Nickel is a silvery-white lustrous metal with a slight golden tinge. Nickel is one of most common alloying elements. About 65% of nickel production is used in stainless steels. Because nickel does not form any carbide compounds in steel, it remains in solution in the ferrite, thus strengthening and toughening the ferrite phase. Nickel steels are easily heat treated because nickel lowers the critical cooling rate.
Nickel based alloys (e.g. Fe-Cr-Ni(Mo) alloys) alloys exhibit excellent ductility and toughness, even at high strength levels and these properties are retained up to low temperatures. Nickel and its alloys are highly resistant to corrosion in many environments, especially those that are basic (alkaline). Nickel also reduces thermal expansion for better dimensional stability. Nickel is the base element for superalloys. These metals have excellent resistance to thermal creep deformation and retain their stiffness, strength, toughness and dimensional stability at temperatures much higher than the other aerospace structural materials.
Types of Nickel Alloys
Nickel-based Superalloys
Nickel-base superalloys currently constitute over 50% of the weight of advanced aircraft engines. Nickel-base superalloys include solid-solution-strengthened alloys and age-hardenable alloys. Age-hardenable alloys consist of an austenitic (fcc) matrix dispersed with coherent precipitation of an Ni3(Al,Ti) intermetallic with an fcc structure. Ni-based superalloys are alloys with nickel as the primary alloying element are preferred as blade material in the previously discussed applications, rather than Co- or Fe-based superalloys. What is significant for Ni-based superalloys is their high strength, creep and corrosion resistance at high temperatures. It is common to cast turbine blades in directionally solidified form or single-crystal form. Single-crystal blades are mainly used in the first row in the turbine stage.
For example, Inconel is a registered trademark of Special Metals for a family of austenitic nickel-chromium-based superalloys. Inconel 718 is a nickel-based superalloy that possesses high strength properties and resistance to elevated temperatures. It also demonstrates remarkable protection against corrosion and oxidation. Inconel’s high temperature strength is developed by solid solution strengthening or precipitation hardening, depending on the alloy. Inconel 718 is composed of 55% nickel, 21% chromium, 6% iron, and small amounts of manganese, carbon, and copper.
Nickel Silver
Nickel silver, known also as German silver, nickel brass or alpacca, is a copper alloy with nickel and often zinc. For example, UNS C75700 nickel silver 65-12 copper alloy has good corrosion and tarnish-resistance, and high formability. Nickel silver is named due to its silvery appearance, but it contains no elemental silver unless plated.
Constantan
Constantan is a copper–nickel alloy consisting usually of 55% copper and 45% nickel and specific minor amounts of additional elements to achieve precise (almost constant) values for the temperature coefficient of resistivity. That means, its main feature is the low thermal variation of its resistivity, which is constant over a wide range of temperatures. Other alloys with similarly low temperature coefficients are known, such as manganin.
This alloy has high electrical resistivity (4.9 x 10−7 Ω·m), high enough to achieve suitable resistance values in even very small grids, the lowest temperature coefficient of resistance, and the highest thermal EMF (also known as the Seebeck effect) against platinum of any of the copper-nickel alloys. Because of the first two of these properties, it is used for electrical resistors, and because of the latter property, for thermocouples. Thermocouples are electrical devices consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature.
For example, constantan is the negative element of the type J thermocouple with iron being the positive. The type J thermocouples are used in heat treating applications. Also, Constantan is the negative element of the type T thermocouple with copper the positive. These thermocouples are used at cryogenic temperatures.
In nuclear reactors, the thermocouples are positioned at preselected locations to measure fuel assembly coolant outlet temperature for use in monitoring the core radial power sharing and coolant. But in this case, thermocouples must withstand neutron irradiation, thus type E (chromel-alumel) or other special thermocouples are preferred.
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