2016 ANNUAL REVIEW Ministry of Science and Technology

(4) Toppling a long-held myth with the development of high-entropy alloys A research team from the Department of Materials Science and Engineering, National Tsinghua University has toppled a longstanding myth with the development of high-entropy alloys, which have become a new field in materials science. The team's results have been published as the special report "Stronger, tougher, and more ductile alloys made from multiple metal types" in Vol. 533, no. 7603 of Nature , and this work has brought credit to Taiwan. Since ancient times, almost all conventional alloys have contained one chief metallic element and small additive quantities of other elements, which improve the alloy's properties. For instance, carbon steel refers to steel to which 0.02-2% carbon has been added, and aluminum alloy contains at least 80% aluminum. This approach to alloy composition has long been limited by the belief-now revealed as a myth-that when a greater content of alloying elements is added, large amounts of brittle compounds will form, and the alloy will be brittle and useless. However, the high-entropy alloys that toppled this myth contain five or more chief metallic elements, and each chief element has an atomic percentage in the range of 5%-35%. The multiple chief elements in these alloys give them high entropy, which can inhibit the formation of compounds and promote the mutual fusion of different elements, yielding alloys that are strong and tough. For instance, the CrMnFeCoNi alloy discussed in the foregoing report has a greater low- temperature strength-toughness combination than traditional materials. While there are approximately 30 types of conventional alloys containing a single chief element, there are vastly more types of high-entropy alloy systems; if 13 metallic elements are available, and five elements, six elements, and so on up to 13 elements are used together, a total of 7,099 combinations are possible: C 5 + C 6 + C 7 + C 8 + C 9 + C 10 + C 11 + C 12 + C 13 =7,099 13 13 13 13 13 13 13 13 13 This would therefore allow the creation of 7,099 alloy systems, and each system includes alloys with equal and unequal atomic ratios, such as CuCoNiCrAlFe and CuCo0.5Ni1.2CrAlFe1.3. High- entropy alloys possess great application potential, and many applications in which conventional materials cannot be satisfactorily used provide high-entropy alloys with an opportunity to shine. Such applications include abrasion-resistant, heat-resistant lathe blades and coatings, jet engine blades resistant to extremely high temperatures, structural materials for use in next- generation nuclear reactors that are resistant to a temperature of 900 ℃ and radiation damage, films with an extremely low resistance temperature coefficient, room temperature superconductors, and anti-adhesion films. Numbers and percentage of one- and multi-year specific-topic research projects in engineering, 2012-2016 Year All projects One-year projects (%) Multi-year projects (%) 2012 7,428 4,115 （ 55.40 ） 3,313 （ 44.60 ） 2013 6,886 3,753 （ 54.50 ） 3,133 （ 45.50 ） 2014 6,740 3,673 （ 54.50 ） 3,067 （ 45.50 ） 2015 6,390 3,418 （ 53.49 ） 2,972 （ 46.51 ） 2016 6,231 3,393 （ 54.45 ） 2,838 （ 45.55 ） Source: High-entropy materials laboratory, Department of Materials Science and Engineering, National Tsinghua University Products made from high entropy alloys MOST Ministry of Science and Technology Ministry of Science and Technology 36 2016 ANNUAL REVIEW