Among structural metals, beryllium (symbol Be) has a unique combination of properties. It has low density (two thirds that of aluminum), high modulus per weight (five times that of ultra strength steels), high specific heat, high strength per density, excellent dimensional stability, and transparency to x-rays. Beryllium is expensive, however, and its impact strength is low compared to values for most other metals.

Beryllium is a steel-gray lightweight metal, used mainly for its excellent physical properties rather than its mechanical properties. Except for magnesium (Mg), it is the lightest in weight of common metals, with a density of 1855 kg/m3. It also has the highest specific heat (1833 J/kg K) and a melting point of 1290°C. It is nonmagnetic, has about 40% the electrical conductivity of copper, a thermal conductivity of 190 W/m K, high permeability to x-rays, and the lowest neutron cross section of any metal having a melting point above 500°C. Also, its tensile modulus (28.9 x 104 MPa) is far greater than that of almost all metals.

Ultimate tensile strength ranges from 228 to 690 MPa and tensile elongation from 1 to 40%, depending on mill form. Thus, because of its low density, beryllium excels in specific strength, and especially in specific stiffness. However, tensile properties, especially elongation, are extremely dependent on grain size and orientation, and are highly anisotropic so that results based on uniaxial tensile tests have little significance in terms of useful ductility in fabrication or fracture toughness in structural applications. From these standpoints, the metal is considered to be quite brittle. Ductility, as measured by elongation in tensile tests, increases with increasing temperature to about 400°C, then decreases above about 500°C. Although resistant to atmospheric corrosion under normal conditions, beryllium is attacked by O2 and N2 at elevated temperatures and certain acids, depending on concentration, at room temperature.

Available forms include block, rod, sheet, plate, foil, extrusions, and wire. Machining blanks, which are machined from large vacuum hot pressings, make up the majority of beryllium purchases. However, shapes can also be produced directly from powder by processes such as cold-press/sinter/coin, CIP/HIP, CIP/sinter, CIP/hot-press, and plasma spray/sinter. (CIP is cold isostatic press, and HIP is hot isostatic press.) Mechanical properties depend on powder characteristics, chemistry, consolidation process, and thermal treatment. Wrought forms, produced by hot working, have high strength in the working direction, but properties are usually anisotropic.

Beryllium parts can be hot-formed from cross-rolled sheet and plate as well as plate-machined from hot-pressed block. Forming rates are slower than for titanium, for example, but tooling and forming costs for production items are comparable.

Structural assemblies of beryllium components can be joined by most techniques such as mechanical fasteners, rivets, adhesive bonding, brazing, and diffusion bonding. Fusion-welding processes are generally avoided because they cause excessive grain growth and reduced mechanical properties.

Beryllium behaves like other light metals when exposed to air by forming a tenacious protective oxide film that provides corrosion protection. However, the bare metal corrodes readily when exposed for prolonged periods to tap water or seawater or to a corrosive environment that includes high humidity. The corrosion resistance of beryllium in both aqueous and gaseous environments can be improved by applying chemical conversion, metallic, or non-metallic coatings. Beryllium can be electro less nickel-plated, and flame or plasma sprayed.

All conventional machining operations are possible with beryllium, including EDM and ECM. However, beryllium powder is toxic if inhaled. Because airborne beryllium particles and beryllium salts present a health hazard, the metal must be machined in specially equipped facilities for safety.

Machining damages the surface of beryllium parts. Strength is reduced by the formation of microcracks and "twinning." The depth of the damage can be limited during finish machining by taking several light machining cuts and sharpening cutting tools frequently or by using nonconventional metal-removal processes. For highly stressed structural parts, 0.05 to 0.10 mm should be removed from each surface by chemical etching or milling after machining. This process removes cracks and other surface damage caused by machining, thereby preventing premature failure. Precision parts should be machined with a sequence of light cuts and intermediate thermal stress reliefs to provide the greatest dimensional stability.

Beryllium is toxic if inhaled or ingested, necessitating special precaution in handling. Most applications are quite specialized and stem largely from the good thermal and electrical properties of the metal. Uses include precision mirrors and instruments, radiation detectors, x-ray windows, neutron sources, nuclear reactor reflectors, aircraft brakes, and rocket nozzles.

Beryllium typically appears in military aircraft and space-shuttle brake systems, in missile reentry body structures, missile guidance systems, and satellite structures. The modulus-to-density ratio is higher than that of unidirectionally reinforced, "high-modulus" boron, carbon, and graphite fiber composites. Beryllium has an additional advantage because its modulus of elasticity is isotropic.

The largest-volume uses of beryllium metal are in the manufacture of beryllium-copper alloys and in the development of beryllium-containing moderator and reflector materials for nuclear reactors. Addition of 2% beryllium to copper forms a nonmagnetic alloy that is six times stronger than copper. These beryllium-copper alloys find numerous applications in industry as non sparking tools, as critical moving parts in aircraft engines, and in the key components of precision instruments, mechanical computers, electrical relays, and camera shutters. Beryllium-copper hammers, wrenches, and other tools are employed in petroleum refineries and other plants in which a spark from steel against steel might lead to an explosion or fire.

One of the largest uses of beryllium metal is in nuclear reactors as a moderator to lessen the speed of fission neutrons and as a reflector to reduce leakage of neutrons from the reactor core. Beryllium is useful in nuclear applications because of its relatively high neutron-scattering cross section, low neutron-absorption cross section, and low atomic weight.

Another large-scale use of beryllium is in the manufacture of beryllium bronze, which has high tensile strength and a capacity for being hardened by heat treatment. Beryllium-copper molds are used in manufacturing plastic furniture with the appearance of wood-grain surfaces.

A small but important use of beryllium is in sheet or foil form as window material in x-ray tubes. Beryllium transmits x-rays 17 times better than an equivalent thickness of aluminum and six to ten times better than Lindemann glass. This, together with its high melting point, makes possible the use of x-ray beams of greater intensity.

Beryllium oxide is used in the manufacture of high-temperature refractory material and high-quality electrical porcelains, such as aircraft spark plugs and ultrahigh-frequency radar insulators. The high thermal conductivity of beryllium oxide and its good high-frequency electrical insulating properties find application in electrical and electronic fields.

Another use of beryllium oxide is as a slurry for coating of graphite crucibles to insulate the graphite and to avoid contamination of melted alloys with carbon. Beryllium oxide crucibles are used where exceptionally high purity or reactive metals are being melted. In the field of beryllium-oxide ceramics, a type of beryllia has been developed that can be formed into custom shapes for electronic and microelectronic circuits. Beryllium oxide has a high thermal conductivity, equal to that of aluminum, and excellent insulating properties, which permits closer packing of semiconductor functions in silicon (Si) integrated circuits.

The lightweight, very high elastic modulus, and heat stability of beryllium make it an attractive material for use as construction material in aircraft and missiles. However, its lack of ductility is a drawback. Were it not for its toxicity and scarcity, beryllium would find use as a rocket fuel because it produces more heat energy per unit volume than any other element. In multistage missiles a small weight reduction in the final stage, such as might be achieved by using beryllium in place of steel, permits a much larger weight reduction in the earlier stages in terms of fuel and structure. Research in the utilization of beryllium metal and beryllium-containing materials for aircraft and missiles is carried out very actively. These and other still-developing applications together with the continuing uses of beryllium in nuclear technologies sustain the ever-mounting production levels of beryllium.

Next post:

Previous post: