Nickel Alloys

Nickel Alloys

 

alloys based on nickel. The capacity of nickel to dissolve a considerable quantity of other metals while retaining its plasticity has led to the formation of numerous nickel alloys. The useful properties of nickel alloys are based to a certain extent on the properties of nickel itself, which—in addition to the ability to form solid solutions with many metals —include ferromagnetism, high corrosion resistance in gaseous and liquid media, and absence of allotropic transformations.

Cupronickel alloys, which in addition to high corrosion resistance have high plasticity, as well as valuable electrical properties, have been fairly widely used since the late 19th century. Structural materials that have found practical application include alloys of the Monel-metal type, which, together with the Cunials, are known for high chemical stability in water, acids, strong alkalies, and air.

Ferromagnetic alloys of nickel (40–85 percent) with iron, which belong to the class of soft-magnetic materials, play an important role in technology. Such materials include alloys that are characterized by extremely high values of magnetic permeability (Permalloy), constancy of magnetic permeability (Perminvar), and the combination of high saturation magnetization and magnetic permeability (Permenorm). Such alloys are used in many areas of technology that require high sensitivity of the working elements to changes in the magnetic field.

Alloys containing 45–55 percent nickel alloyed with small quantities of copper or cobalt have coefficients of linear thermal expansion that are close to that of glass, which is used in cases in which a hermetic seal between glass and metal must be achieved.

Alloys of nickel with 4 or 18 percent cobalt belong to the magnetostrictive materials. The good corrosion resistance of these alloys in river water and seawater makes them valuable materials for the construction of hydroacoustic devices.

In the early 20th century it became known that the oxidation resistance of nickel in air, although sufficiently high by itself, could be improved by the introduction of aluminium, silicon, or chromium. Among alloys of this type that have remained of great practical importance because of their good combination of thermoelectric properties and thermal stability are nickel with aluminum, silicon, and manganese (Alumel) and nickel with 10 percent chromium (Chromel). Chromel-Alumel thermocouples are among the most widespread in industry and the laboratory. Chromel-Copel thermocouples are also of practical importance.

The Nichromes, heat-resistant alloys of nickel with chromium, have found important applications in technology. The most widespread are the Nichromes with 80 percent nickel, which were the most heat-resistant industrial materials until the appearance of the Chromals. Attempts to reduce the cost of Nichromes by reducing their nickel content have led to the production of the Ferronichromes, in which a significant portion of the nickel is replaced by iron. A composition of 60 percent nickel, 15 percent chromium, and 25 percent iron emerged as the most widespread. The stability of most Nichromes in use is higher than that of the Ferronichromes; therefore, the latter are usually used at lower temperatures.

Nichromes and Ferronichromes have a rare combination of high heat resistance and high electrical resistance (1.05–1.40 microhm · m [μΩ·m]). Therefore, together with the Chromals, these materials make up the most important classes of alloys that are used for the production of wire and strips for high-temperature electric heaters. Nichromes alloyed with silicon (up to 1.5 percent), in combination with trace additions of the rare earths, alkaline earths, and other metals, are used in the production of electric heaters. The limiting operating temperature of Nichromes of this type is usually 1200°C, but it is 1250°C for a number of varieties.

Nickel alloys containing 15–30 percent chromium and up to 4 percent aluminum are more heat-resistant than alloys containing silicon. However, it is more difficult to produce wire or strips of uniform composition from aluminum-containing materials, and uniformity is a prerequisite for reliable performance of electric heaters. For this reason, such nickel alloys are used mostly for the production of heat-resistant parts that are not subjected to large mechanical loads at temperatures of up to 1250°C.

Production of the high-temperature Ni-Cr-Ti-Al alloys, called the Nimonics, was begun in Great Britain during World War II (1939–45). These alloys, which are formed by alloying Nichrome (Soviet type Kh20N80) with titanium (2.5 percent) and aluminum (1.2 percent), have an appreciable advantage in heat resistance over the Nichromes and special alloyed steels. In contrast to the previously used high-temperature steels, which are usable up to 750°-800°C, the Nimonics are usable at higher temperatures. Their appearance has been a powerful stimulus for the development of aircraft gas-turbine engines. A large number of complex alloys of the Nimonic type—with titanium, aluminum, niobium, tantalum, cobalt, molybdenum, tungsten, boron, zirconium, cesium, lanthanum, and hafnium—have been produced in a relatively short time; their operating temperatures are in the range of 850°-1000°C. An increase in the complexity of alloying reduces the alloys’ ability to undergo hot pressure working. For this reason, foundry alloys have become widespread, in addition to the wrought alloys. They may be alloyed to a higher degree and therefore may be more heat-resistant (up to 1050°C). However, the foundry alloys are characterized by less uniform properties and a consequently greater variation in their characteristics. Methods for the production of high-temperature composition materials have been developed in which refractory oxides, such as thorium, aluminum, and zirconium oxides, or other compounds are added to nickel. The most widely used material of this type consists of nickel and highly disperse thorium oxides (TD-nickel).

Important roles in technology are played by Ni-Cr, Ni-Mo, and Ni-Mn alloys, which have valuable combinations of electrical properties, such as high specific electric resistance (p = 1.3–2.0 μΩ·m), low temperature coefficient of electric resistance (of the order of 10-5l/°C), and low thermoelectromotive force in a couple with copper (less than 5 millivolts per °C). These alloys are inferior to Manganin in their temperature coefficient of electrical resistance in the room-temperature range, but their specific electrical resistance is 3–4 times greater. The principal area of application of these alloys is the production of small resistors, which require constancy of electrical properties during operation. They are usually produced from microwire or a strip 5–20 microns thick. Alloys based on Ni-Mo and Ni-Cr are also used in the production of small tensoresistors, which are characterized by an almost linear dependence of the change of electrical resistance on the magnitude of elastic deformation.

Nickel-molybdenum or nickel-chromium-molybdemum alloys, which are known abroad under such names as Hastelloy and Remanit, and Soviet alloys of the types N70M28, N70M28F, Khl5N55M16V, and Khl5N65M16V, are used for chemical equipment operating in highly aggressive media, such as hydrochloric, sulfuric, and phosphoric acids of various concentrations at temperatures approaching the boiling point. These alloys are more resistant than any known corrosion-resistant steel.

Numerous other nickel alloys (with chromium, molybdenum, iron, and so on) are being used in practice; they have a favorable combination of mechanical and physicochemical properties—for example, corrosion-resistant alloys for springs and hard alloys for dies. In addition to the nickel alloys proper, nickel is also a component of many alloys based on other metals, such as the alni alloys.

REFERENCES

Bozorth, R. Ferromagnetizm. Moscow, 1956. (Translated from English.)
Materialy v mashinostroenii: Vybor i primenenie. Vol. 3: Spetsial’nye stall i splavy. Moscow, 1968.
Khimushkin, F. F. Zharoprochnye stali i splavy, 2nd ed. Moscow, 1969.
Babakov, A. A., and M. V. Pridantsev. Korrozionnostoikie stali i splavy. Moscow, 1971.

L. L. ZHUKOV

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