An oxidation resistant refractory high entropy alloy protected by CrTaO4-based oxide

Kai-Chi Lo,An-Chou Yeh,Jien-Wei Yeh,Yao-Jen Chang,Hideyuki Murakami

doi:10.1038/s41598-019-43819-x

Kai-Chi Lo, An-Chou Yeh + Show 3 more

Open Access

https://doi.org/10.1038/s41598-019-43819-x

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Abstract

Although refractory high entropy alloys (RHEAs) have shown potentials to be developed as structural materials for elevated temperature applications, most of the reported oxidation behaviours of RHEA were associated with short term exposures for only up to 48 hours, and there is a lack of understanding on the oxidation mechanism of any RHEA to-date. In this work, by using thermogravimetric analysis, isothermal oxidation was conducted on a novel RHEA at 1000 °C and 1100 °C for up to 200 hours, which is an unprecedented testing duration. The external oxide layer strongly influenced the weight gain behaviours, and it consisted of CrTaO4-based oxide with some dispersion of Al2O3 and Cr2O3. At 1000 °C, the inability to form dense CrTaO4-based oxide layer resulted an exponential dependence of weight gain throughout 200 hours. At 1100 °C, mass gain curve showed two parabolic dependences associated with the formation of protective CrTaO4-based oxide layer and the weight gain after 200 hours was 4.03 mg/cm2, which indicates that it is one of the most oxidation resistant RHEAs comparing to literature data to-date. This work can also provide insights on how to further develop RHEA to withstand long term oxidation at elevated temperatures.

Highlights

As the temperature capability of modern Ni- based superalloys has reached its limit for gas turbine engine applications, materials scientists are searching for new materials with higher temperature capabilities
The as-cast microstructure of NV1 had dendrites and contained several phases; back-scattered electron image (BEI) and the chemical composition of the observed phases are shown in Fig. 1(a) and Table 1, respectively
There were distinct differences in the weight gain behaviours of NV1 at 1000 and 1100 °C; X-ray diffraction (XRD) patterns and chemical compositions of all the oxidised samples indicated the major components of the oxide layers at both temperatures were CrTaO4-based oxide with dispersion of Al2O3 and Cr2O3

Summary

Introduction

As the temperature capability of modern Ni- based superalloys has reached its limit for gas turbine engine applications, materials scientists are searching for new materials with higher temperature capabilities. “High-Entropy Alloy (HEA)”[1,2] has been proposed as a new strategy for alloy design with a wide composition space. The most recent study on the oxidation behaviour of AlCrMoTaTi reported by Gorr et al.[14] indicated a mass gain after 48 hours of oxidation at 1000 and 1100 °C was less than 1 and 3 mg/cm[2], respectively; this extremely low mass gain at 1000 °C was claimed to be due to formation of Al2O3 layer on the alloy surface, the associated oxidation. Mechanism was not clear since there was no detailed microstructure characterization These previous oxidation studies were conducted for only up to 48 hours, and there was no indication whether this Al2O3 layer could still be effective after a longer exposure. Such oxidation study for RHEA can provide valuable information on the long-term oxidation behaviours of this class of alloy and how to improve it further

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