Y/Hf-doped AlCoCrFeNi high-entropy alloy with ultra oxidation and spallation resistance

Highlights

• Ultra oxidation and spallation resistant Y/Hf-doped AlCoCrFeNi high-entropy alloy (HEA) is fabricated.

• The HEA consists of disordered body-centered cubic (BCC) A2 phase and ordered BCC B2 phase.

• An exclusive α-Al₂O₃ scale in the early oxidation stage is fast established.

• The oxide scale predominantly consists of α-Al₂O₃ with minor quantity of Y/Hf-rich oxides at α-Al₂O₃ grain boundaries.

• The oxide/substrate interface is smooth and clean even after 1000 h oxidation.

Abstract

In this study, we report a type of ultra oxidation and spallation resistant Y/Hf-doped AlCoCrFeNi high-entropy alloy (HEA). The alloy exhibits a significantly lower oxidation rate at 1100 °C with a k_p value of about 1.9 × 10⁻¹³ g² cm^-4s^-1, which is one magnitude lower than those of the conventional NiCoCrAlY alloys. Additionally, the alloy shows a strong resistance to the oxide scale spallation. The superior oxidation performance for the Y/Hf-doped HEA can be attributed to the fast establishment of exclusive α-Al₂O₃ scale, the beneficial reactive element effect and the large columnar grain size (width) in the oxide scale.

Introduction

Metals with exceptional mechanical strength, oxidation and corrosion resistance at high temperatures, have many important engineering applications. However, it has been established that a number of trade-offs must be accepted when it comes to alloy design. For example, Ni-based superalloys with superior creep strength generally do not have sufficient resistance to oxidation required for the long-term durability at high temperatures. To deal with this problem, oxidation resistant coatings are applied to the superalloys during service [1,2]. In contrast, FeCrAlY, one of the most oxidation resistant alloys, has relatively low strength at high temperatures, and thus restricts its application as structural components [[3], [4], [5]]. Entropy engineering, however, offers a promising approach to design alloys and overcome the trade-off between high temperature strength and oxidation resistance. As will be demonstrated in this work, the Y/Hf-doped AlCoCrFeNi high-entropy alloy exhibits superior oxidation resistance, which is comparable to those of the most oxidation resistant alloys ever reported, but still maintain high mechanical strength and stability at high temperatures.

High-entropy alloys (HEAs) [6], also referred to as multi-principal element alloys or multi-component alloys [7], have attracted much attention due to their unique compositions, microstructures and adjustable properties [[8], [9], [10], [11]]. The HEAs are originally defined as solution alloys with more than five principal elements in equal or near equal atomic percent (at. %), which are proposed to be stabilized by the maximized configurational entropy [[8], [9], [10], [11], [12]]. To date, the definition of HEAs has been extended as solution alloys with the composition that the atomic fraction of each principal element is being between 35 and 5 at. % [8]. Among all HEAs, AlCoCrFeNi exhibits proper yield strength at elevated temperatures and compressive ductility at room temperature, which possibly has the potential for high temperature applications [[13], [14], [15]]. However, it was reported that the oxidation resistance of AlCoCrFeNi is extremely low. For example, the oxidation products contain spinel or mixed oxides, showing a high oxide growth rate and low spallation resistance [16,17]. Butler et.al investigated the oxidation behavior of an equal atomic percent AlCoCrFeNi HEA at 1050 °C [16,17]. A large amount of non-alumina oxides (e.g. Cr₂O₃) formed on the HEA surface and substantial spallation of the oxide scale occurred after 100 h oxidation. Essentially, the AlCoCrFeNi HEA has similar elemental compositions with those of NiCoCrAlY and FeCrAlY. A good oxidation resistance for this type of alloy is expected to be achieved with the minor doping of reactive elements (REs, e.g. Y or Hf).

It is well accepted that the doping of reactive elements (REs, e.g. Y or Hf) in the typical alumina-forming alloys (e.g. NiCoCrAl, NiAl or FeCrAl alloys) can significantly improve the oxidation performance due to the beneficial REs effects [5,[18], [19], [20], [21], [22]]. Therefore, in this work, minor concentrations of Y (0.02 at. %) and Hf (0.02 at. %) are doped into an AlCoCrFeNi HEA to improve its oxidation performance. It will be demonstrated that the Y and Hf-doped AlCoCrFeNi HEA exhibits an ultra low oxidation rate, which is about an order of magnitude lower than those of conventional NiCoCrAlY alloys. Extensive characterization of the oxide scale microstructure, growth rate, compositions and spallation lifetime is conducted to understand the mechanisms.