High Entropy Alloys: From Bulk Metallic Materials to Nanoparticles
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NTRODUCTION
HIGH entropy alloys (HEAs) with multiple elements were proposed in 2004 by breaking through the traditional alloy design strategies.[1] The HEAs have attracted attention recently because of their extraordinary behaviors.[2–5] The FCC HEAs showed unusual mechanical behavior with low stacking fault energy,[2] while the BCC HEAs exhibited excellent high-temperature mechanical properties.[3] Beyond the mechanical properties, the functional properties of HEAs were also attractive, such as the superconductivity[4] and good radiation resistance.[5] The concept of HEAs was initiated by the metallurgists of structural materials. After that, various high entropy materials, including high entropy ceramics[6] and high entropy thermoelectric materials,[7] were successfully generalized and explored. However, most of the previous investigations on HEAs focused on the bulk metallic materials. On the other hand, nanoparticles have been proved to perform excellently in catalysis, chemical sensing, and biomedicine.[8] Recently, Chen et al.[9] proposed the concept of ‘‘polyelemental nanoparticle libraries’’ referring to nanoparticles composed of up to five elements. It is
QINGFENG WU, ZHIJUN WANG, FENG HE, JIE LUO, JUNJIE LI, and JINCHENG WANG are with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, P.R. China. Contact e-mails: [email protected], [email protected] LILIN WANG is with the School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, P.R. China. Manuscript submitted March 13, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
believed that the physical and chemical properties of this kind of nanoparticle can be tuned by mixing different elements purposefully. Besides, the performance of polyelemental nanoparticles can be further enhanced compared with binary particles in terms of catalysis[10] and plasmonics.[11] Therefore, it is important to fabricate the bulk HEAs into nanoparticles. In 2015, Singh et al.[12] synthesized NiFeCrCuCo HEA nanoparticles using a wet chemical synthesis technique for the first time. However, due to the tremendous differences in reaction kinetics of different metal precursors, it is very difficult for different atoms to coalesce into a single particle through this method.[13] Therefore, it is hard to extend this fabrication method to different high entropy systems. Chen et al.[9] also prepared HEA nanoparticles using the scanning-probe block copolymer lithography method, but their equipment is extremely expensive and the nanoparticles were phase separated because of the near equilibrium reaction. Recently, Yao et al.[14] fabricated HEA nanoparticles containing up to eight elements using the carbothermal shock synthesis method. The proposed method conquers the phase separation in the nanoparticles by rapid cooling after thermally shocking the metal salt mixture, implementing the chemical synthesis in extreme conditions. Besides the chemical synthesis, the spark discharge is an effective physical method
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