Microstructure and cyclic oxidation behavior of hot dip aluminized coating on Ni-base superalloy Inconel 718

doi:10.1016/j.surfcoat.2006.07.242

Surface and Coatings Technology

Volume 201, Issue 7, 20 December 2006, Pages 3862-3866

https://doi.org/10.1016/j.surfcoat.2006.07.242 Get rights and content

Abstract

Ni-base superalloy In-718 was coated by hot-dipping into a molten bath containing Al–7wt.%Si. Cyclic oxidation experiments on bare substrate and aluminized alloy were conducted at 1100 °C, covering 240 cycles in static air. After hot dip treatment the coating layers consisted of two phases Al and FeAlSi were detected in the external topcoat to the aluminide/alloy substrate. After oxidation testing, a continuous alumina scale was detected on the surface of the aluminide layer. This coating shows better cyclic oxidation resistance for In-718 alloy than untreated substrate. Cr₂O₃ was found to be the primary oxide phase in the oxidation of bare In-718 alloy. The inward diffusion of Al in the aluminide layer was restricted by the interdiffusion zone. The NiAl phase constituent of the aluminide layer was similar through all of the testing. Only the γ phase could be found below the coating surface and in the subsurface region as aluminum was lost to form the oxide.

Introduction

High temperature materials such as nickel-base superalloys are currently used in numerous high temperature applications which require both mechanical strength and oxidation resistance [1]. Alloy 718 is a niobium-modified nickel-base superalloy that is widely used for high temperature parts of aircraft turbines and steam turbines. The oxidation resistance of nickel-base superalloys is dependent on the selective oxidation of chromium in the alloys through the formation of a protective Cr₂O₃ scale. The effective usage of Cr₂O₃ forming alloys at high temperatures is restricted due to the formation of volatile CrO₃, resulting in a loss of scale at temperatures greater than about 1000 °C [2], [3]. Therefore, the highest useful temperature exposure for Cr₂O₃-forming alloys is lower than 1000 °C. It is well-known that an Al₂O₃ protective layer has better oxidation resistance than Cr₂O₃ [4], and resists spalling at high temperatures up to 1300–1350 °C [5]. Furthermore, Al₂O₃ scales are most likely to provide adequate resistance to oxidation for nickel-base superalloys [2], [6].

Aluminizing the surface of a nickel-based alloy has been proven to be an effective method to form and maintain a protective Al₂O₃ scale, where the coating provides a reservoir of Al [7]. Hot dip aluminizing (HDA), is a diffusion coating formation process that has been widely used to deposit high temperature oxidation- and corrosion-resistant coatings on stainless steels and low-alloy steels [8], [9]. It has the distinctive advantages of high volume, low capital and operating costs and is applicable for large-size workpieces. The first aluminizing of Ni-base turbine blades by hot dip process happened in 1952 [10]. During the HDA process, silicon is normally added to the melt to reduce the thickness of the intermetallic layer [11]. Silicon has also been found to be beneficial for improving hot corrosion resistance of high temperature coatings [12]. However, their ability to provide total protection against high temperature oxidation and increase lifetimes of nickel-base superalloys has not been demonstrated adequately. From a practical point of view, the adherence of scale may also be affected by cyclic thermal stresses generated during heating and cooling, which are often a primary cause of degradation to the protective external oxide scale [13]. Consequently, cyclic oxidation is one of the fundamental methods used to assess high temperature environmental resistance of coatings. In this study, a hot dip aluminizing process has been employed to deposit aluminide coating on Ni-base superalloy Inconel 718. Cyclic oxidation tests at 1100 °C were conducted to evaluate the performance of the aluminide coating and the bare substrate. The microstructure and chemical composition of the coating and the bare substrate after cyclic oxidation have also been investigated to determine the degradation mechanism of the aluminide coating.

Section snippets

Experimental procedures

A commercial, fully-annealed, nickel-base superalloy, Inconel 718, (54.0% Ni–18.0% Cr–18.5% Fe–3.0% Mo–0.9% Ti–0.5% Al–5.1% Nb + Ta, in wt.%), plate was used as the substrate material. Rectangular specimens were cut to dimensions of 15 × 12 × 1 mm by a water-cooled cutting machine. They were degreased in an acetone bath and finally cleaned ultrasonically in an ethanol bath, and dried in air before hot dip aluminizing. The Al–7wt.Si% aluminum alloy (Al–7.0% Si–0.2% Fe–0.3% Mg, in wt.%) was melted in

Microstructures and phases of the aluminide layer

A typical cross-sectional BEI image and element concentration profile, in the coating layer, as measured by EDS, of the as-coated specimen are shown in Fig. 1. It can be seen from the BEI image that the coating consisted of two uniform layers with a total thickness of about 42 μm. The outer layer contains mostly Al with a little Si, which conforms to the bath composition. In contrast, the inner layer contains mostly Al, Si, Fe and Cr with a very small amount of Ni, indicating that this is

Conclusions

The hot dip aluminizing process has been proven to be an effective technique in obtaining an aluminide coating on the Ni-base In-718 superalloy. The adhesive Al₂O₃ scale was formed on the nickel aluminide surface to extend the lifetime of this alloy for up to 240 cycles of cyclic oxidation at 1100 °C. The presence of σ phase and Cr₃₇Nb₂₇Si₃₆ particles in the interdiffusion zone retarded the inward diffusion of Al in the coating. The phase constituent of aluminide layer is similar after the

Acknowledgements

The authors would like to thank Dr. J. W. Lee for the valuable discussions. This work was supported by the National Science Council, Taiwan through Contract No. NSC 94-2216-E-011-009.

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Microstructure and cyclic oxidation behavior of hot dip aluminized coating on Ni-base superalloy Inconel 718

Abstract

Introduction

Section snippets

Experimental procedures

Microstructures and phases of the aluminide layer

Conclusions

Acknowledgements

Mater. Sci. Eng.

Surf. Coat. Technol.

Intermetallics

Mater. Chem. Phys.

Mater. Chem. Phys.

Surf. Coat. Technol.

Surf. Coat. Technol.

Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process.

Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process.

Int. J. Press. Vessels Piping