Precipitation-hardened high-entropy alloys for high-temperature applications: A critical review
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ntroduction The concept of high-entropy alloys (HEAs) was initially proposed independently by Yeh1 and Cantor,2 abandoning traditional single-principal-element alloy systems and adopting a multicomponent design strategy with chemical element concentration for each element ranging between 5 and 35 at.%. In the early stages of developing HEAs, it was widely accepted that the formation of a simple solid-solution phase is preferred rather than complex intermetallic compounds, owing to the increased configurational entropy with the increased number of chemical components. For example, the prototypical equiatomic CoCrFeMnNi “Cantor alloy”2 is a face-centered-cubic (fcc) solid-solution phase, which exhibits excellent ductility, but limited strength.3,4 Second-phase strengthening, as a traditional approach for strength enhancement, is widely used among Ni-based superalloys,5 Al alloys,6 Mg alloys,7 and high-performance steels.8 Precipitation strengthening is also effective for HEAs.9–18 Though the formation of a second phase deviates from the original intention to retain a single solid-solution phase in HEAs, second-phase strengthening leads to unambiguous properties enhancement, which has become a research hotspot at the present time.
Various precipitate phases have been explored and studied so far, such as the L12-type phase (γ′),9–11 σ and μ phases,13,14 Laves phase,15 B2 phase,16,17 and carbides.18 Ni-based superalloys made use of the L12-type Ni3(Al,Ti) phase, which provides high-temperature strength due to its stability at high temperatures.19 Molybdenum contributed to the formation of topologically close-packed (TCP) phases (σ and μ phases), whose hardnesses are extremely high, up to 15 GPa.13 Niobium addition into CoCrFeNi, as an example, induced the formation of a Nb-rich Laves phase, resulting in an increase in yield strength from 147 MPa for CoCrFeNi to 637 MPa for CoCrFeNiNb0.412.15 In Al0.7CoCrFeNi HEA, a high ultimate tensile strength of 1223 MPa can be attributed to the CsCl-type B2 precipitates embedded in the disordered bodycentered-cubic (bcc) matrix.16 It should also be noted that the carbide volume fraction increased as the carbon content increased, leading to a significant strength enhancement.18 The L12-type precipitation-hardened HEAs (PH-HEAs), in particular, exhibit a promising outlook for high-temperature applications. The addition of Al and Ti atoms into single-phase NiCoFeCr HEA can induce the formation of coherent L12 precipitation with a dramatic strength enhancement.10 The crystal structure of
Boxuan Cao, Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong; [email protected] Tao Yang, Department of Mechanical Engineering, City University of Hong Kong, Hong Kong; [email protected] Wei-hong Liu, Department of Materials Science and Engineering, Harbin Institute of Technology, China; [email protected] C.T. Liu, Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong; [email protected] doi:10.1557/mrs.2019
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