Heavy Alloy

any of a group of tungsten-based composition materials containing (by weight) up to 10 percent nickel and iron (tungsten-nickel-iron type alloys), in proportions from 7:3 to 1:1, or nickel and copper (tungsten-nickel-copper alloys), in proportions from 3:2 to 1:1. Small amounts of chromium, molybdenum, rhenium, cobalt, and other metals are sometimes also present.

Heavy alloys have a two-phase structure, in which tungsten grains, the γ-phase, are uniformly distributed in a nonrefractory matrix of nickel and iron or copper, the α-phase. Iron and copper limit the solubility of tungsten in nickel, inhibiting the formation of the β-phase (Ni₄W), and they lower the temperature at which the α-phase begins to melt.

Heavy alloys are ductile and easily cut and shaped. Their properties depend on the extent and graininess of the α-phase, the Ni: Fe or Ni: Cu ratio, the alloying elements, and the production conditions. Tungsten-nickel-copper alloys are not as strong as tungsten-nickel-iron alloys because of the formation of a coarse dendritic structure when they are cooled below the sintering temperature, but they are more practical to manufacture because the temperature at which the α-phase begins to melt is about 100°C lower.

Heavy alloys have a density of at least 16.5–17 g/cm³ at 20°C. They have a thermal coefficient of linear expansion of (4.0–5.5) × 10^–6 between 20° and 400°C, a tensile strength of up to 150 kilograms-force per sq mm (kgf/mm²; 1 kgf/mm² = 10⁷ newtons per sq m), a compressive strength of up to 120 kgf/mm², a yield point of up to 140 kgf/mm², a relative elongation of up to 30 percent, and an impact strength of at least 1 kgf-m/cm² for unmatched samples. They are corrosion-resistant and are good absorbers of γ rays and X rays.

Heavy alloys are produced from mixtures of powdered metals using techniques of powder metallurgy. During the process of sintering at 1350°-1500°C in the presence of the liquid phase, the tungsten powder recrystallizes to form almost spherical particles that are dozens of times larger than the particles of the initial powder. The properties of heavy alloys are improved by subsequent pressure working and heat treatment.

Heavy alloys have a wide range of application because of their

advantageous combination of desirable properties. They are used in making screens that are more effective than lead as shields against penetrating radiation, and in the production of containers for radioactive isotopes, such as ⁹⁰Sr. In addition, they are used in the manufacture of balances and counterweights for aircraft, counterweights for self-winding clocks and watches, rotors for gyroscopes, inertial masses, cores for armor-piercing shells, dies for electrical upsetting processes, and matrix inserts for hot pressing of brass and bronze rods. Heavy alloys are used as electrode materials in chipless metalworking processes and resistance welding and as temperature compensators in silicon semiconductor devices. The range of uses for heavy alloys is continually and rapidly expanding.