Oxidation behaviour of a novel refractory high entropy alloy at elevated temperatures

doi:10.1016/j.intermet.2020.106711

Intermetallics

Volume 119, April 2020, 106711

https://doi.org/10.1016/j.intermet.2020.106711 Get rights and content

Highlights

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A new refractory high entropy alloy were oxidised up to 1400 °C for 100 h.
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Microstructural evolution strongly affected the oxidation of the studied alloy.
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(Al, Cr)(Nb, Ta)O₄ external oxide layer was protective against oxidation.
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Strong Ti-rich nitride presence was observed deep beneath the oxidised regions.

Abstract

The refractory high entropy alloy (RHEA) has shown great potentials for high temperature applications beyond modern Ni-based superalloy. However, its oxidation behaviours are rarely reported and understood. In this work, the oxidation behaviours of a novel RHEA “NV1”, Cr-17.6Al-20.3Mo-15.2Nb-2.9Si-13.4Ta-5.4Ti (in at%), were examined at 1200, 1300, and 1400 °C up to 100 h. At 1200 °C, the oxidation kinetics curve yielded toward parabolic behaviour owing to the formation of a rutile-type complex oxide layer with Al₂O₃ and Cr₂O₃ dispersions; breakaway oxidation contributed by Cr₂O₃ evaporation occurred at 1300 °C; a single power-law behaviour governed the oxidation kinetics curve at 1400 °C, and mullite was identified within the oxide layer. This work provides guidelines for understanding the oxidation mechanisms and improving oxidation resistance of RHEA at elevated temperature.

Introduction

Research on the new materials for high temperature application with capabilities beyond Ni-based superalloy has drawn great attention. Following the recently developed concept of high entropy alloy [1], Senkov et al. [2] proposed RHEA as a potential material for high temperature applications. By combining multiple refractory elements in similar atomic fractions, the RHEA exhibits attractive mechanical properties at 1200 °C and above [3,4], which is beyond the temperature limitations of Ni-based superalloys [5]. But for high temperature applications in the atmosphere, oxidation is an important issue. To the best of authors’ knowledge, there were very limited reports on the oxidation mechanisms and associated microstructure characterization for RHEAs to date [[6], [7], [8], [9], [10], [11], [12], [13], [14]], especially for temperatures at 1200 °C and above. Currently, the oxidation resistance of RHEA is still inadequate for high temperature applications since there is no RHEA that can spontaneously form protective oxide layers, such as Al₂O₃, SiO₂, and Cr₂O₃, at high temperatures. In our previous work [14], a novel RHEA “NV1” was presented to possess parabolic mass gain behaviours at 1100 °C during oxidation; this article presents the oxidation behaviours of NV1 at 1200, 1300, and 1400 °C up to 100 h. The study on such unprecedent temperature range and testing duration would shed more light on understanding the oxidation mechanisms of RHEA.

Section snippets

Material and methods

The ingot of NV1 was prepared by means of cold crucible levitation melting with nominal mixture of high purity (>99.9 wt%) elements and machined into specimens by electrical discharge machining. The composition of the as-cast ingot was determined with Shimadzu Lab Center XRF-1800. The as-cast microstructure of NV1 showed dendritic microstructure, with Mo-rich body centred cubic phase as dendrite arm, Cr-rich hexagonal Laves phase and minor Al-rich tetragonal phase as inderdendritic region. More

Results

The isothermal oxidation kinetics curves of NV1 alloy at 1200, 1300, and 1400 °C are shown in Fig. 1a. After 100 h of oxidation, the mass gain per initial surface area is 6.00, 9.91, and 18.43 mg/cm² at 1200, 1300, and 1400 °C, respectively. A single mass gain dependence is observed within 100 h of oxidation at 1200 °C, which can be described by Equation (1): $Δ m^{2.421} = 0.865 t$

Interestingly, three different trends of mass gain dependence are observed during the oxidation at 1300 °C, which can be

Discussion

NV1 alloy exhibits distinctly different oxidation behaviours at 1200, 1300, and 1400 °C, Fig. 1a. Since XRD patterns indicate the major component of the oxide near the top surface is rutile-type oxide for all specimens (Fig. 2), the microstructural evolution could play an important role in the different oxidation behaviours between and within different temperatures.

At 1200 °C, a complex OL bearing mainly rutile-type oxide with Al₂O₃ and Cr₂O₃ dispersion is formed, Fig. 3a. The rutile-type oxide

Conclusions

NV1 alloy exhibits distinctly different oxidation behaviours at 1200, 1300, and 1400 °C up to 100 h: a single parabolic behaviour was observed at 1200 °C owing to the formation of a complex rutile-type external OL with Al₂O₃ and Cr₂O₃ dispersion; the deterioration of the complex OL caused by Cr₂O₃ evaporation resulted a 3 stages oxidation at 1300 °C; both Cr₂O₃ evaporation and mullite formation led to a single power-law behaviour at 1400 °C. In addition, nitride particles are detected in all

CRediT authorship contribution statement

Kai-Chi Lo: Conceptualization, Methodology, Data curation, Writing - original draft, Visualization, Investigation, Writing - review & editing. Hideyuki Murakami: Supervision, Writing - review & editing. Jien-Wei Yeh: Supervision. An-Chou Yeh: Conceptualization, Methodology, Supervision, Writing - review & editing.

Acknowledgements

This study was a collaboration between National Tsing Hua University, Taiwan and National Institute for Materials Science (NIMS), Japan. This work is supported by NIMS under the International Cooperative Graduate Program; Ministry of Science and Technology (MOST) in Taiwan under Grant MOST106-2923-E-007-002-MY2, MOST107-2218-E-007-012, and MOST107-3017-F-007-003; and the “High Entropy Materials Center” from The Featured Areas Research Center Program within the framework of the Higher Education

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View full text

Oxidation behaviour of a novel refractory high entropy alloy at elevated temperatures

Highlights

Abstract

Introduction

Section snippets

Material and methods

Results

Discussion

Conclusions

CRediT authorship contribution statement

Acknowledgements

Intermetallics

Intermetallics

J. Alloy. Comp.

J. Alloy. Comp.

J. Alloy. Comp.

Intermetallics

Intermetallics

J. Alloy. Comp.

Surf. Coat. Technol.

Appl. Surf. Sci.

J. Eur. Ceram. Soc.

J. Non-Cryst. Solids

Nanostructured high‐entropy alloys with multiple principal elements: novel alloy design concepts and outcomes

Adv. Eng. Mater.

Microstructure and properties of aluminum-containing refractory high-entropy alloys

JOM (J. Occup. Med.)