The mechanical behavior of beryllium at high pressure

Abstract

The pressure dependence of the flow and fracture behavior of polycrystalline beryllium has been studied at room temperature and constant strain rate. Specimens were strained in tension under constant hydrostatic pressures up to 10 kilobars (~150,000 psi) and the true stress and strain obtained from direct measurements of load and specimen cross section during the tests. The structural changes accompanying deformation and fracture were examined by both optical and transmission electron microscopy. Two types of well characterized beryllium—ingot and powder metallurgy—were studied in order to determine the influence of metallurgical variables. The yield behavior is found to be unaffected by pressure, whereas the fracture stress and strain increase with increasing pressure. At the higher pressures, plastic instability (necking) develops with 3-dimensional ductility and leads to area strains at fracture as high as 70%. Deformation mechanisms observed to operate, in addition to basal and prism slip, are non-basal slip and a new phenomenon, micro-kinking. An increase in strain hardening rate observed for the ingot Be under pressure is shown to be associated with the formation of observed sessile vacancy loops during plastic straining. The dominant fracture mode of the ingot Be is transgranular cleavage. For the pm Be, fracture is predominantly intergranular up to 6 kilobars and then becomes transgranular with a corresponding increase in the pressure dependence of fracture stress. These differences are shown to be associated with the presence of oxide particles at the grain boundaries in the pm material. The measured pressure dependence of fracture stress compared with that predicted from current theory points to a possible effect of pressure and/or strain on the effective surface energy for crack propagation. The theory is not in good agreement with the observed incidence of micro-cracks and the pressure dependence of fracture at low pressures. The observations on pm Be indicate the need to incorporate the intergranular fracture mode in any general theory of the pressure dependence of fracture.