| Popis: |
Graphitic bodies that contain a fine dispersion of zirconium or molybdenum carbide particles and that are hot-worked at temperatures high enough so that the carbide phase is in the liquid state have been found to possess good density, impermeability, and strength. The carbide-bearing bodies, originally prepared by molding, are deformed in compression at temperatures above the carbide-graphite eutectic temperature. The excess carbide is squeezed from the free surface of the body, which results in a predominantly graphitic body with properties approaching those of the pyrolytic graphites. The initial carbide content of 10 vol.% is reduced to about 6 vol.% when hot-worked to true strain levels of the order of 60 per cent. The initial stage of hot-working is rapid and corresponds to the effective elimination of open porosity and gas permeability. The second stage (beyond about 30 per cent strain) is slower, but the preferred orientation, thermal expansivity, and strength continue to change appreciably with increasing strain. The microstructure consists of fine carbide particles, generally less than 5 μ, distributed uniformly in an oriented graphite matrix whose structure differs from that of conventional molded or hot-worked graphites. Molybdenum-bearing bodies show the highest strengths with moduli of rupture in excess of 20,000 psi. |
