Research

Our Research on High-Entropy Alloys

Why high-entropy alloys? High-entropy alloys (HEAs), or multi-component alloys with equiatomic or close-to-equiatomic compositions, emerge as novel metallic materials with great potential to be used as high temperature materials, or coating materials requiring high hardness and high wear resistance. Although being highly concentrated multi-component alloy systems, simple solid solutions tend to form in HEAs, with the absence of intermetallic compounds (note, this is certainly composition dependent, and it by no means indicate intermetallic compounds cannot form in HEAs). This seemingly unexpected phenomenon is partly due to the significant entropic contribution, which lowers the Gibbs energy of the solid solution phases at elevated temperatures. 

What we do on HEAs? Normally, the formed solid solutions in HEAs are of fcc and/or bcc structures. The mechanical behaviour of HEAs are greatly affected by the phase constitutions in that generally fcc-typed HEAs are ductile but soft, while the bcc-typed HEAs are hard but brittle. How to simultaneously achieve both high strength and high tensile ductility in HEAs is a great challenge in the field, and this constitutes one of the main interests of this research group. Alloying strategies for HEAs and their deformation mechanisms at both room temperature and elevated temperatures, will be systematically studied to gain an in-depth understanding for this relatively new material. Our target is to develop HEAs as new generation of structural materials for high-temperature applications. From time to time (this is more or less funding oriented), we also work on applying HEAs as functional materials, such as thermoelectric materials or bio-materials. We are also quite keen on the idea of using HEAs as alternative binder in hard metals and also novel coating materials. We hope to be able to share with you some exciting news coming out from this research group. 

At the moment, we mainly work on two types of HEAs, eutectic HEAs (EHEAs) and refractory HEAs (RHEAs), which can be reflected from our recent publications in these two areas. We are also getting into the field of additive manufacturing (AM), using AM not only as a new processing route, but also as a way to tune the micro-structure and phase constitutions.

Some of the PAST research topics in this research group can be summarized as follows. More information on our ongoing research projects can be seen in the publications from this group. 

1) Phase selection rules in multicomponent alloys with equiatomic or close-to-equiatomic compositions

References:

(introductory paper) Phase selection rules for complex multi-component alloys with equiatomic or close-to-equiatomic compositions, Chinese Journal of Nature, 35(2), 85-96, 2013, http://nature.shu.edu.cn/EN/abstract/abstract13304.shtml

(amorphous phase or solid solution phase?) Phase stability in high entropy alloys: Formation of solid-solution phase or amorphous phase, Progress in Natural Science: Materials International, 21(6), 433–446, 2011, http://www.sciencedirect.com/science/article/pii/S100200711260080X

(amorphous or intermetallic compounds?) More than entropy in high-entropy alloys: Forming solid solutions or amorphous phase, Intermetallics, 41, 96-103, 2013, http://www.sciencedirect.com/science/article/pii/S0966979513001404

(fcc or bcc solid solution?) Effect of valence electron concentration on stability of fcc or bcc phase in high entropy alloys, J. Appl. Phys. 109, 103505, 2011, http://dx.doi.org/10.1063/1.3587228

2) Meta-stability of the solid solution phases in HEAs

References:

（thermo-mechanical treatments and thermodynamics calculations) Entropy-driven phase stability and slow diffusion kinetics in an Al0.5CoCrCuFeNi high entropy alloy,Intermetallics, 31,165–172, 2012, http://www.sciencedirect.com/science/article/pii/S0966979512002531; 

Phase stability and tensile properties of Co-free Al0.5CrCuFeNi2 high-entropy alloys,  Journal of Alloys and Compounds, 584, 2014, 530, http://www.sciencedirect.com/science/article/pii/S0925838813022688

3) Unique solidification behavior

References:

(eutectic structures) Anomalous solidification microstructures in Co-free AlxCrCuFeNi2high-entropy alloys, Journal of Alloys and Compounds, 557, 77-81, 2013, http://www.sciencedirect.com/science/article/pii/S0925838813000303; 

Sunflower-like solidification structure in a near-eutectic high-entropy alloy, Materials Research Letters, 1(4), 2013, 228, http://www.tandfonline.com/doi/full/10.1080/21663831.2013.844737#.Ul094FB_N8E

4) Mechanical behavior

References:

(micro-compression) Micromechanical characterization of casting-induced inhomogeneity in an Al0.8CoCrCuFeNi high-entropy alloy, Scripta Materialia, 64(9), 868–871, 2011, http://www.sciencedirect.com/science/article/pii/S1359646211000212

(tension) Phase stability and tensile properties of Co-free Al0.5CrCuFeNi2 high-entropy alloys,  Journal of Alloys and Compounds, 584, 2014, 530, http://www.sciencedirect.com/science/article/pii/S0925838813022688

5) Solid solutioning behavior

References:

(lattice distortion & Vegard's law) Solid solutioning in equiatomic alloys: Limit set by topological instability, Journal of Alloys and Compounds (letter), 583, 2014, 410, http://www.sciencedirect.com/science/article/pii/S0925838813021002