Effects of Si and Ni powder refining on the formation mechanism of nickel silicides induced by mechanical alloying
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INTRODUCTION
MECHANICAL alloying (MA) has been found to be a versitle technique, which allows production of a broad range of intermetallic materials.[1–11] In a variety of highly exothermic reaction mixtures, combustion reactions have been, in general, initiated by ball milling. The major advantages of this processing method are higher energy efficiency, higher productivity, and shorter processing cycle times than with conventional reaction processes.[12] In our previous study, MA of mixtures of elemental Ni and Si with high-Cr hardened steel balls was carried out using a Spex mill and a cylindrical, partially stabilized zirconia (PSZ) vial.[13,14] The total MA times (tT) for the complete formation of Ni2Si, Ni5Si2, and NiSi have been found to be 120, 360, and 720 minutes, respectively, which were significantly longer than those for other metal silicides.[13–17] This reason was mainly attributed to the relatively lower negative value of the heat of formation of the product involved. The formation mechanism of nickel silicides via MA has been, thus, concluded to be a self-propagating hightemperature synthesis (SHS) reaction in the early period of milling, followed by a slow solid-state interface diffusion reaction. However, milling times for the complete transformation to stochiometric nickel silicide without the coexistence of elemental powders via MA have been found, in the current work, to be significantly decreased by introducing premilled, as-received elemental powders. Therefore, the present article illustrates the effects of Si W.H. LEE, Professor, formerly with the School of Metallurgical and Materials Engineering, Kookmin University, Seoul 133-791, Korea, is now with the Department of Advanced Materials Engineering, Sejong University, Seoul 143-747 Korea. C.S. BYUN, Research Scientist, and S.H. LEE and D.K. KIM, Professors, are with the Department of Materials Engineering, Taejon National University of Technology, Taejon 330-717, Korea. Manuscript submitted May 19, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
and Ni powder refining on the formation mechanism of nickel silicides via MA. Experimental studies were designed to achieve a better conversion of reactants to the product via MA by investigating the effects of the size and shape of starting elemental Ni and Si powders on the energy efficiency, productivity, and processing cycle time for the formation of Ni2Si, Ni5Si2, and NiSi. II. EXPERIMENTAL PROCEDURE The experimental work on MA of mixtures of elemental Ni and Si powders was carried out using a Spex 8000D mixer/mill (SPEX Industries, Inc.) and a cylindrical, PSZ vial (38-mm i.d. and 52-mm long). Elemental powders and high-Cr hardened steel balls (12.7 and 6.4 mm in diameter) were placed in the vial in an Ar-flushed glove box to avoid oxidation. The mean powder particle size of the as-received Ni powder was 48.8 m, with a spherical shape; that of the as-received Si powder was and 19.1 m, with an angular shape. The purity of the powders was better than 99.95 pct. Each as-received Ni and Si pow
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