Y/Hf-doped AlCoCrFeNi high-entropy alloy with ultra oxidation and spallation resistance
Graphical abstract
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.
Section snippets
Sample preparation
The AlCoCrFeNi alloys were prepared by using equiatomic Al, Co, Cr, Fe, and Ni metals of high purity (99.99 wt. %) with the addition of reactive elements Y (0.02 at.%) and Hf (0.02 at.%) as raw materials and then placed on a water-cooled Cu hearth under a high-purity argon atmosphere. The alloys were re-melted five times to achieve compositional homogeneity.
Isothermal oxidation test
The AlCoCrFeNiYHf ingot was cut into 10 × 10 × 3 mm3 square plates using a SiC abrasive cutting blade in a precision cut-off machine
The microstructure of as-cast AlCoCrFeNiYHf HEA
Fig. 1 shows the morphology of the HEA after etching. As shown in Fig. 1a, dendritic (DR) and interdendritic (ID) regions are uniformly distributed in the equiaxed dendritic grain structure, due to solute segregation during dendritic solidification [25]. In the grains of HEA, a periodic, fine-scale two-phase microstructure can be observed from the DR and ID regions, which can be attributed to the spinodal decomposition mechanism (Fig. 1b, c and d) [25,26].
A combination of HAADF-STEM (Fig. 2a
Discussion
In this study, it has been clearly demonstrated that the AlCoCrFeNiYHf HEA exhibits an extremely low oxide growth rate and strong oxide/substrate interface adhesion, which eventually contribute to the superior spallation resistance of oxide scale for the long-term oxidation. The mechanisms will be discussed as follows.
Conclusions
In the present work, an ultra oxidation and spallation resistant AlCoCrFeNi high-entropy alloy with Y/Hf doping was fabricated and characterized. The following conclusions can be drawn :
- 1
The AlCoCrFeNiYHf HEA consists of a B2 structure matrix and a A2 structure precipitates. Ni and Al are riched in B2 phase, and Fe and Cr preferentially segregate in the A2 phases, while Co is uniformly distributed.
- 2
An exclusive α-Al2O3 scale is fast established on the AlCoCrFeNiYHf HEA in the early oxidation
CRediT authorship contribution statement
Jie Lu: Conceptualization, Data curation, Writing - original draft. Ying Chen: Writing - review & editing. Han Zhang: Methodology, Formal analysis. Na Ni: Writing - review & editing. Ling Li: Methodology, Formal analysis. Limin He: Resources. Rende Mu: Resources. Xiaofeng Zhao: Conceptualization, Writing - review & editing. Fangwei Guo: Methodology, Formal analysis.
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.
Acknowledgement
This work was financially support by National Natural Science Foundation of China (51971139).
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