High-entropy alloys: An interview with Jien-Wei Yeh
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High-entropy alloys: An interview with Jien-Wei Yeh www.mrs.org/fall2016 Interviewed by N. Balasubramanian
For millennia, metallic alloys were produced with one or two dominant metals to which other elements were added—such as carbon to iron, or tin to copper—in the required quantities to obtain targeted properties. Jien-Wei Yeh, a professor at the National Tsing Hua University, Hsinchu, Taiwan, thought of making alloys with five or more metals in equal (or nearly equal) proportions so that there were many principal elements. He called them high-entropy alloys (HEAs) since they owe their stability to high-mixing entropy, which increases with the number of elements. Counterintuitively, some of the HEAs form a single phase with no intermetallic compounds (which are often brittle). Thus, CoCrFeMnNi forms a face-centered-cubic (fcc) phase, although Co is hexagonal; Cr and Fe, body-centered cubic (bcc); Ni, fcc; and Mn has a complex crystal structure. CoCrFeMnAl forms a bcc structure, although Al is fcc. HEAs have interesting mechanical properties—high strength and ductility, unusual temperature dependence of strength-magnetic properties, irradiation resistance, and many functional properties. The field has grown from the first article published in 2003 to six in 2004, to nearly 1000 journal articles to date. To Yeh, ideas are more important than awards, but his work has been recognized by the Industry-University Cooperation Award of Education Ministry in 2000, Ho-Chin-Tue Award of Metallurgy by Tung-Ho Steel Company in 2003, Academic Excellence Awards of National Tsing Hua University, Academic Research Awards (first class) of the National Science Council of Taiwan, Fellowship of the Materials Research Society-Taiwan in 2015, and honorary membership of the Indian Institute of Metals in 2016. Yeh shares his insights with the readers of MRS Bulletin in an interview with N. Balasubramanian during a conference visit in Bangalore, India. This interview is published as a curtain raiser for the 2016 MRS Fall Meeting symposium on HEAs, beginning November 27, in Boston.
MRS BULLETIN: How did you think of multicomponent alloys? J.W. YEH: Before the creation of “highentropy alloys,” I had engaged in the development of new routes to improve metallic materials in my studies at the National Tsing Hua University. The first was using thermomechanical treatment on 7000 series Al alloys for grain refining, strengthening, and toughening in my master’s thesis. I concentrated on the toughening mechanisms in 7000 series alloys in my PhD research. Then I invented a twin-disk method to produce Al flakes at a cooling rate of about 106~9 K/s when I was an associate professor. However, I was not satisfied because it still needed degassing, compacting, consolidating, and wrought processing for the flaky powder to produce bulk Al alloys. The
complicated process not only increased the cost, but also downgraded the original merits of the fine microstructure and properties of rapidly solidified powders. After this, I invented a reciprocating extrusion
