Fatigue behavior of Pt-modified aluminum coated single crystal superalloy at 900 ℃

https://doi.org/10.1016/j.ijfatigue.2023.107596Get rights and content

Highlights

Abstract

The purpose of this paper is to investigate the effect of different diffusion-resistance bond coats on the fatigue performance of second-generation single crystal superalloys. The β-(Ni,Pt)Al coating with Re-base diffusion barrier decreased the fatigue performance of the substrate, while the γ′ coating improved the fatigue performance. Oxidation is the most important factor leading to fatigue failure of the bond coat at high temperature. Stress concentration due to the diffusion barrier and TCP accelerated the β-(Ni,Pt)Al coating failure. Solution of refractory elements is of significant importance for the γ′ coating to resist plastic deformation and prevent fatigue crack propagation.

Introduction

Ni-based single crystal (SX) superalloys, due to their excellent thermal stability and mechanical properties, have been extensively applied to manufacture rotating components in advanced aero- and land- based gas turbine engines [1], [2]. In order to meet the requirements of increasing power efficiency and reducing exhaust emissions, turbine blades made of SX superalloys must operate at higher inlet temperature. Protective coatings, used either alone or as a bond coat for thermal barrier coatings (TBCs), are required to avoid high temperature oxidation damage to the superalloy [3]. Upon exposure at operational temperatures, a thermally grown oxide (TGO), predominately exclusive and intact α-Al2O3 scale, develops on protective coatings. Pt-modified aluminide coatings have been shown to possess much superior high temperature oxidation resistance compared to the unmodified ones [4]. The presence of Pt in the coating can promote the selective oxidation and adhesion of α-Al2O3 scale [5], improve the stability of alumina and decrease the oxidation rate [6].

In contrast to the good oxidation resistance, at high temperatures, metallic bond coats usually cause interdiffusion of elements due to significant concentration differences between the coating and the superalloy (e.g. Al and Ni), which can lead to the formation of harmful topologically-close-packed (TCP) phases and rapid growth of the secondary reaction zone (SRZ) [7]. The inward diffusion accelerates the depletion of Al in the bond coat and the formation of SRZ degrades mechanical properties of the superalloy. In order to resolve the interdiffusion problem, bond coat with a diffusion barrier (DB) are currently being explored [8], [9], [10]. The Re-base diffusion barrier can effectively inhibit the elemental interdiffusion during the isothermal oxidation at high temperature. The thickness of SRZ and the quantity of TCP precipitated phases in the superalloy were significantly decreased [8]. In addition, recent studies have found that bond coats having a γ-γ′ structure alleviate interdiffusion with the superalloy [11], [12], [13], [14]. The former shows a three-layer structure consisting of an outer layer, a diffusion barrier and the interdiffusion zone (IDZ), while the latter exhibits a relatively simple structure.

Despite their excellent diffusion and oxidation resistance, bond coats can affect the mechanical properties of the superalloy substrate [15], [16], [17], [18]. Micro-voids formed due to the interdiffusion of elements, and the coalescence of micro-voids was the main reason for the formation of the fatigue crack [15]. The Pt-Al bond coat had a positive effect on the fatigue behavior at 800 °C. The fine grains in the IDZ effectively disperse the stress concentration of the crack tip under low stress. Under high stress, Al diffusion into the substrate and the rafting γ′ formed near the bond coat, thus inhibiting crack propagation [16]. The generation of brittle TCP particles may be the main cause of lifetime degradation due to stress concentration and dislocation accumulation. In addition, the increasing number of TCP phases reduced the fatigue life of single crystal superalloys and the interface between the substrate and TCP phase may be the source of fatigue cracks under fatigue loading [17]. The bond coat had no effect on fatigue life, because fatigue cracks initiated at the surface for both coated and uncoated samples [18]. Conflicting findings suggest that the effect of bond coats on the fatigue behavior of single crystal superalloy is unclear.

In the present study, two kinds of diffusion-resistance bond coats, the β-(Ni,Pt)Al coating with Re-base diffusion barrier (referred as β coating) and γ′ coating, were deposited on single crystal superalloy. A rotating-bend fatigue test was carried out on both coated and uncoated samples. The fracture morphologies and cross-sectional microstructure were characterized. The effects of oxidation, coating microstructure and coating thickness on fatigue behavior of superalloy were discussed.

Section snippets

Single crystal superalloy and bond coated preparation

The substrate material of this study was a second-generation single crystal superalloy. The typical microstructure consists of 68.6% cuboidal γ′-precipitates embedded in a face-centered cubic γ matrix. The nominal chemical composition is shown in Table 1. The shape and dimensions of fatigue specimens are shown in Fig. 1. Symmetry axis of the fatigue specimen was fabricated along [0 0 1] orientation (below 10 deg. deviation). The hardness of the material is 550 Hv [19]. To eliminate surface

Characteristics of the coatings

Fig. 2 shows the cross-sectional morphologies and the elemental distributions of main elements of with the β coating and the γ′ coating. As shown in Fig. 2a, the β coating exhibits a three-layer structure: the outer layer, the intermediate diffusion barrier and the interdiffusion zone. The outer layer is the oxidation-resistant functional region that is composed of the β-NiAl phase, while the intermediate layer consists of a Re-rich precipitation phase. The Re-rich particle belonged to σ phase

Discussion

According to the experimental results in section 3.3, both β and γ′ bond coats reduce the maximum stress intensity factor of the cast pores inside the single crystal superalloy. However, the initiation of fatigue cracks in coated samples are all related to the coating. Fatigue behavior of the two coating samples is not the same. In order to clarify the fatigue crack initiation mechanism of both Pt-modified aluminum coated samples, the following discussion will be made from the perspective of

Conclusions

In this study, different bond coats were deposited on single crystal superalloys. The fatigue behaviors of the β coating sample and the γ′ coating sample were investigated at 900 °C. The fatigue crack initiation and propagation mechanism in the bond coat have also been discussed. Based on the experimental results, the following conclusions can be drawn:

  • (1)

    The β-(Ni,Pt)Al coating with Re-base diffusion barrier decreased the fatigue performance of the substrate, while the γ′ coating improved the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. U21A2044, 52272057, 52001022); and the Science Center for Gas Turbine Project (Grant No. P2021-A-IV-002-001, P2022-B-IV-008-001).

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