Effect of Aluminum Content on Plasma-Nitrided Al x CoCrCuFeNi High-Entropy Alloys
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INTRODUCTION
SURFACE hardening by means of plasma nitriding[1–3] has been successfully 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 nitrided difficultly by conventional nitriding techniques.[1] 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 by ion and energetic particle bombardment, and thus nitrogen mass transfer from the plasma into the component subsurface can be achieved effectively. In view of the preceding discussion, 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. By definition, HEAs may contain at least five principal metallic elements, each of which has a concentration between 5 and 35 at. pct.[4] Based on previous research,[4–10] HEAs have four core effects[6] largely influencing their microstructures and properties, which are much less pronounced in traditional alloys based on one or two major metal elements. (1) High-entropy effect: High mixing entropy could effectively simplify the microstructures that tend to WEI-YEH TANG, Ph.D. Student, and JIEN-WEI YEH, Professor, 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 July 22, 2008. Article published online April 2, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
consist of simple solid solution phases such as fcc or bcc structures instead of complicated intermetallic compounds. (2) Sluggish diffusion effect: Sluggish diffusion of atoms in the multiprincipal-element matrix lowers the phase transformation rate and enhances nanoprecipitation and nanocrystalline or even amorphous structures. (3) Severe-lattice-distortion effect: Severe lattice distortion due to atomic size difference among all different elements further influences mechanical, physical, and chemical properties. (4) Cocktail effect: Solid solution phase with multiprincipal elements has an atomic-scale composite effect on properties, wherein the interactions among different elements themselves gives an extra effect besides the rule of mixture. The HEAs with suitable alloy design have been found to provide versatile properties such as high hardness and superior resistance to temper softening, wear, oxidation, and corrosion; thus, they are promising for a number of applications such as tools, molds, dies, and mechanical parts.[4–10] Although this new field is still in the beginning stage, it has triggered at least 15 global research groups and 60 published papers. Among the developed HEA systems, AlxCoCrCuFeNi alloys have been reported to have a gradual structure change from
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