf could be achieved at a coating temperature of 1100 °C in the CrCl3·6H2O activated packs containing either only Hf or both Al and Hf. The coating structure produced depended on the pack composition. It is suggested that compositions suitable for codepositing Al and Hf could be effectively identified by comparing the vapour pressures of HfCl4 and HfCl3 with that of AlCl in the packs activated by chloride salts. It has also been experimentally demonstrated that, although W could not be deposited from the vapour phase, a high volume of fine W particles can be entrapped into the NiAl coating layer formed through the outward Ni diffusion using a modified pack cementation process, resulting in the formation of a composite coating layer with W particles evenly distributed in a NiAl matrix. It is suggested that this modified pack process could be similarly applied to produce nickel aluminide coatings containing uniformly distributed fine particles of other refractory metals that may not be codeposited with Al from the vapour phase.

Formation of Hf- and W-modified aluminide coatings on nickel–base superalloys by the pack cementation process

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