Densification and structural change of mechanically alloyed W-Cu composites
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INTRODUCTION
IN recent years, the use of W-Cu composite heavy alloys has increased in both commercial and military applications. Most heavy alloys consist of W particles embedded in a matrix of other metals such as iron (Fe), nickel (Ni), or Cu.[1] The W-Cu composites may have potential uses as heat dissipation materials in the microelectronics field,[2] as divertor plates in fusion reactors,[3] or as warhead materials for military ordnance applications. It has been postulated that if high-density W could be combined with ductile Cu, the resultant W-Cu composite may be a more suitable material for shaped-charge liners. The alloying of W and Cu proves to be difficult. The assessed equilibrium phase diagram[4] in Figure 1 reveals that the two metals are completely immiscible in both solid and liquid phases. This limitation excludes the use of conventional alloying techniques to develop full-density W-Cu alloys. Several alternate processing techniques have been examined to develop W-Cu composites.[5] Most commonly, WCu is alloyed through the sintering of finely divided W-Cu powder mixtures. Both solid-state and liquid-phase sintering (SSS and LPS) techniques can be used depending on the composite and its desired characteristics. The SSS of W-Cu may be performed at any temperature, but the LPS of WCu requires a sintering temperature that is at least higher than the melting point of Cu (Tm ⫽ 1358 K). In some cases, when mechanical alloying is used, LPS may occur at lower temperatures. Specifically, as the powder particle size is reduced through welding and fracture during milling,[6,7] the powder structure is often refined to nanoscale. As a result, LASZLO J. KECSKES, BRADLEY R. KLOTZ, KYU C. CHO, and ROBERT J. DOWDING are with the Weapons and Materials Research Directorate, United States Army Research Laboratory, Aberdeen Proving Ground, MD 21005-5069. MATTHEW D. TREXLER, Graduate Student, is with the Materials Science and Engineering Department, Georgia Institute of Technology, Atlanta, Georgia. Manuscript submitted December 27, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
diffusion paths are decreased; this, in turn, reduces the required temperature.[8] The objective of this effort was to mix W and Cu precursors intimately using mechanical alloying and then to sinter the resultant mixture into near-full-density, disk-shaped WCu pellets. The effect of the milling time and the type of medium used on the development of the composite microstructure was of primary concern. Powder mixtures of 80W20Cu wt pct (58W-42Cu at. pct) were ball milled in air or in hexane for times ranging from 0 min to 48 h (2880 min). The milled powders were then cold pressed into green compacts and sintered into dense pellets. Density measurements, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were used in concert to discern the extent and effectiveness of the densification and mechanical alloying of the samples. The precursor and product microstructures were examined by
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