beryllium-based alloys. The industrial use of beryllium alloys began in the 1950’s. The production of beryllium articles by means of plastic flow is difficult because beryllium has low plasticity owing to a hexagonal structure and the presence of impurities. During the plastic flow of beryllium, slippage occurs first in grains that are favorably oriented toward the applied stress. An unfavorable orientation of adjoining grains causes the emergence of considerable stresses at their junction that lead to the formation of fissures. These imperfections in the structure of beryllium (small number of facets and directions of slippage) are removed in certain beryllium alloys, which are formed by the introduction of a so-called plastic matrix (one of the metals Ag, Sn, Cu, Si, Al, or others). The matrix envelops the beryllium grains and facilitates the relaxation of the stresses on the boundaries of unoriented grains and the development of plastic flow. In cases of low content of the plastic matrix in the beryllium, mainly the beryllium is deformed, and the matrix is a stress relaxer. When the plastic-matrix content is considerable (for example, alloys of Be with Al), the plastic flow is brought about primarily because of the plastic metal. Beryllium alloys with increased plastic-matrix content are easily deformed (rolled, drawn, or forged) but have less strength than beryllium alloys that have a reduced plastic matrix content or than beryllium itself.
Beryllium alloy systems of the Be-Ag type, which contain 1.9–3.7 percent Ag, have increased plasticity; those that contain 20–40 percent Ag have increased resistance to impact loads. The addition of 2.7–2.9 percent Sn to Be substantially improves its mechanical properties in extruded and rolled states at room temperature. When 3 percent Cu and Ni is used as a plastic matrix in the process of obtaining billets, there is formation of brittle beryllides (for example, Be2Cu and Ni5Be21). The addition to Be-Cu alloys of 0.25 percent P, which retards the diffusion of Cu and Be, prevents the formation of beryllide and increases the plasticity. Industrial Be-Al systems containing 24–43 percent Al, called “Lock-alloy,” have been developed in the USA by the Lockheed company (see Table 1).
|Table 1. Properties of alloys of the Be-Al system In the pressed state|
|Aluminum content (percent)||Yield point In elongation (MN/m2)||Tensile strength (MN/m2)||Elastic modulus (GN/m2)||Elongation per unit length (percent)|
Be-Al alloy systems have a number of merits: they are lighter than aluminum and magnesium alloys, more plastic and less sensitive to surface defects than beryllium, and they do not require chemical cleansing after cutting treatment. The wide range of values for the elastic, strength, and plasticity moduli that may be achieved in these alloys considerably enlarges their sphere of application.
The attempt to obtain beryllium alloys with greater strength than beryllium or beryllium alloys with a plastic matrix leads to the creation of alloys strengthened by a dispersed phase. The strengtheners are intermetallic compounds, carbides, nitrides, and oxides. The mechanical (chiefly strength) properties of these beryllium alloys are increased by the introduction of a finely dispersed strengthening phase. The presence of a dispersed phase leads to the emergence of stresses in the beryllium matrix (in the case of separation from a solid solution) or hinders the propagation of slippage (in the case of formation of intermetallic compounds). Both processes increase the strength characteristics. The degree of strengthening depends on the amount and type of strengthening phase, on its binding with the matrix, and on the size of its particles and the distance between them. Industrial beryllium, which contains a considerable amount of beryllium oxide, is essentially a dispersion-strengthened alloy. Beryllium alloys have been developed in which beryllides serve as the strengtheners. The Be-Fe and Be-Co alloy systems have the best strength properties; Be-Cu and Be-Ni alloys are less strong but more plastic. At 400° C the ultimate strength of a beryllium alloy with 5 percent Co is 430 meganewtons per sq m (MN/m2), and for one with 3 percent Fe it is 410 MN/m2. The data on the stress-rupture strength of a Be alloy with 1 percent Fe are presented in Table 2.
|Table 2. Stress-rupture strength of beryllium alloys with 1 percent Fe In the hot-molded state|
|Test temperature (°C)||Stress-rupture strength (MN/m2)|
|10hrs||100 hrs||1,000 hrs|
The increase in the strength properties of beryllium alloys strengthened with a dispersed phase is accompanied by a decrease in plasticity, which considerably complicates the technology of manufacturing items. Objects and semifinished products of beryllium alloys are produced primarily by the methods of powder metallurgy, less frequently by casting. High-strength dispersion-strengthened beryllium alloys are obtained by pressure shaping hot-extruded billets in steel casings at temperatures of 1010°-1175° C. Beryllium-alloy products include rods, tubes, cones, plates, and sections. An important achievement in the area of the creation of beryllium-based materials that can operate for long periods at 1100°-1500° C and for short periods at 1700° C is the development of intermetallic compounds of beryllium with other metals. The principal trend in the use of beryllium alloys is toward aircraft construction material.
REFERENCESDarwin, G., and J. Buddery. Berillii. Moscow, 1962. (Translated from English.)
Berillii. Edited by D. White and J. Burke. Moscow, 1960. (Translated from English.)
Conference Internationale sur la métallurgie du Beryllium, Grenoble, 17–20 mai 1965. Paris, 1966.
The Metallurgy of Beryllium: Proceedings of an International Conference Organized by the Institute of Metals, London, 16–18 October, 1961. London . (Monograph and Report Series, no. 28.)
Tugoplavkie metallicheskie materialy dlia kosmicheskoi tekhniki. Moscow, 1966. (Translated from English.)
V. F. GOGULIA