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High-entropy alloys (HEAs) are a new class of multi-component alloys composed of 5 or more principal constituent elements each with a concentration between 5 and 35 atomic %. Research indicates that HEAs can be considerably lighter, with a higher degree of fracture resistance, tensile strength and hardness, corrosion and oxidation resistance than conventional alloys as well as have exceptional high-temperature strength and structural stability. Although HEAs have existed since before 2004, research interest has been renewed due to research performed in 2014 at North Carolina State University, and as a team with members from DOE's Lawrence Berkeley and Oak Ridge National Laboratories.

High entropy effects

HEAs are so named because of the high configuration entropy exhibited by such alloys. The resultant substance can become a simple solid solution due to its high configurational entropy. The higher entropy of mixing in these alloys facilitates the formation of solid solute phases with simple structures (such as face-centered cubic or body-centered cubic) and thus reduces the number of phases to one phase at the right temperatures.

Further, it has been shown that diffusion is limited in these systems, allowing for more easily attained supersaturated states and nano-sized precipitates.

Production

High entropy alloys are mostly produced using distinct methods that depend on the initial phase - starting either from a liquid, solid, or gas state.

• Melt casting
  • The component metals (with purities higher than 99.9%) are melted using vacuum arc remelting and cast into billets. For elements with a low melting point and are easy to evaporate (e.g. Mg, Zn, and Mn), the arc-melting process may not be the best choice.
• Mechanical alloying (high energy ball milling) of solids
  • High purity (> 99.5% pure) metallic powders with particle size below 45 microns are mixed in equiatomic composition and milled in a planetary ball-mill for 10 to 100 hours in an inert atmosphere and then sintered by spark plasma sintering.
• Mixing elements from gas state
  • Sputtering or molecular beam epitaxy (MBE) are used to carefully control different elemental compositions to get high entropy metallic or ceramic films.

Properties and potential uses

HEAs, because of their high form-ability and strength combined with low density, are expected to replace superalloys in energy sectors and aero-space applications. Because HEAs are a cocktail of metallic elements, a wide range of materials can be produced which can serve future requirements at a lower cost with superior mechanical properties.

Recent research has also indicated that magnetic properties of high entropy alloys could also be promising.

References

1. ^ Tsai, Ming-Hung; Yeh, Jien-Wei (2014). "High-Entropy Alloys: A Critical Review" (Free PDF download). Materials Research Letters 2 (3): 107. doi:10.1080/21663831.2014.912690.
2. ^ Zhang, Yong; Zuo, TingTing; Tang, Zhi; Gao, M.C.; Dahmen, K.A.; Liaw, P.K.; Lu, Z.P. (1 November 2013). "Microstructure and properties of high-entropy alloys". Progress in Materials Science 61: 1–93. doi:10.1016/j.pmatsci.2013.10.001.
3. Huang KH, Yeh JW. A study on multicomponent alloy systems containing equal-mole elements . Hsinchu: National Tsing Hua University; 1996.
4. Yeh JW, Chen SK, Lin SJ, Gan JY, Chin TS, Shun TT, Tsau CH, Chang SY. Nanostructured high-entropy alloys with multiple principal elements: novel alloy design concepts and outcomes. Adv Eng Mater. 2004;6:299–303. doi: 10.1002/adem.200300567
5. Shipman, Matt (10 December 2014). "New 'high-entropy' alloy is as light as aluminum, as strong as titanium alloys". Phys.org. Retrieved 9 January 2015.
6. Lavine, Marc S. (2014). "A metal alloy that is stronger when cold". Science 345: 1131. Bibcode:2014Sci...345Q1131L. doi:10.1126/science.345.6201.1131-b. Retrieved 9 January 2015.
7. Lyn (4 September 2014). "A metallic alloy that is tough and ductile at cryogenic temperatures". Berkeley Lab News Center (University of California, Berkeley). Retrieved 9 January 2015.
8. Gludovatz, B.; Hohenwarter, A.; Catoor, D.; Chang, E. H.; George, E. P.; Ritchie, R. O. (5 September 2014). "A fracture-resistant high-entropy alloy for cryogenic applications" (Free PDF download). Science (AAAS) 345 (6201): 1153–1158. Bibcode:2014Sci...345.1153G.doi:10.1126/science.1254581. PMID 25190791. Retrieved 9 January 2015.
9. Singh S, Wanderka N, Murty BS, Glatzel U, Banhart J. Decomposition in multi-component AlCoCrCuFeNi high-entropy alloy. Acta Mater. 2011;59: 182–190. doi: 10.1016/j.actamat.2010.09.023
10. Youssef, Khaled M.; Zaddach, Alexander J.; Niu, Changning; Irving, Douglas L.; Koch, Carl C. (9 December 2014). "A Novel Low-Density, High-Hardness, High-entropy Alloy..." (Free PDF download). Materials Research Letters: 1. doi:10.1080/21663831.2014.985855.
11. Ji, Wei; Wang, Weimin; Wang, Hao; Zhang, Jinyong; Wang, Yucheng; Zhang, Fan; Fu, Zhengyi (January 2015). "Alloying behavior and novel properties of CoCrFeNiMn high-entropy alloy fabricated by mechanical alloying and spark plasma sintering".Intermetallics 56: 24–27. doi:10.1016/j.intermet.2014.08.008.
12. Zhang et.al (Scientific Reports Volume: 3 Published: MAR 15 2013,DOI: 10.1038/srep01455)

See also

• Amorphous metal
• Nanocrystalline material
• Hume-Rothery rules

• Materials science
• Chemical physics
• Thermodynamic entropy
• Statistical mechanics