Microstructure characterization of Al x CoCrCuFeNi high-entropy alloy system with multiprincipal elements
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I. INTRODUCTION
FOR thousands of years, the development of practical alloy systems has been based mainly on one principal element as the matrix, such as iron-, copper-, and aluminumbased alloys, limiting the number of applicable alloy systems.[1,2] Even though a substantial amount of other elements is incorporated for property/processing enhancement, superalloys with higher elevated-temperature strengths and better thermal resistance, widely developed since the 1930s, are still based on one principal element such as nickel.[3] Since the 1970s, intermetallic compounds of Ti-Al, Ni-Al, and Fe-Al binary systems have attracted much attention because of their extremely high specific strengths and thermal resistance.[4] However, these new compounds with enhanced performance are just based on two principal metallic elements. In the last two decades, many researchers have explored a wide range of bulk amorphous alloys, including Pd-, Ln-, Zr-, Fe-, Mg-based, etc. alloys.[5–13] The design concept of multicomponent bulk amorphous alloys was, once again, based on one principal element. Some other metallic glasses with multicomponents have been prepared by melt spinning; however, they are still based on one principal group of transition metals.[14–17] The main reason for not incorporating multiprincipal elements into alloy preparation is the anticipated formation of CHUNG-JIN TONG, Postgraduate Student, YU-LIANG CHEN, Postdoctoral Candidate, and JIEN-WEI YEH and SU-JIEN LIN, Professors, are with the Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan. Contact e-mail: [email protected] SWE-KAI CHEN, Professor, is with the Materials Science Center, National Tsing Hua University. TAO-TSUNG SHUN, Researcher, is with the Materials Research Laboratory, Industrial Technology Research Institute, Chutung 310, Taiwan. CHUN-HUEI TSAU, Associate Professor, is with the Institute of Materials Science and Manufacturing, Chinese Culture University, Taipei 111, Taiwan. SHOU-YI CHANG, Assistant Professor, is with the Department of Materials Engineering, National Chung Hsing University, Taichung 402, Taiwan. Manuscript submitted July 2, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
many intermetallic compounds and complex microstructures.[18] Brittleness of the alloys and also difficulty in processing and analysis are expected with the compound formation and complex microstructures, which discourage other new alloy designs with multiprincipal elements. However, solid solutions with multiprincipal elements will tend to be more stable at elevated temperatures because of their large mixing entropies. Following Boltzmann’s hypothesis on the relationship between the entropy and system complexity, the change in configurational entropy during the formation of a solid solution from three elements with an equimolar ratio is already larger than the entropy changes for fusion of most metals.[19] Consequently, alloys containing a higher number of principal elements will more easily yield the formation
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