Microstructural Stability of As-Cast and Directionally Solidified AlCoCrFeNi 2.1 Eutectic High-Entropy Alloys at Elevate
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TRODUCTION
SINCE 2004, high-entropy alloys (HEAs), composed of more than four principal elements in equal or near-equal atomic percentage, have attracted extensive research attention because of their unusual structural properties.[1–8] Then, many single-phase FCC or BCC HEAs were exploited. However, it is difficult for the single-phase HEAs to achieve a balance of ductility (FCC) and strength (BCC). Luckily, the eutectic high-entropy alloy (EHEA) proposed by Lu et al., like AlCoCrFeNi2.1 EHEA,[9–11] can balance the ductility and strength because of the mixing effect of dual phases (FCC and BCC). Subsequently, many EHEAs are exploited, such as CoCrFeNiNbx,[12–14] LEI WANG, XINYUAN WU, CHENGLI YAO, YUNPENG ZHANG, YUHUI GE, and GUOJUN ZHANG are with the School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, P.R. China. Contact e-mails: [email protected]; [email protected] JUN SHEN is with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, P.R. China. Contact email: [email protected] Manuscript submitted January 21, 2020. Article published online September 21, 2020 METALLURGICAL AND MATERIALS TRANSACTIONS A
CoCrFeNiTax,[15,16] CoCrFeNiZrx,[17] CoCrFeNi2.0 (Zr0.6, Nb0.74, Hf0.55, Ta0.65)[18] and others.[19–27] Among these EHEAs, the AlCoCrFeNi2.1 EHEA possesses an excellent combination property.[7–11,28–36] In addition, due to the sluggish diffusion kinetics and high softening resistance of HEA at elevated temperatures, the AlCoCrFeNi2.1 EHEA has been regarded as a promising high-temperature structural material. As a high-temperature structural material, AlCoCrFeNi2.1 EHEA needs to be microstructurally stable when it is exposed to high temperatures for long times. In other words, the microstructure like the lamellar structure does not degrade significantly at elevated temperatures. From the point view of thermodynamics, the lamellar structures are not stable because of the great amount of interface energy. Generally, the total interface energy could be decreased by reducing the interfacial area. The interfacial area can be decreased by increasing the average interlamellar spacing, which can be achieved by directional solidification (DS) at slower growth rates. Thus, it is significant to investigate the microstructural stability of as-cast and directionally EHEA at elevated solidified AlCoCrFeNi2.1 temperatures.
VOLUME 51A, NOVEMBER 2020—5781
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EXPERIMENTS
The AlCoCrFeNi2.1 (molar ratio) eutectic high-entropy alloy was prepared by vacuum induction melting. The alloy rods with 7 mm diameter and 105 mm length, cut from the homogenized cast ingot by electro-discharge machining (EDM), were directionally solidified at the withdrawal rates (solidification rates) of 6, 15, 30, 60 and 120 lm/s. The melting temperature was 1660 C ± 10 C. The molten length of specimen was 90 mm. The pulling length of the DS specimen was 80 mm. At the end of the experiment, the crucible was quickly dropped into the liquid Ga-In-Sn alloy to retain t
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