Researchers propose new class of high-temperature alloy; eutectic high-entropy alloys with both high ductility and high strength
High-entropy alloys (HEAs), which represent an emerging effort in materials science and engineering, are multi-principal-element (at least four) alloys that are promising for high-temperature applications due to their high resistance to softening at elevated temperatures and sluggish diffusion kinetics.
However, HEAs so far have either high strength or high ductility; achieving both has been a challenge. Further, the inferior castability and compositional segregation of HEAs have been obstacles for commercialization. Now, researchers in China are proposing a new strategy for designing high-entropy alloys using the eutectic alloy concept to accomplish both attributes. As reported in a paper in Nature’s open access journal Scientific Reports, an eutectic high-entropy alloy (EHEA) showed an “unprecedented” combination of high tensile ductility and high fracture strength at room temperature. The excellent mechanical properties could be kept up to 700 °C.
|High Entropy Alloys|
|In an introduction to Entropy’s 2013 special issue on HEAs, guest editor Prof. Jien-Wei Yeh, National Tsing Hua University, Taiwan, explained that in principle, with at least five major elements, HEAs thus have high mixing entropy at the liquid state or random state.|
|High mixing entropy can enhance the formation of solution-type phases, and in general leads to a simpler microstructure, he wrote.|
|He noted that HEAs have a broad range of structure and properties and may find applications in structural, electrical, magnetic, high-temperature, wear-resistant, corrosion-resistant, and oxidation-resistant components.|
To the best of our knowledge, there is no report of HEAs possessing an excellent balanced strength and tensile ductility. Naturally, a composite way can be expected to achieve this balance. However, simply introducing a combination of fcc and bcc phases, without a proper structural design, could not solve the problem. Moreover, the inferior castability and compositional segregation, which are common for HEAs, further downgrade their mechanical properties and cast shadow on their engineering applications.
In order to address to these important technical issues that HEAs are currently facing, we proposed here to use the eutectic alloy idea to design HEAs with the composite structure, or it can be said to use the high-entropy alloy concept to design eutectic alloys.
Furthermore, we proposed to design the eutectic alloys with a mixture of soft fcc and hard bcc phases, to achieve the balance of high fracture strength and high ductility. Apart from being a new way to obtain the composite structure in HEAs, eutectic alloys are also known to be good candidate high-temperature alloys, because the eutectic solidification structure has the following features: 1) near-equilibrium microstructures that resist change at temperatures as high as their reaction temperature; 2) low-energy phase boundaries; 3) controllable microstructures; 4) high rupture strength; 5) stable defect structures; 6) good high-temperature creep resistance; 7) regular lamellar or rod-like eutectic organization, forming an in-situ composite.
In addition, eutectic alloys are known to have better castability. Also importantly, since the eutectic reaction is an isothermal transformation and hence there exists no a solidification temperature range, both the segregation and shrinkage cavity can be alleviated. Accordingly, if eutectic HEAs with the composite fcc/bcc structure can be prepared, they are expected to possess the advantageous mechanical properties and castability of eutectic alloys, clearing obstacles for their technological application.—Lu et al.
An eutectic reaction is the moment at which a mixture of two or more distinctively different solid phases simultaneously solidify from the parent liquid—e.g., liquid → α + β. The components form a joint super-lattice with a unique atomic percentage ratio between the components. The eutectic temperature is the lowest possible melting temperature over all of the mixing ratios for the involved components.
In an eutectic system, the growth of solid phases depends on the simultaneous rejection of constituents to the liquid phase, which causes adjustments of the melt composition and hence, mass transport by diffusion normal to the growth direction. This is a complex process involving interactions of heat and mass transfer; the result is a unique microstructure resulting from cooperative growth, connected to the atomic transfer in the melt ahead the solid/liquid interface.
Modern pistons are often made of aluminum-silicon alloys of eutectic, and partly hyper-eutectic composition, which can be cast easily and also forged. MAHLE notes that the eutectic alloy M124 is the classic piston alloy, and has been used for decades. Hyper-eutectic alloys enable pistons with even great wear resistance.
Hypereutectic pistons are lighter than forged steel and with a lower coefficient of thermal expansion, allowing tighter tolerances. The lighter weight also can enable less reciprocating mass inside the engine, promoting greater efficiency, high rpm capability, and immediate engine response, notes GM.
To verify their novel design strategy the team from Dalian University of Technology, in collaboration with colleagues from Chalmers University in Sweden, Yanshan University, City University of Hong Kong, and The Hong Kong Polytechnic University, prepared a ~2.5 kg ingot of AlCoCrFeNi2.1. The ingot exhibited a uniform and fine lamellar microstructure characteristic of eutectic alloys.
The EHEA bulk material exhibited an excellent castability with few casting defects—significantly differentiating with other bulk HEAs, the team reported. They then machined a tensile test specimen from the bulk alloy ingot.
The room temperature tensile test showed a fracture stress of 944 MPa and an elongation of 25.6%; these values converted to 1186 MPa and 22.8% in the true stress/strain condition.
Of note, the authors said, was that the EHEA exhibited an ultra-high strain hardening behavior, having the proof stress of 75 MPa but reaching the fracture stress close to 1.2 GPa.
At 600 and 700 °C, the proof stress, fracture stress and elongation were 95 MPa, 806 MPa, 33.7%, and 108 MPa, 538 MPa, 22.9%, respectively, both in the true stress/strain condition. The combination of high strength and high ductility for this EHEA could be maintained at least up to 700 °C, the researchers reported.
The mechanical properties of the as-cast EHEA could be further tuned by thermomechanical treatments. Simple cold rolling with a thick reduction of 8% increased the engineering/true proof stress and fracture stress to 275 MPa and 1145 MPa/1265 MPa, respectively, while remaining a decent ductility of 10.4%/9.9% in elongation.
The researchers then compared their EHEA to NiAl and NiAl/Cr(Mo) eutectic alloys.
The reference alloys were chosen because the current EHEA can be regarded as a highly concentrated pseudo-binary NiAl/(Co, Cr, Fe) eutectic alloy, and they are also known to be promising high-temperature alloys, due to the lower density and melting point, much higher thermal conductivity, and excellent oxidation resistance at high temperatures. The major disadvantages for NiAl are the low ductility at room temperature and the low strength at high-temperatures, which also constitute the motivation to develop NiAl/Cr(Mo) eutectic alloys, where the in-situ formed composite structure is expected to improve the mechanical properties of NiAl.—Lu et al.
Compared to the eutectic alloys, the EHEA exhibited a much higher fracture strength, and simultaneously much higher ductility at room temperature.
… our alloy design strategy resulted in the discovery of an EHEA with a fine lamellar fcc/B2 structure and balanced high fracture strength and high ductility. This novel strategy is promising to solve the problems inhibiting the engineering application of HEAs as high-temperature materials, including the inferior castability and compositional segregation, and the conflict between high fracture strength and high ductility. In addition, this novel strategy also brought breakthrough to NiAl-based alloys in significantly enhancing the ductility at room temperature while simultaneously achieving the high strength. The EHEA concept, combining the advantages of both HEAs and eutectic alloys, and at the same time eliminating the disadvantages of these two types of alloys, leads to a promising new class of high-temperature alloys. Using this new alloy design strategy, an industrial scale HEA with uniform organizations and compositions can be prepared by the conventional casting method in a low cost way, allowing for large-scale industrial applications.—Lu et al.
Yiping Lu, Yong Dong, Sheng Guo, Li Jiang, Huijun Kang, Tongmin Wang, Bin Wen, Zhijun Wang, Jinchuan Jie, Zhiqiang Cao, Haihui Ruan & Tingju Li (2014) “A Promising New Class of High-Temperature Alloys: Eutectic High-Entropy Alloys,” Scientific Reports 4, Article number: 6200 doi: 10.1038/srep06200
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Special 2013 issue of Entropy on High Entropy Alloys
“Pistons and Engine Testing”, MAHLE GmbH (Ed.) 2012