We have sintered ultrafine-grained tungsten powders using microwave radiation. This has the advantage that it delivers energy uniformly throughout the green compact at exceptionally fast rates, and eliminates the problem of non-uniform shrinkage. Rods of tungsten up to 6” in length and 1.5” in diameter, have been sintered to high densities in less than 30 minutes using this approach. When microwave sintering is combined with a post sinter HIP, near theoretical densities are achieved with a grain size of ~ 2 micron. Although the grain size achieved in the phase pure material by microwave sintering of ultrafine-grained powders was much smaller than what has been obtained to date by conventional processing, further reduction of the sintered grain size was achieved by modifying the composition of the starting material. A secondary phase was introduced during the powder synthesis stage to “pin” the grain boundaries, and microwave sintering was used to reduce the densification time. Further, due to the reduced grain growth during sintering, the sintering temperature has been reduced to as low as 1450 oC. We have adopted a novel approach where a small amount of a second phase, oxides of refractory metals such as Y and Hf, is introduced at the molecular level, thereby accomplishing the best possible state of dispersion. The primary particle size of as synthesized tungsten powder containing secondary phases was reduced to 27 nm compared to 124 nm for phase pure tungsten powder. The final grain size of as sintered tungsten composites was ~ 0.5 microns.

Acknowledgment: The work described here was sponsored by two components of the US Army: Army Research Laboratory, Aberdeen Proving Ground, and Picatinny Arsenal, New Jersey