High- and medium-entropy alloys (HEAs/MEAs), also called as multicomponent alloys, are a new class of materials that break through the traditional alloy design concept based on single principal element. However, they do not break away from the magic spell of strength–ductility trade-off. Therefore, designing HEAs/MEAs with both high strength and high ductility still remains a great challenge nowadays. This article provides a review on the recent progress in mechanical properties of face-centered cubic (FCC) HEAs/MEAs. First, several traditional strengthening strategies are briefly reviewed, focusing on the strengthening mechanisms and the optimized mechanical properties. Subsequently, various novel strategies for achieving strength–ductility synergy in HEAs/MEAs are summarized, which include lowering the stacking fault energy, regulating the short-range order, promoting transformation-induced plasticity, and constructing heterogeneous microstructures. The basic ideas and related underlying mechanisms from these strategies are discussed. Finally, the current challenges and the future outlooks are emphasized and addressed systematically. In brief, the present review is expected to provide a useful guide for the design of HEAs/MEAs with superior mechanical properties. 

Strategies for achieving strength‐ductility synergy in FCC high‐ and medium‐entropy alloys

Additively manufactured (CoCrNi)94(TiAl)6 medium-entropy alloys (MEAs) were fabricated by laser-directed energy deposition. The strengthening induction mechanism in the additively manufactured (CoCrNi)94(TiAl)6 MEAs was investigated using electron backscatter diffraction and transmission electron microscopy. The results showed that the addition of TiAl powder led to increases of 29.6% and 43.5% in the ultimate tensile strength (UTS) and yield strength (YS), respectively, at 298 K, and a decrease of 17.1% in the elongation. For the samples tested at 77 k after the addition of the TiAl powder, the UTS and YS improved by 26.6% and 28.0%, respectively, and the elongation increased by 26.3%. The microstructural observation results indicated that the matrix grains changed from initial columnar grains with a uniform growth direction to fine dendrites. Numerous TiO clad Al2O3 strengthening nano-precipitates with a novel core–shell structure were found in the additively manufactured (CoCrNi)94(TiAl)6 MEAs, and both featured a face-centered cubic structure. No crystallographic orientation was observed between the nano-precipitates and the matrix. At 298 K, the dislocation line bypassed the nano-precipitates during the plastic deformation period and Lomer–Cottrell locks appeared near the nano-precipitates, thereby strengthening the additively manufactured (CoCrNi)94(TiAl)6 MEAs. At 77 K, plastic deformation resulted in the synergistic deformation of the nano-precipitates and matrix, the formation of a large number of dislocations, and a twinning induced plasticity effect, which further increased the strength and plasticity of the additively manufactured (CoCrNi)94(TiAl)6 MEAs. 

Laser-directed energy deposition of a high performance additively manufactured (CoCrNi)94(TiAl)6 medium-entropy alloy with a novel core-shell structured strengthening phase

Medium-Entropy Alloy (MEA) is a new metal material with high strength, high hardness, high corrosion resistance and excellent high temperature properties. With its excellent performance makes it promising for a wide range of applications in the fields of aerospace, automotive, energy and biomedicine. In addition, the continuous breakthroughs in preparation process have promoted the development of MEA with unique appeal. So far, outstanding results have been achieved in the preparation technology of MEA, and the scientific field of improving the performance of MEA by adjusting the process technology has received much attention. No work has been reported specifically on MEA preparation methods and enhancement mechanisms, and a review is needed to summarize the current state of research in this area. In this paper, we review the research progress of MEA in both preparation process and strengthening mechanism, and introduce in detail the arc melting, induction melting and mechanical alloying and deposition methods with CoCrNi as the main focus, and analyze the advantages and disadvantages. Finally, the prospects and challenges for the development of optimally designed MEA are presented. 

Review on the preparation methods and strengthening mechanisms of medium-entropy alloys with CoCrNi as the main focus

Recently, medium and high-entropy alloys with heterogeneous features have exhibited excellent synergistic effects of strength and toughness. In this work, we have produced a series of cast CrCoNi medium entropy alloys with various heterogeneous microstructures through Al and Ti elements micro-alloying. The heterogeneous microstructure and deformation mechanism are systematically analyzed. The result showed that Al/Ti ratio ranging from 0.5 to 2 has a significant influence on the microstructure and mechanical properties of the CrCoNi-based alloy. The present findings provided efficient guidance for developing of microstructure tailoring method in high and medium-entropy alloys. 

Synergistic effect of Al and Ti micro-alloying in CoCrNi-based medium entropy alloy

High-pressure and high-temperature induced densely discontinuous nanoprecipitates in multi-principle element alloy

A novel high-strength and high-ductility CoCrNi medium entropy alloy by Ti/Mo co-doping

In this research, a class of Co-free Fe 3 Cr 2 NiAl x (x = 0–1.1) multi-principle-element alloys (MPEAs) with ultrafine/nano precipitates and serration behavior were fabricated. By varying Al element levels, the microstructure changed from a dual-phase structure of face-centered-cubic (FCC) plus body-centered-cubic (BCC) to a tri-phase structure of plate-like FCC, BCC and a uniformly dispersed nano-sized ordered B2 precipitates within BCC, then transformed to another dual-phase structure of BCC and the dual-morphology ultrafine/nano B2 precipitates within BCC. The increasing Al content resulted in a significant improvement in nanohardness (∼8.01 ± 0.32 GPa) and compressive yield strength (∼1284 ± 35 MPa) with keeping a large ductility (>60%), attributed to the uniform distribution of dual-morphology coherent B2 precipitates. The quantitative contributions of different strengthening mechanisms have been estimated in Fe 3 Cr 2 NiAl 1.1 MPEA. A notable phenomenon has been discovered that Fe 3 Cr 2 NiAl x (x = 0.1–0.3) MPEAs showed the serration behavior near the yielding point of compressive stress-strain curves. In the compressive experiments with three different strain rates, as Al content increased, the serration type evolved from type B + C to type B. Serration behavior on stress-strain curves of alloys, impedes the engineering applications of alloys. It is critical to understand serration behavior of MPEAs in phenomenon and theory. This work has broader applicability to understand the effect of Al content on the microstructure and mechanical deformation behavior in MPEAs. 

On the characteristic of ultrafine/nano precipitates and serration behavior in an Al-doped Fe3Cr2Ni multi-principle-element alloy

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A novel high-strength and high-ductility CoCrNi medium entropy alloy by Ti/Mo co-doping

On the characteristic of ultrafine/nano precipitates and serration behavior in an Al-doped Fe3Cr2Ni multi-principle-element alloy