Microstrain characteristics of isostatically hot-pressed beryllium

Abstract

Polycrystalline beryllium specimens were fabricated from powders by using significantly higher pressures and lower temperatures than are normally employed for commercially-compacted beryllium. The procedures involved encasement of a partially-compacted powder in a collapsible steel container; the container was externally isostatically pressurized by (a) hot gas (helium), or (b) a solid pressure-transmitting medium (steel or Al 2O 3, etc.) enclosed in a cavity and acted upon by the ram of a forging press. A finer initial powder-particle size and distribution were found to result in a smaller as-fabricated grain size. Microyield strength of as-fabricated, low-oxide beryllium decreased with decreasing grain size. However, upon heat treating at 924 °C for 30 min after pressing, microyield strength increased with decreasing grain size. Microyield strength also increased with decreased processing temperature, increased beryllium oxide content, and small alloying additions of chromium or silver. Flow stress of isostatically pressed beryllium was found to be a parabolic function of microstrain. The flow curves were characterized by three distinct ranges of microstrain, viz., 1. (1) heterogeneous basal glide at low microstrains, 2. (2) dispersion hardening, most probably by a dislocation pile-up mechanism at intermediate levels of microstrain, and 3. (3) prismatic glide and cross-slip at microstrains greater than 50 × 10 −6. Microflow behavior of isoforged beryllium was a sensitive function of the time held at pressure during final consolidation, probably due to the effect of pressure on the solubility of precipitates.