Key Takeaways & Executive Findings
- •• Thermal exposure at 1100°C paradoxically improves the high-cycle fatigue life of PtAl-coated single-crystal superalloys at 900°C. • The improvement is attributed to a shift in crack initiation from surface microcracks to internal micropores. • Formation of a protective Al2O3 layer within microcracks and γ′ rafting near the interdiffusion zone hinder surface crack growth. • Findings provide critical insights for refining HCF life prediction models for coated superalloys in turbine blade applications.
Abstract
The as-deposited coating–substrate microstructure has been identified to substantially influence the high-cycle fatigue (HCF) behavior of Ni-based single-crystal (SX) superalloys at 900°C, but the impact of degraded microstructure on the HCF behavior remains unclear. In this work, a PtAl-coated third-generation SX superalloy with sheet specimen was thermal-exposed at 1100°C with different durations and then subjected to HCF tests at 900°C. The influence of microstructural degradation on the HCF life and crack initiation were clarified by analyzing the development of microcracks and coating–substrate microstructure. Notably, the HCF life of the thermal-exposed coated alloy increased abnormally, which was attributed to the transformation of the fatigue crack initiation site from surface microcracks to internal micropores compared to the as-deposited coated alloy. Although the nucleation and growth of surface microcracks occurred along the grain boundaries in the coating and the interdiffusion zone (IDZ) for both the as-deposited and the thermal-exposed coated alloys, remarkable differences of the microcrack growth into the substrate adjacent to the IDZ were observed, changing the crack initiation site. Specifically, the surface microcracks grew into the substrate through the cracking of the non-protective oxide layers in the as-deposited coated alloy. In comparison, the hinderance of the surface microcracks growth was found in the thermal-exposed coated alloy, due to the formation of a protective Al2O3 layer within the microcrack and the γ′ rafting in the substrate close to the IDZ. This study will aid in improving the HCF life prediction model for the coated SX superalloys.
1. Introduction
Platinum–aluminide (PtAl) bond coat has been frequently employed on the single-crystal (SX) turbine blades based on its superior oxidation and corrosion resistance [1]. PtAl-coated components operate at elevated temperatures, and obvious interdiffusion occurs between the coating and the substrate owing to the chemical concentration gradients, leading to significant microstructural degradation within the coating, the interdiffusion zone (IDZ), and the substrate near the IDZ [2]. Many previous studies have found that high-temperature mechanical properties were deteriorated significantly for the coated alloys after thermal exposure [2–4]. Consequently, it becomes crucial to reveal the influence of coating–substrate microstructural degradation on the high-temperature mechanical properties of the SX superalloys.
High-cycle fatigue (HCF) failure is the predominant failure mode for turbine blades [5]. Particular attention has been paid to investigate the influence of the as-deposited coating on the HCF behavior of Ni-based SX superalloys [6–7]. The fatal fatigue cracks preferentially nucleated at the coating surface in the as-deposited coated SX superalloy, which was closely related to the coating–substrate microstructure and oxidation [8–9]. Wang et al. [8] found that the as-deposited PtAl coating of β phase with diffusion barrier decreased the HCF life for the SX superalloy, while the as-deposited PtAl coating of γ′ phase improved the HCF life. The topological close-packed (TCP) phases in the diffusion barrier region and the IDZ deteriorated the HCF property of the PtAl (β phase) coated alloy. Moreover, PtAl coating of β phase suffers from an abrupt brittle-to-ductile transition at a critical temperature, ranging from 650 to 800°C [10]. However, recent studies [11–12] have found that the role of the as-deposited PtAl coating of β phase on the HCF property changed from negative to positive as the experimental temperature decreased from 900 to 700°C, due to the different oxidation behavior at the tip of microcrack under different temperatures. In a word, it indicates that the coating–substrate microstructure influences the HCF behavior of the as-deposited coated alloy significantly.
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Dong Sun, Siliang He, Longfei Li, Song Lu, Weiwei Zheng, Jonathan Cormier, Qiang Feng (2025). High-cycle fatigue life improvement of a PtAl-coated third-generation Ni-based single-crystal superalloy after thermal exposure. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-025-3157-z
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Frequently Asked Questions
What is the main finding of this study?
The study reveals that thermal exposure at 1100°C paradoxically improves the high-cycle fatigue life of a PtAl-coated single-crystal superalloy at 900°C, due to a shift in crack initiation from surface microcracks to internal micropores.
Why does thermal exposure improve HCF life?
Thermal exposure leads to the formation of a protective Al2O3 layer within surface microcracks and γ′ rafting near the interdiffusion zone, which hinders the growth of surface microcracks, thereby shifting crack initiation to internal micropores and increasing fatigue life.
What is the significance of this research for turbine blade applications?
The findings provide critical insights into the role of microstructural degradation on fatigue behavior, which can improve the accuracy of HCF life prediction models for coated superalloys used in turbine blades.
What are the key microstructural features affecting crack initiation?
Key features include the coating–substrate microstructure, the interdiffusion zone, the formation of oxide layers within microcracks, and γ′ rafting in the substrate near the IDZ.
How does this study contribute to the field of superalloys?
It clarifies the influence of thermal exposure on the HCF behavior of coated single-crystal superalloys, offering a mechanistic understanding that can guide coating design and life prediction methodologies.
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