On sinterability of nanostructured W produced by high-energy ball milling
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K.S. Kumar Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India
B.S. Murty Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Madras, Chennai 600036, India
B. Sarma Defense Metallurgical Research Laboratory, Hyderabad, India
S.K. Pabia) Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India (Received 31 May 2006; accepted 18 September 2006)
The present investigation reports for the first time a dramatic decrease in the sintering temperature of elemental W from the conventional temperature of 艌2500 °C to the modest temperature range of 1700–1790 °C by making the W powder nanostructured through high-energy mechanical milling (MM) prior to sintering. The crystallite size of the initial W powder charge with a particle size of 3–4 m could be brought down to 8 nm by MM for 5 h in WC grinding media. Further milling resulted in a high level of WC contamination, which apparently was due to work hardening and the grain refinement of W. A sintered density as high as 97.4% was achieved by sintering cold, isostatically pressed nanocrystalline (8 nm) W powder at 1790 °C for 900 min. The microstructure of the sintered rods showed the presence of deformation bands, but no cracks, within a large number of W grains. The mechanical properties, when compared with the hardness and elastic modulus, of the sintered nano-W specimen were somewhat superior to those reported for the conventional sintered W.
I. INTRODUCTION
Tungsten can be an excellent high-temperature structural material for applications in temperatures up to 2480 °C because of its attractive properties, compared to a high melting point of 3420 °C, a density of 19.3 g/ml, a hardness of 9.75 GPa, an elastic modulus of 407 GPa, a low coefficient of thermal expansion, good thermal conductivity, and low vapor pressure.1 However, the consolidation of a conventional microcrystalline W powder with an average particle size of 3–4 m is difficult,2–5 and normally electrical resistance sintering under hydrogen atmosphere at temperatures of 2500 °C or more is used for this purpose.4–8 The process is expensive and
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0166 1200 J. Mater. Res., Vol. 22, No. 5, May 2007 http://journals.cambridge.org Downloaded: 17 Mar 2015
usually only simple shapes like filaments, rods, or bars are sintered by this method. Several studies have shown that the onset of sintering can take place at a significantly lower temperature in the case of nanoparticles compared to conventional microcrystalline powders.9–15 For instance, the sintering of both metals and ceramic nanoparticles was found to start at temperatures of 0.2–0.4 Tm (Tm ⳱ melting temperature) compared to 0.5–0.8 Tm for the conventional powders. It is believed that in the case of nanostructured powders the grain boundary sliding, dislocation motion, grain rotation, and viscous flow can sig
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