Formation and Stability of Equiatomic and Nonequiatomic Nanocrystalline CuNiCoZnAlTi High-Entropy Alloys by Mechanical A
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INTRODUCTION
RESEARCH on high-entropy alloys (HEAs) began with the work of Yeh et al. in 2004[1] and became an interesting field that promised materials with high strength, good thermal stability, high wear, and corrosion resistance at room temperature as well as at high temperature. So far, various techniques have been adopted to synthesize HEAs, such as vacuum arc melting,[2–8] rapid solidification,[9] coating,[10–12] and mechanical alloying (MA).[13,14] Other than MA, in all other processes, the authors reported segregation and precipitations along with simple crystal structures, which are common while handling elements with a wide range of melting points. MA is a high-energy ballmilling process that can lead to stable microstructures with better homogeneity than other nonequilibrium processing techniques. The extension of solid solubility with good homogeneity and room-temperature processing are the main advantages of MA over the casting route, especially with multicomponent systems with large differences in the melting points.[15,16] Moreover, MA also can yield a nanocrystalline structure, which is useful to improve the mechanical properties in these alloys. In the current work, an attempt was made to investigate the phase formation and stability of the mechanically alloyed nanocrystalline Cu-Ni-Co-Zn-Al-Ti S. VARALAKSHMI, Doctoral Student, and M. KAMARAJ and B.S. MURTY, Professors, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Madras, Chennai 600036, India. Contact e-mail: [email protected] Manuscript submitted October 23, 2009. Article published online July 1, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
equiatomic and nonequiatomic HEAs. The configurational entropy is maximum at equiatomic composition, and a significant decrease in the high-entropy effect is expected when we deviate from the equiatomic composition.[17] Therefore, the current study has been carried out to investigate the influence of deviation on equiatomic composition on structure formation during MA and its thermal stability in HEAs.
II.
EXPERIMENTAL DETAILS
The elemental powder blends of Cu, Ni, Co, Zn, Al, and Ti have been mechanically alloyed in a high-energy ball mill (Pulverisette-P5; Fritsch GmbH, IdarOberstein, Germany) with equiatomic (binary CuNi to hexanary CuNiCoZnAlTi) and nonequiatomic (CuxNiCoZnAlTi; x = 0 to 50 at. pct) compositions. All elemental powders are of 325 mesh (
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