Powder-Route Synthesis and Mechanical Testing of Ultrafine Grain Tungsten Alloys
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INTRODUCTION
ULTRAFINE grain tungsten’s high density, compressive strength, and propensity for shear banding make it an attractive material for penetration applications, although the amount of available data supporting its unique mechanical properties remains limited due to processing constraints.[1,2] One major constraint has been the limited dimensions of W specimens whose grain sizes are refined by severe plastic deformation techniques such as wire drawing and high-pressure torsion.[3–7] For example, Wei and coworkers used highpressure torsion to prepare a W specimen whose grain size was refined to about 170 nm, but this technique is limited to specimen geometries of ~1 mm thickness and ~10 mm diameter.[7,8] Much higher aspect ratio specimens are required for traditional ballistic testing.[9] Furthermore, severe plastic deformation techniques (e.g., cold rolling and equal-channel angular extrusion) are capable of yielding larger specimens, but generally have produced W specimens with coarser ultimate grain sizes (>500 nm).[6,10,11] It is thus of interest to synthesize ultrafine grain W specimens using more readily scalable methods, for example based on powder processing.
ZACHARY C. CORDERO and MANSOO PARK, Graduate Students, and CHRISTOPHER A. SCHUH, Professor and Department Head, are with the Department of Materials Science and Engineering, MIT, Cambridge, MA 02139. Contact e-mail: schuh@ mit.edu EMILY L. HUSKINS, Postdoctoral Fellow, is with the Oak Ridge Institute for Science and Education Postdoctoral Fellowship Program, Army Research Laboratory, Aberdeen Proving Ground, MD 21005. STEVEN LIVERS, Graduate Student, and MEGAN FRARY, Associate Professor, are with the Department of Materials Science & Engineering, Boise State University, Boise, ID 83725. BRIAN E. SCHUSTER, Mechanical Engineer, Team Leader, is with the Experimental and Computational Penetration Mechanics Team, Weapons and Materials Research Directorate, Army Research Laboratory. Manuscript submitted November 6, 2013. Article published online April 4, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
Previous efforts to synthesize ultrafine grain W articles using powder processing have generally found that pressure-assisted sintering and alloying are necessary for achieving high relative densities without grain growth.[6,12–19] Hot isostatic pressing, for example, was used to consolidate 50 nm W particles to >95 pct relative density between 1093 K and 1193 K (820 C and 920 C).[20] Since these temperatures are below the grain growth onset temperature of unalloyed tungsten [1273 K to 1373 K (1000 C to 1100 C)], grain growth was suppressed, and the compacts retained grain sizes as small as 150 nm.[21] Attempts to replicate these results using other pressure-assisted sintering techniques such as field-assisted sintering (FAS) used lower stresses; whereas the hot isostatic pressing study used an isostatic pressure of 1 GPa, the maximum uniaxial stress reported in FAS studies is 266 MPa.[6,16–19] Consequently, these FAS studies generally required higher soa
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