P. 0. Box 2008 Oak Ridge, Tennessee

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1 ALTERNATIVE PROCESSING METHODS FOR TUNGSTENBASE COMPOSITEMATERIALS E. K. Ohriner and V. K. Sikka Metals and Ceramics Division Oak Ridge National Laboratory P. 0. Box 2008 Oak Ridge, Tennessee Tungsten composite materials contain large amounts of tungsten distributed in a continuous matrix phase. Current commercial materials include the tungsten-nickel-iron with cobalt replacing some or all of the iron, and also tungsten-copper materials. Typically, these are fabricated by liquidphase sintering of blended powders. Liquid-phase sinteering offers the advantages of low nprocessing costs, established technology, and generally attractive mechanical properties. However, liquid-phase sintering is restricted to a very limited number of matrix alloying elements and a limited range of tungsten and alloying compositions. In the past few years, there has been interest in a wider range of matrix materials that offer the potential for superior composite properties. These must be processed by solid-state processes and at sufficiently low temperatures to avoid undesired reactions between the tungsten and the matrix phase. These processes, in order of decreasing process temperature requirements, include hotisostatic pressing (HIPing), hot extrusion, and dynamic compaction. The W i n g and hot extrusion processes have also been used to improve mechanical properties of conventional liquidphase-sintered materials. The results of laboratory-scale investigations of solid-state consolidation of a variety of matrix materials, including titanium, hafnium, nickel aluminide, and steels are reviewed. The potential advantages and disadvantages of each of the possible alternative consolidation processes are identified. Postconsolidation processing to control microstructure and macrostructure is discussed, including novel methods of controlling microstructure alignment. INTRODUCTION Tungsten composite materials such as tungsten-nickel-iron and tungsten-coppermaterials have been successful because they have useful properties and are readily fabricated by low-cost sintering processing. There is interest in developing high-density tungsten composites with properties not achieved by these existing materials. This now requires that new types of binder phases be considered. Most of these cannot be sintered due to undesirable interactions between the tungsten and binder phases at suitable sintering temperatures. There is interest in other methods of consolidation that are not limited by elevated-temperature phase equilibria considerations. The consolidation methods that have been used include hot-isostatic pressing (HIPing), hot extrusion,

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3 and dynamic compaction. The uses and relative advantages and disadvantages of these possibie alternative consolidation processes are discussed. Sinteriu Liquid-phase sintering is the most important commercial method used to fabricate tungsten composites. The advantages of this process include: low processing cost, the use of relatively simple equipment, and the ability to obtain near-net-shape components. The compositionsfor which liquid-phase sintering is appropriate are limited by restrictions imposed by equilibrium phase diagrams. In the case of the common tungsten sintered with a binder consisting of a mixture of nickel, iron, and in some cases, cobalt, the nickel content of the binder addition must be about 50% to avoid the formation of brittle intermetallics. This can be extended to as low as 35% by quenching [ 11. Compositions in the tungsten-nickel-manganese system can also be liquid-phase sintered [2]. The tungstencopper and tungsten-nickel-copper systems are also well established systems for liquid-phase sintering. More recently, the intermetallics of Ni3Al and Fe3Al and alloys of the two have been shown to be amenable to liquid-phase sintering [3]. Solid-state sintering has many of the same restrictions on binder compositions as does liquid-phase sintering. It does allow for the use of lower tungsten contents which cannot be adequately processed in the liquid phase due to gravitational effects such as slumping and macrosegregation due to density differences. In some cases, notably copper binders, this can be overcome by the use of liquid-phase infiltration of a sintered-tungsten skeleton. Hot-Isostatic Pressing HIPing has been used for consolidation of composites of tungsten with both nickel-base [4,5] and copper [6] binder materials. Considerations include the fraction of the tungsten and binder phases as well as the processing temperatures, pressures, and times. Some of the nonconventional binder materials have also been processed by HIPing. HIping of tungsten with a hafnium binder presents problems with interactions to form intermetallic as well as with the porosity that results from this interaction [A.In another study of W-l5% Hf, cracking in the hot-isostatically pressed compacts was attributed to residual stresses from the HIP can [8]. Extrusion Hot extrusion of canned powder has been used for consolidation of a range of tungsten composite materials. A composite of tungsten with a steel binder has been consolidated by extrusion at 1ooo"Cwith minimal interaction of the components [9]. Tungsten-hafnium composite materials have also been consolidated at temperatures in the range of 1100 to 1400 C. The low end of the temperature range does limit the formation of intermetallic phases to trace amounts while still achieving complete densification. Elongation of the tungsten phase can be controlled by varying the extrusion ratio as seen in Figures 1 and 2. &Did Densification Me&& Hotexplosive consolidation of W-5wt % Ti has been used to obtain densities of 98 to 99% of the theoretical density [lo]. In this method, an exothermic mixture surrounding the powder sample provides rapid heating, and compaction is achieved using explosive-driven anvils. Heating times prior to compaction on the order of 1 min were needed to approach a reasonably uniform temperature distribution in a 50-mm-dim by 9-mm-thick sample. Issues of particle bonding and nonuniform microstructures and interfaces remain to be addressed.

4 Figure 1. Tungsten-hafnium composite extruded at a ratio of 6:1. Figure 2. Tungsten-hafniumcomposite extruded at a ratio of 40:1.

5 Dynamic magnetic compaction has been used to obtain fully dense tungsten steel composites at a consolidation temperature of 1ooo"C [ref Repeated magnetic pulses generate compressive stresses estimated to be on the order of 300 MPa. Either sintering in hydrogen or HIPing at 1ooo"C was needed to achieve full bonding of the tungsten particles to the steel. Hot-dynamic processing using a solid-pressure transmitting media (Ceracon) has been used to consolidate tungsten powder with a chemically vapor-deposited (CVD)nickel-iron coating. Full density was achieved with specimens of 25 mm d i m by 25 mm length using preheat temperatures in the range of 10o0 to 1500 C. Scale-up to larger sizes required the use of increased tem.perature and resulted in some less desirable microstructures. The Same process was also used to consolidate tungsten powder with a CVD hafnium coating to produce a near fully dense (98%) W-2 wt 96 Hf composite [ 121. Rods of tungsten with 1 vol 96 Ni binder have been produced using explosive consolidation of nickel and tungsten powder in a double-tube shock-loading configuration [ 131. Locally,fully dense material was produced, but greater control of shock pressure is needed to obtain a uniform microstructure even in small sizes. Powder Requiremen g The requirements on particle size and particles mixing are generally more stringent in the solid-state processes than for liquid-phase sintering. The particle size of the binder material has been an issue in many of the studies of tungsten composites. Ideally, the binder particle size is significantly smaller than the tungsten particle size in order to minimize the quantity of tungsten-tungsten particle contacts in the consolidated material. The blending, deagglomeration,and milling of the starting powders are important issues in the preparation of composite materials using solid-state processes, generally more so than for liquid-phase sintering. Coating of tungsten powder has also been employed to address this issue [5,12]. Comparison of Consolidation Metho& The consolidation methods are compared in Table I in the order of decreasing process temperature. In general, the compressive pressures increase as the processing temperature decreases in order to achieve the densification needed. Equilibrium phases present at the sintering temperature can be considered to be present in a liquid-phase-sintered material. Thus, only matrix materials that do not form intermetallics at these temperatures are likely candidates for this process. This would also hold true, in general, for solid-state sintering. In the case of HIPing, it is possible to use lower process temperatures at which the kinetics of reaction of the components are reduced. In the case of extrusion, the use of higher compressive stresses, as well as the introduction of large shear stresses, can produce densification at reduced temperatures and with short hold times at temperature, thus avoiding undesired reactions in the composite. This may be extended even further with dynamic compaction to the extent that lower temperatures and preheat times can be used. Costs can generally be expected to increase as process pressures increase due to the increased sophistication of the needed equipment and possibly due to the increased cost of starting materials. STCONSOLIDATION PROCESSING Postconsolidation processing can be used to obtain particular microstructures or properties. Swaging of liquid-phase-sintered materials is conventionally used to increase strength, and more advanced deformation processing has also been developed, which results in large aspect ratios of the tungsten particles within the composite [ 141. This has also been demonstrated by re-extrusion of tungsten-steel and tungsten-hafnium composites consolidated by extrusion initially [9]. Deformation processing can produce composites with pronounced deformation or recrystallization textures. 4

6 Table I. Comparison of Advantages and Disadvantages of Consolidation Processes for Tungsten Composites Process Liquid-Phase Sintering Solid-state Sintering Hot-Isostatic Pressing Extrusion Dynamic Compaction Process Ternperam Very High High Moderate LOW LOW to Very High Matrix Materials Very Few Few Some May Most cost Low Low Moderate Moderate High Scale-up Constraints Minor Minor Minor Minor Major In addition to the control of microstructure, novel macrostructures have also been produced. Tungstencopper and tungsten-nickel macrocompositeshave been produced by brazing of foils [15]. Evaluations showed some benefit from thinner layers, raising issues of fabrication from very thin foils as well as braze-joint integrity and thickness. Rolling of liquid-phase-sintered tungsten-nickel-iron composite was employed to produce an elongated tungsten phase that was ballistically tested at orientations of 0,45, and 90" [ref. la]. Although no significant effect was found, it is suggested that in other tungsten composite materials, with greater difference in the flow stress of the tungsten and binder phases, an effect of alignment may be important. An alternative method of alignment uses extrusion to obtain an axisymmetric alignment of the microsmcture that may give more favorable deformation characteristics. A longitudinal cross section of such an extruded bar of a tungsten-hafnium composite is shown in Figure 3. The particular nature of the alignment can be controlled by the selection of extrusion parameters. There is a range of methods for consolidating tungsten composite materials. In the general order of increasing applied stress and decreasing process temperature, these methods include liquid-phase sintering, solid-state sintering, HIPing, hot extrusion, and a number of dynamic consolidation methods. The lower temperature processes axe frequently suitable to materials that would form undesirable reaction products at the higher process temperatures. Postconsolidation processing offers the potential to control microstructureand macrostructure alignment. ACKNOWLEDGMENTS The authors thank D. Kapoor, U.S. Army Armament Research and Development Engineering Center, for his encouragement and useful discussions; G. Mackiewicz-Ludtka and S. C. Deevi for reviewing the paper; K. Spence for editing; and M.L. AtchIey for preparing the manuscript. This research was performed at the Oak Ridge National Laboratory which is managed by Lockheed Martin Energy Systems under contract DE-AC-840R21400 with the U.S. Department of Energy.

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