Improvement in oxidation behavior of Al0.2Co1.5CrFeNi1.5Ti0.3 high-entropy superalloys by minor Nb addition

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Abstract

High-entropy superalloys (HESAs) can replace commercial Ni-based superalloys. For high-temperature applications, the oxidation resistance of HESAs must be considered. Herein, the oxidation mechanism of HESA with sufficient minor Nb addition was conducted at 900 °C under atmosphere. With 0.9 at% Nb added in the base metal, oxidation resistance was significantly improved, which was confirmed by a furnace test and thermogravimetry. The isothermal oxidation resistance was enhanced by approximately 66%, owing to the presence of the Nb-rich layer. This improvement was observed and analyzed by backscattering electron images through scanning electron microscopy and wavelength dispersive spectroscopy with a field-emission electron probe microanalyzer. The mechanism of oxide formation was elucidated by X-ray diffraction for various exposure time durations. With Nb minor addition, the microstructures of the present alloy were found to be composed of γ matrix and γ′ precipitates and the mechanical properties were slightly higher than those without Nb.

Introduction

High-entropy alloys (HEAs) in metallic materials have become well-known owing to their unique properties and more extensive design possibilities. HEAs are a novel concept of alloy design, first proposed by Yeh et al., in 2004 [1]. HEAs have five or more principle elements with four unique core effects [2,3]: high-entropy effect, severe lattice distortion, sluggish diffusion, and cocktail effect. Therefore, HEAs have many superior properties [29], such as excellent thermal stability [4], outstanding mechanical properties [[5], [30], [31]], good wear resistance [6], high corrosion resistance [[7], [32]]. The research of HEAs has gotten an increasing amount of attention in recent years [[33], [34], [35]]. High-entropy superalloys (HESAs) combine the concept of HEAs with the concept of superalloys to create new high temperature alloys [[8], [9], [10], [11], [36], [37]]. HESAs have outstanding thermal stability, mechanical properties, and oxidation resistance at the evaluated temperature [[38], [39]]. It has high potential to replace some superalloys for the high temperature applications [[40], [41]]. The functional alloy of Al0.2Co1.5CrFeNi1.5Ti0.3, designated as “FTA”, was published in 2015, consisting of γ (FCC matrix) and γ’ (L12 coherent precipitates) phase [12]. Its phase composition is similar to that of commercial Ni-based superalloys, and it also has excellent mechanical properties. After oxidation at 900 °C, FTA can form dense Cr2O3, which provides the main oxidation resistance, maintaining high-temperature performance.

However, the oxidation resistance of Al0.2Co1.5CrFeNi1.5Ti0.3 is insufficient compared with those of Ni-based superalloys, such as superalloy Inconel 718 [12]. Meanwhile, HESAs exhibit superior mechanical properties at the evaluated temperature. The improvement of oxidation resistance still needs to be investigated for high-temperature applications. In previous studies, it has been shown that aluminized coatings, which is a popular industrial process, can also be used on HESAs. The oxidation resistance of FTA was improved at temperatures above 900 °C, while that temperature is considered to be the degraded temperature of chromia. After 441 h, there was still no spallation observed at 900 °C [13]. Additionally, pack-aluminization, two-step aluminization-chromization, siliconization [14] and the sol-gel method [15] are common industrial methods for improving oxidation resistance in high-temperature applications.

This study develops a more efficient method for directly improving the oxidation resistance of HESAs by alloy design rather than the complex industrial process. Furthermore, mechanical properties need to be on the same level for the original purpose of alloy design to be carried out. With minor Nb addition, the oxidation resistance is improved owing to the formation of a Nb-rich layer in Ni-based superalloys [16,17]. However, excessive Nb addition causes a decrease in oxidation resistance and the formation of a topological closed-packed (TCP) phase, which worsens the mechanical properties at elevated temperatures [42]. Until now, only a few studies have reported the effect of Nb on HESAs. This study discusses the effect of Nb on the oxidation behavior and tensile properties of HESAs. This method of alloy design directly enhances the oxidation resistance and eliminates complex industrial processes.

Section snippets

Experimental procedure

Calculation of phase diagram (CALPHAD) was used to perform the thermodynamic simulation of the equilibrium phase diagram in Thermo-calc software with the database of TCNI8. CALPHAD was an efficient method for predicting the formation of phases. Two HESAs were selected for further experiment, with the elemental composition shown in Table 1. Al–Co–Cr–Fe–Ni–Ti–Nb HESAs were prepared by vacuum arc melting and casting in a water-cooled Cu-molding under an argon atmosphere. The as-cast ingot was the

Phase diagrams and microstructures

CALPHAD results represent the calculation of the equilibrium phase diagram. Al0.2Co1.5CrFeNi1.5Ti0.3 is a well-studied HESA, and it has been confirmed to be composed of only γ matrix and γ′ precipitate [12,18,19]. Recently, there have been many publications of data from this HESA, which can beused as reference for subsequent thermodynamic calculations. The Nb thermodynamic calculations of equilibrium phase diagram for (Al0.2Co1.5CrFeNi1.5Ti0.3)1-xNbx for a variety of conditions is shown in

Conclusion

The method proposed herein efficiently enhanced the oxidation resistance of HESAs at the evaluated temperature. Al0.2Co1.5CrFeNi1.5Ti0.3Nb0.05 with 0.9 at% Nb consisted of only γ and γ′, which was identical to the microstructure of Al0.2Co1.5CrFeNi1.5Ti0.3. The formation of Nb-rich layer in Al0.2Co1.5CrFeNi1.5Ti0.3Nb0.05 significantly improved oxidation resistance. The oxidation weight gain reduced by 40% with Nb addition. The 4-μm Nb-rich layer was formed between Cr2O3 and Al2O3 in the (Cr0.1Ti

Prime novelty statement

The prime novelty of this research is the significant improvement in oxidation resistance of high-entropy superalloys with minor Nb addition. Until now, only a few studies have reported the effect of Nb on high-entropy superalloys. The oxidation weight gain reduced by 40% with 0.9 at% of Nb added. The 4 μm Nb-rich layer is explored by FE-EPMA mapping and BSE image. In addition to the improvement of oxidation resistance, the tensile properties are slightly improved, too.

CRediT authorship contribution statement

Jun-Jie Yang: Conceptualization, Investigation, Data curation, Writing - original draft. Chia-Ming Kuo: Writing - review & editing. Po-Ting Lin: Software, Writing - review & editing. Hung-Chih Liu: Software, Writing - review & editing. Cheng-Yao Huang: Methodology. Hung-Wei Yen: Resources, Supervision. Che-Wei Tsai: Methodology, Project administration, Supervision.

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.

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Acknowledgments

We are pleased to acknowledge the financial support for this research by Ministry of Science and Technology, R.O.C (MOST 107-2218-E-007-012 and 108-2218-E-007-005). The support provided by the High Entropy Materials Center from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan is greatly appreciated.

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