Quantitative Analysis of Heterogeneous Microstructure and Diversified Strengthening Mechanisms in Spark Plasma Sintered
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ing mechanical consolidation into in situ synthetic reactions, spark plasma sintering (SPS) is a promising process to produce fully dense bulk material of intermetallics.[1] To date, the literatures reporting the SPS have demonstrated its adaptability for fabricating monolithic molybdenum disilicide (MoSi2) from elemental Mo and Si powder blend.[2,3] Compared with the traditional processes such as solid-state reaction,[4] hotisostatic pressing,[5] and combustion synthesis,[6] the SPS is characterized by a very high heating rate up to ~1273 K (1000 C) per minute, which greatly improved the costeffectiveness, but inevitably caused a significant temperature gradient leading to a heterogeneous microstructure.[7,8] By now, the microstructure development of MoSi2 has been studied by X-ray diffraction.[3] Nevertheless, very few works have been conducted to model, in a quantitative way, the inhomogeneous characteristics of microstructure and the resulting diversity of strengthenQIAODAN HU, Associate Professor, and JIANGUO LI, Professor, are with School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. PENG LUO, formerly with the Defence Materials Technology Centre, Department of Mechanical Engineering, University of Melbourne, VIC 3010, Australia, is now Researcher, with the School of Mechanical Engineering, Shanghai Dian Ji University, Shanghai 201306, P.R. China. Contact e-mail: [email protected] YOUWEI YAN, Professor, is with the School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P.R. China. Manuscript submitted March 26, 2014. Article published online February 11, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
ing mechanisms in MoSi2. Motivated by an improved understanding of the microstructure vs properties relationship in fully dense bulk MoSi2 produced by SPS, electron backscatter diffraction (EBSD) was used in this study to reveal grain structure and phase composition. The strength property of MoSi2 was evaluated by the Vickers micro-hardness (HV) testing. Elemental powders of Mo and Si (purity ~99.0 pct and lot number ~200, Sinopharm Chemical Reagent) were dry-mixed for 4 hours with a mass ratio of ball to powder at ~8:1. Subsequently, 10 g of the mixed powders were charged into a graphite die with the cavity of sizes Ø20 9 40 mm, and heated to 1673 K (1400 C) under the circumstance of vacuum by an SPS apparatus (Sumitomo Coal Mining 1050, with a DC pulsed pattern at 12:2). Moreover, mechanical consolidation was simultaneously undertaken with the application of a uniaxial pressure ~30 MPa retained for the period of 15 minutes. The sample for metallographic characterization was mechanically ground along its transverse section using the diamond pastes of ~3 and ~1 lm, respectively, then polished in colloidal silica suspensions (~0.05 lm) by a VibroMet Buehler polisher at 220 VAC 50 Hz. EBSD characterization was carried out in a field-emission scanning electron microscope (FEGSEM, FEI Sirion NOVA NanoSEM 230 at
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