Initial-stage Sintering Kinetics of Nanocrystalline Tungsten
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materials show enhanced diffusion and, hence, result in lower sintering temperature and time in comparison with their microcrystalline counterparts.[1,2] Nanocrystalline tungsten has been sintered at a low temperature of 1973 K (1700 C) with 95 pct theoretical density in comparison with a high conventional sintering temperature (‡2773 K (2500 C)) for microcrystalline tungsten.[3] Recently, Wang et al.[4] reported as low a sintering temperature as 1373 K (1100 C) for nanocrystalline tungsten powders in hydrogen atmosphere without pressure, which was further reduced to 1273 K (1000 C) using spark plasma sintering (SPS).[5] Nanocrystalline tungsten powder of submicron sizes was also used to produce near fully dense (above 99.7 pct theoretical density) tungsten heavy alloy.[6] Tungsten is the most used high-temperature refractory metal in various applications, retention of the nanocrystalline/ultrafine grain (UFG) structure after sintering is a concern, and several processing strategies using newly developed consolidation techniques (e.g., SPS) have been reported earlier.[7–9] Nano-grained tungsten (~50 nm) with 91 pct theoretical density was produced by Zhou et al.[10] using resistance sintering under ultra-high pressure. Recently, finegrained tungsten heavy alloy were reported by Li et al.[11] using SPS.
AJEET K. SRIVASTAV, Research Scholar, and B.S. MURTY, Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India. Contact e-mail: [email protected] M. SANKARANARAYANA, Scientist, is with the Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500 058, India. Manuscript submitted February 18, 2011. Article published online November 9, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
There is a need to understand the densification mechanism in nanocrystalline tungsten powder during sintering to get the near fully dense sintered nanocrystalline/UFG tungsten. Trusov et al.[12] investigated the initial-stage sintering behavior of ultrafine metallic powders. The reported activation energy (134 kJ/mol) for tungsten powder of 40-nm average particle size is lower than that for surface (~300 kJ/mol) and volume diffusion (~580 kJ/mol). Thus, surface and volume diffusion cannot explain such behavior. The low activation energy is an indication that mechanisms other than surface and volume diffusion are operative during initial-stage sintering of nanopowders, where agglomeration can play a critical role.[13,14] In contrast to nanopowders, grain boundary diffusion may play a major role in densification of high-energy ball milled nanocrystalline tungsten as a result of the nonequilibrium nature of grain boundaries.[15] Non-isothermal sintering (constant heating rate) proposed by Young and Cutler[16] is the most widely used method to study the initial-stage sintering kinetics of metals and ceramics. The present investigation deals with the study of initial-stage sintering kinetics of high-energy ball milled nanocrystalline tungsten using dilatom
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