Electron metallography of powder beryllium — a hot-pressing mechanism and the role of oxygen impurities

Abstract

The application of electron microscopy to the study of hot-pressing beryllium powder is described. Microstructural examination revealed that the principal impurity in consolidated metal was beryllium oxide which was introduced into the material as a surface layer on the original powder particles. The oxide in the compact, although distributed as a network of fine precipitates surrounding relatively precipitate-free regions, was not concentrated at the final grain boundaries. As the oxide was found to be stable in beryllium, behaving as a form of reference marker, it was deduced that this network was the remains of the oxide present on the surface of the powder particles. Clusters of very small grains were observed in material pressed at low temperatures, indicating that nucleation takes place in the initial stages of compaction, the formation of nuclei being related to the highly deformed contact regions between projections on the particle surfaces. As consolidation proceeds, the grains have grown and the final compact contains a grain structure superimposed on, but not directly related to, the original oxide network. It is therefoie concluded that the hot-pressing process is analogous to hot-working, with definable stages of nucleation and recrystallisation taking place, and that consolidation has been facilitated by promotion of the bulk flow characteristics during the working. The improved strength of beryllium of powder origin, as compared with metal fabricated from cast ingot, is ascribed mainly to a finer grain size, the oxide having a beneficial effect in minimising grain growth. It is shown that ~ 0.2 wt % oxygen is sufficient to maintain a thermally stable structure up to temperatures ~ 10 °C below the melting point of beryllium, oxide levels in excess of this being of little further benefit with respect to grain size control. It is also concluded that the distribution of oxide precipitates has by no means been optimised to achieve maximum lattice hardening and since the limited solubility found for oxygen in beryllium appears to preclude the possibility of modifying the precipitate dissemination by thermal treatment, it is suggested that recycling the powder may be advantageous.