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REVIEW This section of Journal of Materials Research is reserved for papers that are reviews of literature in a given area.

L12-strengthened high-entropy alloys for advanced structural applications Tao Yang, Yilu Zhao, Weihong Liu, Jijung Kai, and Chaintsuan Liua) Center for Advanced Structural Materials, College of Science and Engineering, City University of Hong Kong, Hong Kong, China (Received 6 March 2018; accepted 18 May 2018)

Advanced alloys with both high strength and ductility are highly desirable for a wide range of engineering applications. Conventional alloy design strategies based on the single-principle element are approaching their limits in further optimization of their performances. Precipitation-hardened high-entropy alloys (HEAs), especially those strengthened by coherent L12-nanoparticles, have received considerable interest in recent years, enabling a new space for the development of advanced structural materials with superior mechanical properties. In this review, we highlight recent important advances of the newly developed L12-strengthened HEAs, including the aspects of computation-aided alloy design, unique properties, atomiclevel characterization, phase evolution, and stability. In particular, we focus our attention on elucidating fundamental scientific issues involving the alloying effects, precipitation behaviors, mechanical performances, and the corresponding deformation mechanisms, all of which provide a comprehensive metallurgical understanding and guidance for the design of this new class of HEAs. Finally, future research directions and prospects are also critically assessed.

I. INTRODUCTION

From the beginning of our civilization, especially in modern society, high-performance structural materials with combined high strength and large ductility are pursued urgently for improving engineering reliability and energy efficiency. However, most conventional alloys designed based on one single-principle element gradually show a serious embrittlement effect with increasing strength, following the long-standing dilemma of strength-ductility trade-off.1,2 Multicomponent highentropy alloys (HEAs), emerging as a new class of metallic materials, have been proposed recently.3,4 HEAs were typically defined as those alloys having at least five major metallic elements each having an at.% between 5 and 35 at.%. Based on the entropy concept, the socalled HEAs are also generally defined as alloys having a high configurational entropy (DSmix) larger than 1.5R (calculated by DSmix 5
R(XA ln XA 1 XB ln XB 1. . ., where R is the gas constant, XA means the molar fraction of constituent A in the whole alloy, XB means the molar fraction of constituent B, and so on). By breaking through the traditional “single-element” base idea, the compositionally complex HEAs greatly widen the alloy design space for tailoring phase structure, stacking-fault energy, a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.186 J. Mater. Res., 2018

and associated deformation mechanisms i