Ductility in Tungsten: A Treatise on Processing, Characterization, and Analysis of Plastically Deformed Material

Abstract

The goal of this work was to increase the room temperature ductility of polycrystalline tungsten to over 10% tensile elongation. In conjunction with this, the objective goals were to determine the underlying deformation mechanisms and the microstructure influences that alter the mechanical behavior.

In this work, centimeter diameter polycrystalline tungsten rods were plastically deformed to strains between 1.15-4.6 through equal channel angular extrusion (ECAE) at 320^oC. Microstructure characterization was done by optical and scanning electron microscopy; the texture was analyzed by X-ray diffraction. Mechanical behavior was characterized through Vickers hardness measurements and three-point bend testing. From the three-point bend test yield strength, ultimate flexural strength and fracture energy were determined. Failure mechanisms were analyzed by examination of through fracture surfaces and crack paths on specimens following failure.

The results show that bulk polycrystalline tungsten can be processed to strain >4 at 320^oC by ECAE at slow strain rates. This processing with ECAE produced bulk material with a ductility of ~19% at room temperature, while increasing the strength and fracture energy far above those of the as received material.

Through comparisons of fracture energy results from fracture toughness data on single crystals, it was determined that room temperature ductility is due to the orientation of the {110} planes along the principal stress direction. Microstructure refinement and grain elongation produced by ECAE undoubtedly contribute to the strength and improved fracture resistance. Severe plastic deformation processing also altered the mode of crack propagation from intergranular in the as received material from along subgrain boundaries to transgranular with increased grain boundary delamination.

This work shows that it is possible to plastically deform polycrystalline tungsten at relatively low temperatures to large amounts of strain, and that this processing beneficially affects the mechanical behavior by increasing the ductility, strength and fracture energy.