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Development of Ductile, Bulk Tungsten for Next Generation Munitions and Warheads

Description:

OBJECTIVE: To increase the ductility of bulk tungsten and develop a scalable manufacturing method for production. DESCRIPTION: Tungsten is an attractive material for military use due to its very high density (19.3 g/cc), melting point (3410C), strength, corrosion resistance, and benign environmental impact. It is mainly limited in use due to its brittleness (low ductility) and subsequently high ductile-to-brittle transition temperature (DBTT) range. The temperature where tungsten usually transitions from brittle behavior to ductile behavior is between 280C and 330C. The reason for the brittleness of tungsten at room temperature can be largely attributed to grain size, grain boundaries, and impurities. Past efforts to increase the ductility of tungsten have focused on alloying, grain refinement, extreme working, area reductions (dislocation densities), impurity reductions, and heat treatments. For example, traditional alloying of tungsten with nickel, iron, and cobalt can produce a strong ductile material, but at the sacrifice of density. Ductile tungsten currently exists in wire form through working and area reduction (i.e. filaments), but this does not lend itself practically to military applications where large bulk sizes are needed. One of the most successful and significant methods of lowering the DBTT has been by alloying tungsten with Rhenium. However, the high cost of Rhenium makes this method non-ideal. Alternative low cost alloying additions are needed, and the basic mechanisms behind why Rhenium imparts ductility need to be fully explored. Once the W-Re system is understood, materials and processing techniques can be exploited to develop low cost, ductile tungsten that is simple to process by conventional equipment, near the density of pure tungsten, and in sizes which are practical for munitions use. Increasing the quasi-static and high strain rate ductility of tungsten will have multiple military payoffs, allowing for more usage opportunities as well as enhanced performance in legacy applications. Such payoffs include: 1) An alternate EFP material to tantalum at a lower cost with increased performance. Tungsten provides a 2.6 g/cc density increase over tantalum. 2) An alternate shape charge liner material to copper and molybdenum with increased performance. Tungsten provides a 9.02 g/cc density increase over molybdenum and 10.36 g/cc over copper. 3) Alternate material to depleted uranium in kinetic energy penetrators. DU is currently the material of choice regarding KE penetrators, but due to political pressures, environmental, and health concerns, an alternate material must be developed. Developing ductile tungsten will enhance launch survivability. 4) Enhanced performance in small caliber applications against targets at obliquity. Current rounds have ballistic issues upon target impact at unspecified angles. 5) More complex geometries for warhead packages. Machining and forming of less brittle tungsten will allow for more a more widespread integration as a warhead item in legacy munitions. 6) Logistics optimization through the use of a single material in multiple munitions packages. PHASE I: The objective of Phase I is to develop a synthesis and processing technology to fabricate low cost (Rhenium free), bulk, ductile tungsten. Metrics to be met are density = 18.0 g/cc, diameter = 1" & length 4"to 6", 2 deliverable rods, and an elongation = 5% at room temperature. Extensive microstructural, compositional, and mechanical characterization, as well as quasi-static mechanical testing shall be performed as per the applicable ASTM standards. However, high strain rate ductility is the ultimate goal. PHASE II: The objective of Phase II is to further refine and optimize synthesis and processing technology & materials properties to meet Phase II metrics of density =18.0 g/cc, diameter = 4" & length 8"to 10", 5 deliverable rods, and an elongation =10% at room temperature. Characterization shall be performed as outlined in Phase I. PHASE III: The objective of Phase III is to transition the fabrication technology to produce large quantities of optimized ductile tungsten from Phase II. The metrics to be met are a price increase no greater than 10% over the cost of commercial tungsten. Included military applications that will benefit from the technology developed are shape charge liners, explosively formed penetrators, kinetic energy penetrators, and small caliber munitions. Commercially, ductile tungsten can be utilized in mining, aerospace, and counterweight applications. REFERENCES: 1) Lassner, E., & Schubert, W. (1999). Tungsten; properties, chemistry, technology of the element, alloys, and chemical compounds. New York, NY: Plenum Publishers. 2) Zhang, Y., Ganeev, A. V., Wang, J. T., Liu, J. Q., & Alexandrov, I. V. (2009). Observations on the ductile-to-brittle transition in ultrafine-grained tungsten of commercial purity. Materials Science and Engineering A, 37-40. 3) Mathaudhu, S. N., deRosset, A. J., Hartwig, K. T., & Kecskes, L. J. (2009). Microstructures and recrystallization behavior of severley hot-deformed tungsten. Materials Science and Engineering A, 28-31. 4) Lavernia, E. J., Xiong, Y., & Liu, D. (2011). Study on ductile-brittle transition temperature of tungsten alloys. ARDEC private communication
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