Nickel is a chemical element with the symbol Ni and atomic number 28, placing it in the transition metals group. Valued for its lustrous silver-white color, corrosion resilience, and unusually high melting point, it is often alloyed with other metals to enhance its properties. Nickel is alloyed with elements like copper, chromium, and iron to create an extensive range of alloys such as austenitic stainless steels, cupronickel, and Monel®. These alloys benefit from the corrosion resistance, heat resistance, ductility/malleability, and strength of nickel.
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This article will discuss what nickel is, its composition, types, properties, and applications.
Nickel is a transition metal element, number 28 in the periodic table. In wide use, it is applicable to alloying, imparting beneficial properties to the resultant materials which, in some cases, are still referred to as nickel.
Nickel&#;s history dates back to 3,500 BC in China where it was used in ornamental objects. Its identification as a distinct element was by Swedish chemist Axel Fredrik Cronstedt, who isolated nickel from a mineral called "kupfernickel" in . Initially mistaken for a copper ore, "kupfernickel" proved to be a previously unknown metal. In the early 19th century, it was alloyed with silver to create "nickel silver." Nickel silver is a metal resembling silver in appearance, but considerably tougher and lower cost. Nickel's resistance to corrosion led to its widespread use in electroplating applications. The alloying of nickel in stainless steel by Harry Brearley in revolutionized various industries and created a whole new class of materials that are still being extended. Nickel's versatility, from adornments to essential industrial applications, has solidified its significance in modern society.
Natural nickel consists of five stable isotopes: nickel-64 (0.91%), nickel-62 (3.59%), nickel-61 (1.13%), nickel-60 (26.10%), and nickel-58 (68.27%). It has a face-centered cubic crystal structure. It is a unique element that can only be broken down by nuclear reactions and high-energy bombardment.
Nickel is produced through a combination of mining, smelting, and refining processes, as it rarely occurs in the pure metallic state. It is generally found as a mineral salt. Nickel ores, typically containing nickel, copper, and other minerals, are mined. The ore is processed to extract nickel salts through crushing, grinding, and density sorting. The resultant nickel-salts concentrate is smelted in a reducing environment, to remove impurities and convert them into nickel matte. Refining is achieved through electrolysis or solvent extraction to purify the nickel from the unrefined and potentially complex alloy as-mined. The refined nickel is then generally added to a secondary smelting operation as an alloying agent. It is often in a carbon arc or induction furnace heated crucible, under a vacuum or inert atmosphere to enhance purity.
Nickel possesses a range of attractive and beneficial characteristics, both in its pure state and imparted to alloys of which it is a constituent.
When refined and polished, nickel offers an undisturbed reflection of visible light. This implies that a mirror finish offers a &#;true color&#; reflection which is generally described as a &#;silver&#; finish. In its natural, unrefined metallic state, nickel is often contaminated with other metals which alter its reflectivity. Combination with copper, for example, lends a red hue to the reflective property.
In its unrefined state, it is generally distributed as salts, mixed in with ores of other metals and various minerals. When first smelted, it will often look dull gray in the relatively pure state. It may also take on the color of the other metals the nickel is mixed with&#;commonly copper, lending red-brown coloration of the more readily oxidized copper. The refined and purified element is lustrous silver as shown in Figure 1 below:
Carpenter Technology&#;s 718 is a precipitation hardenable (PH) nickel-base superalloy. It is made of a γ matrix strengthened by a combination of Ni3(Ti,Al)-γ&#; (gamma prime) and Ni3Nb-γ&#;&#; (gamma double prime) precipitates and also contains carbides and the Nb-rich δ phase. It is a workhorse alloy in the aerospace and oil and gas industries because of its exceptional high-temperature strength and creep resistance, as well as good corrosion resistance. Critical to manufacturing an alloy with such high performance is understanding the processing-properties-microstructure relationship, and materials characterization sits at the center of that understanding.
Carpenter Technology&#;s materials characterization expertise is centered in the R&D Characterization Laboratory. Here, samples of 718 are prepared and polished for either light optical observation (LOM) or scanning electron microscopy (SEM). For LOM, samples are usually etched with Waterless Kallings or Lucas&#; Reagent, specialty etchants to reveal the grain size and microstructure. Some features are too fine for optical microscopy and must be observed by SEM. These observations about the alloy&#;s microstructure reveal critical information about its processing and properties, and the link between them.
A fully recrystallized microstructure with a fine uniform grain is necessary to yield good mechanical properties. When grains are too fine, the alloy strength suffers, and when grains are too coarse, the stress rupture properties suffer. Large un-recrystallized grains (URGs) have a much larger size than the average (Figure 2) and indicate that the solution temperature was too low to achieve full recrystallization. Unrecrystallized grains severally affect stress rupture properties, but their presence in the billet can be identified during sonic inspection.
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Gamma double prime (γ&#;&#;)
Unlike most nickel-base superalloys that are γ&#; strengthened, 718 is mainly strengthened by γ&#;&#; precipitates in the peak age condition. γ&#;&#; has an ordered DO22 structure and a Ni3Nb composition and appears as nanometer scale discs under the transmission electron microscope (TEM). (Figure 3).
Gamma prime (γ&#;)
γ&#; plays a minor role in the strengthening of 718. It has a Ni3(Al,Ti) composition, is spherical in shape, and appears as small dots within the matrix.
Delta Phase (δ)
δ is a Nb-rich phase with an ordered orthorhombic structure and appears as long needles on grain boundaries, twin boundaries, or within grains (Figure 4-6). A small amount of delta phase is beneficial to control grain growth during solution and aging and can deliberately be precipitated during a heat treatment called &#;delta dump.&#; However, too much delta phase is detrimental and decreases the fracture toughness, strength, and creep resistance.
Carbides
In addition to the formation of γ&#;&#; and γ&#; phases during aging, small secondary carbides precipitate and grow, increasing 718 hardness. Small secondary carbides are visible using the SEM. In addition to the small secondary carbides, large blocky carbonitrides are present in the microstructure. Ti-rich carbonitrides appear peach in color, while Nb-rich carbonitrides are grey. These large, blocky carbides can act as crack initiation sites, reducing fatigue performance. Under the right heat treat conditions, these carbonitrides can precipitate out as a grain boundary film that degrades ductility.
Fifty-five years after its invention, 718 remains the predominant superalloy choice for many high temperature and extreme environment applications. In gas turbines and jet engines, where increased operating temperatures mean increased engine efficiency, 718 is used extensively in parts that are subject to high temperature fatigue, stress rupture, and creep loading. There is also a large market for 718 in the oil and gas industries where the high pressure and high temperature corrosive fluids used in drilling require high strength and corrosion resistance. With its high performance under extreme environments, 718 remains relevant and integral to power generation, aerospace, and oil and gas industries.
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