Tungsten single crystal sheet.

Abstract

With the rapid advances in growing large single crystals from commercial purity tungsten powder using the Arc-Verneuil technique, fabrication of single crystal sheet becomes more of a reality, owing to progress in our understanding of the associated anisotropic substructural changes that occur during rolling. Starting single crystal billet sizes of 1J X f X 12 in. already have been produced, with the potential final size unlimited because of the unique method of traversing the hot plasma source both reciprocally and vertically from the seed. Compared to polycrystalline material tungsten, sheet fabricated from single crystals has immediate advantages because of the absence of grain boundaries. Single crystal sheet can be rolled at temperatures between 700° and 1000°C; it is free of delaniination; it possesses strength levels comparable to polycrystalline tungsten sheet and still has as much as 14% elongation at room temperature; it has the bend, ductile-to-brittle, transition temperature that is below room temperature; and it can be welded without affecting the monocrystalline nature of the crystal. Because of the anisotropic nature of single crystals, rolling planes and directions must be considered when selecting the orientation that best favors processing. This results in directional mechanical properties of the sheet which might be disadvantageous for some applications. Although still on a laboratory scale, the possibility of advancing the technology to produce large-size sheet looks promising.