Microstructures and Mechanical Performance of Plasma-Nitrided Al 0.3 CrFe 1.5 MnNi 0.5 High-Entropy Alloys

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TRODUCTION

CONVENTIONAL alloys such as Fe, Al, Cu, Ti, Mg, and Ni-based alloys are based on one principal element.[1,2] Intermetallic compounds such as Ti-Al, Ni-Al, and Fe-Al compounds, developed from the 1970s, are based on two principal elements.[3] To activate a new alloy field, Yeh et al.[4] and Ranganathan[5] proposed the concept of ‘‘high-entropy alloys’’ (HEAs),[4,5] which has attracted considerable research.[4–18] By definition, an HEA has at least five principal metallic elements, each at a concentration between 5 and 35 at. pct. Based on previous research, Yeh proposed that the microstructure and properties of HEAs are affected by four effects more pronouncedly than conventional alloys:[7] (1) high entropy—it enhances the formation of simple solid solution phases, such as fcc or bcc structures, and thus simplifies the microstructures; (2) severe lattice distortion—it arises from the atomic size difference among different elements in solid solution phases and might markedly influence mechanical, physical, and chemical properties; (3) sluggish diffusion—it is due to the inefficient cooperative diffusion of various species and thus slows down phase WEI-YEH TANG and MING-HAO CHUANG, PhD Students, SU-JIEN LIN and JIEN-WEI YEH, Professors, are with the Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan R.O.C. Contact e-mail: [email protected] Manuscript submitted April 28, 2009. Article published online April 3, 2012 2390—VOLUME 43A, JULY 2012

transformations; and (4) cocktail—it comes from the ideal mixing and inevitably excessive interactions among the multiprincipal elements in solid solution phases, and thus has a composite effect on properties. HEAs were found to have versatile properties, which might be unique for their potential applications such as tools, molds, dies, and mechanical parts.[4,6–12] Plasma nitriding[19–21] is one of the successful surface hardening techniques used to improve the wear resistance, fatigue life, and corrosion resistance of parts such as cams, gears, or molds without affecting the softer but tougher interior of the parts. Particularly, plasma nitriding is used to deal with the materials that are difficult to nitride by conventional nitriding techniques.[19] Due to the sputtering effect of the positive ions in the glow discharge, the protective oxide film formed on the surfaces of such materials as stainless steels, aluminum alloys, and titanium alloys can be effectively removed, and thus nitrogen atoms can be effectively transferred from the plasma into the component subsurface. With this merit, plasma nitriding is expected to be useful to treat high-entropy alloys (HEAs) containing a large amount of strong oxide forming elements such as aluminum, chromium, silicon, and titanium. Among developed HEA systems,[4–17] AlxCoCrCuFeNi (in mole ratio) alloys exhibit a gradual change from fcc phase to bcc phase and a corresponding increase in hardness from Hv 120 to 650 as the aluminum content increases from x = 0 to 3.0. They have potential uses in str

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