Beryllium alloys

Dilute alloys of base metals that contain a few percent of beryllium in a precipitation-hardening system are the principal useful beryllium alloys manufactured today. Although beryllium has some solid solubility in copper, silver, gold, nickel, colbalt, platinum, palladium, and iron and forms precipitation-hardening alloys with these metals, the copper-beryllium system and, to a considerably lesser degree, the nickel-beryllium alloys are the only ones used commercially.

Other than the precipitation-hardening systems, small amounts of beryllium are used in alloys of the dispersion type wherein there is little solid solubility (aluminum and magnesium). Various amounts of beryllium combine with most elements to form intermetallic compounds. Development of beryllium-rich alloys have been chiefly confined to the ductile matrix beryllium-aluminum, beryllium-copper solid solution alloy with up to 4% copper, and dispersed-phase type alloys having relatively small amounts of compounds (0.25 to 6%), chiefly as beryllium oxide or intermetallics, for dimensional stability, elevated temperature strength, and elastic limit control.

Beryllium-Copper Alloys

Commercial alloys of the beryllium-copper group are divided into cast or wrought types, usually in ternary combination such as copper-beryllium-cobalt (see Table B.1). Alloys with higher beryllium content have high strength while those with low beryllium have high electrical and thermal conductivity.

Age-hardenable copper-beryllium-cobalt alloys offer a wide range of properties, because they are extremely ductile and workable in the annealed condition and are strong and hard after precipitation or aging treatment. Cobalt is added to inhibit grain growth and provide uniform heat-treatment response. These alloys also have inherent characteristics of substantial electrical and thermal conductivity and resistance to corrosion and wear, being protected by beryllium oxide films which impart this property to all materials containing beryllium. These age-hard enable alloys resist dimensional change and fatigue.

TABLE B.1

Composition of Principal Beryllium-Copper Alloys

Primary applications are found in the electronics, automotive, appliance, instrument, and temperature-control industries for electric-current-carrying springs, diaphragms, electrical switch blades, contacts, connectors, terminals, fuse clips, and bellows (foil, strip, and wire), as well as resistance-welding dies, electrodes, clutch rings, brake drums, and switch gear (rod, plate, and forgings). With 1.5% beryllium or more, the melting point of copper is severely depressed and a high degree of fluidity is encountered, allowing casting of intricate shapes with very fine detail. The characteristic is important for plastic injection molds.

For special applications specific alloys have been developed. Free machining and nonmagnetic alloys have been made, as well as high-purity materials. A precipitation-hardening beryllium-Monel for oceanographic application, containing about 30% nickel, 0.5% beryllium, 0.5% silicon, and the remainder copper, illustrates one of a series of alloys having strength, corrosion resistance to seawater, and antifouling characteristics.

New applications in structural, aerospace, and nuclear fields are submarine repeater cable housings for transoceanic cable systems, wind tunnel throats, liners for magneto hydrodynamic generators for gas ionization, and scavenger tanks for propane-Freon bubble chambers in high-energy physics research. Important developing applications for beryllium-copper are trunnions and pivot bearing sleeves for the landing gear of heavy, cargo-carrying aircraft, because these alloys allow the highest stress of any sleeve bearing material.

Beryllium-copper master alloys are produced by direct reduction of beryllium oxide with carbon in the presence of copper in an arc furnace. Because Be2C forms readily, the master alloy is usually limited to about 4.0 to 4.25% beryllium. The master alloy is remelted with additional copper and other elements to form commercial alloys. Rolling billets up to 680 kg have been made by continuous-casting techniques.

Beryllium-copper alloys can be fabricated by all the industrial metalworking techniques to produce principally strip, foil, bar, wire, and forgings. They can be readily machined and can be joined by brazing, soldering, and welding.

Annealing, to provide high plasticity at room temperature, is accomplished by heat-treating (from 790 to 802°C for the high-beryllium alloys or 900 to 930°C for the low-beryllium alloys) and water quenching. Precipitation-hardening is accomplished by heating to (400 to 480°C; low beryllium) and (290 to 340°C; high beryllium).

Alloys with Nickel and Iron

Nickel containing 2% beryllium can be heat-treated to develop a tensile strength of 1700 MPa with 3 to 4% elongation. Little commercial use is made of the hard nickel alloys although they have been employed, principally as precision castings, for aircraft fuel pumps, surgical instruments, and matrices in diamond drill bits. Another nickel alloy with 2.0 to 2.3% beryllium, 0.5 to 0.75% carbon, and the balance of nickel and refractory metals has been used for mold components and forming tools for glass pressware of optical and container quality. Thermal conductivity, wear resistance, and strength, coupled with unusual machina-bility for a nickel-beryllium alloy, make this alloy particularly advantageous for glassware tooling.

Wrought beryllium-nickel contains about 2% beryllium, 0.5% titanium, and the balance nickel. Casting alloys contain a bit more beryllium (2 to 3%) and, in one alloy, 0.4% carbon. Arsenic in the case of beryllium-copper alloys, mechanical properties vary widely, depending on temper condition — from 310 to 1586 MPa in tensile yield strength and Rb 70 to Rc 55 in hardness at room temperature. The alloys retain considerable yield strength at high temperature: 896 to 1172 MPa at 538°C. They also have good corrosion resistance in general atmospheres and reducing media. Because beryllium is toxic, special precautions are required in many fabricating operations. The wrought alloy is used for springs, bellows, electrical contacts, and feather valves, and the casting alloys for molding plastics and glass, pump parts, seal plates, and metal-forming tools.

Attempts have been made to add beryllium to a number of ferrous alloys. Small amounts are used to refine grains and deoxidize ferritic steels in Japan, and promising properties have been developed for austenitic and martensitic steels. Stainless steels (iron-nickel-chromium) may be made maraging by adding 0.15 to 0.9% beryllium, developing strengths as high as 1800 MPa as cast or 2300 MPa as rolled while retaining their oxidation- and corrosion-resistant characteristics. Amounts of 0.04 to 1.5% beryllium have been added to various iron alloys containing nickel, chromium, cobalt, molybdenum (Mo), and tungsten for special applications such as watch springs.

Beryllium-Base Alloys

Three types of beryllium-base alloys are of interest. These consist of dispersed-phase types containing up to 4% beryllium oxide; ductile-phase or duplex alloys of beryllium and aluminum, particularly 38% aluminum in beryllium; and solid solution alloys of up to 4% copper in beryllium.

Dispersed-phase alloys containing oxides, carbides, nitrides, borides, and intermetallic compounds in a beryllium matrix are chiefly characterized by increased strength and resistance to creep at elevated temperatures. Major commercial alloys in this series are of the fine-grain, high-beryllium oxide (4.25 to 6%), hot-pressed types such as materials used for inertial guidance instruments characterized by high dimensional stability, high-precision elastic limit (55 to 103 MPa), and good machinability.

The 62% beryllium, 38% aluminum alloy previously discussed under aluminum alloys was developed as a structural aerospace alloy to combine high modulus and low density with the machining characteristics of the more common magnesium-base alloys. This alloy in sheet form has at room temperature about 344 MPa ultimate strength, 324 MPa yield strength, and about 8% elongation. It is also produced as extrusions. It has a duplex-type microstructure characterized by a semicontinuous aluminum phase. Other alloys of the beryllium-aluminum type have been reported, but the 62% beryllium, 38% aluminum alloy is the most used.

Intermetallic Compounds

Beryllium, combined with most other elements, forms intermetallic compounds with high strength at high temperature (up to 552 MPa modulus of rupture at 1260°C), good thermal conductivity, high specific heat, and good oxidation resistance. The beryllium oxide film formed by surface oxidation is protective to volatile oxides (molybdenum) and to elements of high reactivity (zirconium and titanium) at temperatures 1100 to 1500°C and for short times up to 1650°C.

The beryllides are of interest to the nuclear field, to power generation, and to aerospace applications. Evaluation of the intermetallics as refractory coatings, reactor hardware, fuel elements, turbine buckets, and high-temperature bearings has been carried out.