Designing High Entropy Alloys with Dual fcc and bcc Solid-Solution Phases: Structures and Mechanical Properties
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INTRODUCTION
ALLOY design in the past was limited to either one principal element (e.g., the Ni-based superalloys) or two (e.g., the TiAl-based superalloys). Their properties were optimized by a minor addition of other elements. In 2004, a new concept of alloy design based on multiprincipal elements was introduced independently.[1,2] Cantor et al.[1] prepared the alloys with up to 20 elements in equal atom fraction and reported the CoCrFeNiMn alloy with a single fcc solid-solution phase. In contrast to the general understanding of binary and ternary phase diagrams, in which many intermetallic compounds are formed within the center, Yeh et al.[2] pointed out that the high mixing entropy of alloys with multiprincipal elements might make the solid-solution phases more stable. A high entropy alloy (HEA) was then defined as an alloy containing at least five principal elements, each of which has a composition range between 5 and 35 at. pct and possesses a high
ZHAOWU TANG, SHANG ZHANG, RUIPENG CAI, QING ZHOU, and HAIFENG WANG are with the State Key Laboratory of Solidification Processing, Center of Advanced Lubrication and Seal Materials, Northwestern Polytechnical University, Xi’an, 710072 Shaanxi, P.R. China. Contact email: [email protected] Manuscript submitted October 8, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
mixing entropy at their liquid state or high-temperature solid-solution state.[3] Nowadays, HEAs play an important role in alloy design, ascribing to their superb properties such as excellent fracture resistance at cryogenic temperatures,[4] exceptional combination of strength and ductility,[5,6] and outstanding wear resistance.[7] Although the terminology of such alloys is under debate (e.g., HEAs[3] followed by the current work, multiprincipal alloys,[1] or complex concentrated alloys[8]), studies have become focused on the center region of phase diagrams where uncountable possible combinations of four, five, six, or even greater numbers of elements are available[9] to obtain potential alloys with desired properties. In this sense, the classical trial and error method becomes challenging because a large number of experimental trials are needed to find appropriate HEAs.[10] This is the reason why designing HEAs becomes important.[8,11–13] According to Gao et al.,[13] there are three main approaches for designing HEAs, i.e., the empirical parameters,[14–39] the CALPHAD method,[9,40–42] and the first-principle calculations.[43–49] The empirical parameters can provide some simple design rules. Their reliability depends on the parameters themselves and the data collection of reported HEAs. The CALPHAD method is the most effective way for calculating the phase diagrams of HEAs, as long as reliable thermodynamic databases are available. The extrapolation of binary and ternary phase diagrams into high order
systems, however, is not reliable. Furthermore, there are no thermodynamic databases for all the reported HEAs. The first-principle calculations are able to predict phase equilibria with the atomic
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