Structural Evolution of Nanocrystalline Nickel-Tungsten Alloys Upon Mechanical Alloying with Subsequent Annealing
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INTRODUCTION
Ni-W alloys are recognized as compounds with such desirable properties as good mechanical strength, attractive wear and oxidation resistance, and catalytic activities.[1,2] These alloys are nowadays considered not only as environmentally safe alternatives to hard chromium plating processing of which requires environmentally hazardous solutions,[3] but also as attractive substitutes for electrodeposited nickel in the fabrication of micro (MEMS) and nano (NEMS) electromechanical systems. Moreover, the tribological properties of Ni-W alloys make them superb materials for use in sliding contacts such as microgears in MEMS, mold inserts, etc.[4,5] The alloys can also be utilized as the substrates in high-temperature superconductors.[1] Many efforts have been devoted to the fabrication of Ni-W alloys by means of melting-based methods[6–10] or
MAHBOOBEH NAZARIAN-SAMANI, formerly Graduate Student with the Department of Materials Science and Engineering, University of Semnan, Semnan 35131-19111, Iran, is now Researcher with the Faculty of Mechanical Engineering, K.N. Toosi University of Technology (KNTU), Tehran 19395-1999, Iran. ROOHALLAH MOBARRA, Researcher, is with the Faculty of Mechanical Engineering, K.N. Toosi University of Technology (KNTU). ALI REZA KAMALI, Research Fellow, is with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, U.K. MASOUD NAZARIAN-SAMANI, Senior Researcher, is with the Faculty of Mechanical Engineering, K.N. Toosi University of Technology (KNTU), and also with the School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran. Contact e-mails: [email protected], [email protected] Manuscript submitted December 18, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
thin/thick films.[5,11–15] Powder metallurgical processes, especially mechanical alloying (MA), have the additional advantage of giving rise to a good combination of strength and ductility.[15–20] MA appears to be a very promising technique as it offers the opportunity not only for microstructural refinement, but also for acquiring materials of high structural homogeneity. Depending principally on the composition and/or experimental circumstances, various phases including an amorphous phase, intermetallic compounds, or stable/ unstable nanocrystalline supersaturated solid solutions could be produced during the Ni-W MA or annealing processes. Aning et al.[20] investigated a wide variety of Ni-W alloys containing 15 to 50 pct W* to find out that *All compositions are in atomic percent, unless indicated.
there is a critical limit for amorphization in these alloys. They also evaluated the influence of milling media (Al2O3 and steel milling tools) and contamination on the progress of various phases upon MA. While most previous studies of Ni-W alloys have concentrated on the W-rich products of the binary Ni-W alloying system,[18,19,21–24] a clear understanding is yet to be gained of the amorphization and subsequent crystallizatio
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