Effects of Composition, Processing and Structure on ...

Elemental nickel is predominantly utilized as an alloying element to enhance the corrosion resistance of commercial iron and copper alloys, with only about 13% of annual consumption allocated to nickel-base alloys. Approximately 60% of nickel is employed in stainless steel production, alongside another 10% in the production of alloy steels, and around 2.5% in copper alloys. Nickel is also integral to special-purpose alloys, including those for controlled expansion, electrical resistance, magnetic properties, and shape memory applications.

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Nickel and its alloys find applications across various industries, including chemical processing, pollution control, power generation, electronics, and aerospace, capitalizing on their exceptional resistance to corrosion, oxidation, and heat.

Nickel exhibits ductility and can be processed using conventional methods into cast, P/M, and various wrought products like bars/wires, plates/sheets, and tubes. The commercially pure nickel exhibits moderately high melting temperature (1455 °C), density (8.902 g/cm³), and elastic modulus (204 GPa). It is ferromagnetic, displaying a Curie temperature of 358 °C (676 °F) and possesses good electrical (25% IACS) and thermal conductivity (82.9 W/m K, or 48 Btu/ft h °F).

Nickel serves mainly as an alloying agent to improve corrosion resistance in commercial iron and copper alloys; only about 13% of annual consumption is used in nickel-based alloys. About 60% is dedicated to stainless steel production, with 10% in alloy steels and 2.5% in copper alloys. Additionally, nickel is utilized in special-purpose alloys, including controlled expansion, electrical resistance, magnetic, and shape memory alloys.

Effects of Alloying Elements in Nickel Alloys

Nickel possesses a face-centered cubic crystal (fcc) structure, granting it excellent ductility and toughness. As nickel demonstrates extensive solid solubility for numerous alloying elements, the microstructure of nickel alloys typically includes the fcc solid-solution austenite (γ), where precipitate particles can form.

Nickel can form a complete solid solution with copper and exhibits nearly complete solubility with iron, dissolving about 35% chromium, around 20% each of molybdenum and tungsten, and between 5 to 10% of elements like aluminum, titanium, manganese, and vanadium. This capability enables a robust, ductile fcc matrix to accommodate substantial amounts of alloying elements in diverse combinations, thereby providing solution hardening and enhancing corrosion and oxidation resistance. The solution hardening extent has been correlated with the atomic size disparity between nickel and the alloying element, influencing the solute's ability to disrupt dislocation movement.

Elements like tungsten, molybdenum, niobium, tantalum, and aluminum (when retained in solution) act as strong solution hardeners. Tungsten, niobium, tantalum, and molybdenum are particularly effective at temperatures above 0.6 Tm (with Tm denoting melting temperature), where creep strength becomes diffusion-controlled. In contrast, iron, cobalt, titanium, chromium, and vanadium are regarded as weaker solution-hardening elements. Often, aluminum and titanium are combined to generate the age-hardening precipitate, Ni3(Al, Ti).

Some alloying elements possess the capacity to partition to γ, thereby influencing interface mismatches and precipitate-coarsening kinetics while contributing a solution-hardening component to strength. Titanium is particularly effective at both room and elevated temperatures in this regard.

Moreover, the presence of titanium, niobium, and tantalum can further alter mechanical properties by promoting the formation of similar types of precipitates. Increased titanium contents can lead to the transformation of γ to the hexagonal close-packed (hcp) η-phase, Ni3Ti, exhibiting an acicular or cellular morphology. Higher niobium concentrations can drive γ to shift to the commercially vital metastable body-centered tetragonal (bct) phase γ". Conversely, allowing the equilibrium orthorhombic phase, Ni3Nb, to form can diminish hardening. The phases precipitated and their efficacy in fortifying the microstructure rely on alloy composition, specific heat treatments, precipitate volume fraction, and service conditions.

Carbides. While nickel itself does not form carbides, it solubilizes numerous elements that readily form carbides in nickel alloys (MC, M6C, M7C3, M23C6). The MC carbides (where M represents W, Ta, Ti, Mo, Nb) are typically large, blocky, and undesirable. M6C carbides (M = Mo, W) may precipitate as small platelets or as blocky particles at grain boundaries, aiding grain control but impairing ductility and stress rupture behavior. M7C3 (M = Cr) can be beneficial when precipitated as discrete particles but is often detrimental as grain boundary particles of M23C6 (M = Cr, Mo, W), where they can enhance creep rupture resistance.

Should carbides agglomerate or create grain-boundary films during heat treatment or under elevated temperatures, they can severely compromise ductility and lead to embrittlement. As observed in stainless steels, the precipitation of chromium carbides at boundaries may result in intergranular corrosion due to the chromium-depleted zones adjacent to grain boundaries becoming anodic compared to other grains.

This grain-boundary sensitization can be addressed through several methods:

• By avoiding the chromium-carbide aging temperature range (425 to 760 °C) during processing,
• Using stabilization heat treatments to sequester carbon with more stable carbide formers (such as niobium, tantalum, titanium), and
• By minimizing carbon content in the base alloy.

Nickel alloys

Nickel is alloyed to extend the corrosion resistance and heat resistance of elemental nickel. Even with a significant presence of alloying elements, the robust, ductile fcc austenitic matrix is retained.

Nickel alloys can be broadly categorized into two primary application domains: those focused on corrosion resistance, particularly in wet environments, and those aimed at heat resistance. This classification is somewhat artificial, as corrosion-resistant alloys can exhibit commendable strength above room temperature, and heat-resistant alloys possess notable corrosion resistance. Special-property alloys, often referenced for their impressive corrosion resistance, heat resistance, and high strength, are described separately.

Corrosion-Resistant Nickel Alloys. Commercially pure nickel grades, from Nickel 200 to 205, demonstrate high resistance to a plethora of corrosive media, especially in reducing conditions, and also thrive in oxidizing environments where a passive nickel oxide surface film can be sustained. These alloys are employed extensively in the chemical processing and electronics sectors.

These alloys are forged at temperatures ranging from 650 to 1200 °C, annealed between 700 and 925 °C, and strengthened through cold working. For instance, processed sheets in the annealed state exhibit tensile properties characterized by a tensile strength of 460 MPa, yield strength of 148 MPa, and elongation of 47%. Cold rolling can elevate tensile strength to 760 MPa, yield strength to 635 MPa, albeit with an elongation decrease to 8%.

Nickel alloy 200, with a nominal carbon content of 0.08% (0.15% maximum), is not recommended for use beyond 315 °C due to embrittlement resulting from graphite precipitation in the temperature range of 425 to 650 °C. Purified nickel is commercially applicable for specialized electrical purposes.

The low-alloy nickels. These alloys contain a minimum of 94% Nickel. The 5% manganese solid-solution addition in Nickel 211 provides protection against sulfur in service environments. Even a mere 0.005% sulfur can induce liquid embrittlement at unalloyed nickel grain boundaries within the 640 to 740 °C range.

Duranickel, specifically alloy 301 (Ni-4.5Al-0.6Ti), boasts corrosion resistance akin to commercially pure nickel while benefiting from the strengthening effects of precipitating γ. The alloy's additions sufficiently adjust its composition to lower the Curie temperature, rendering it weakly ferromagnetic at ambient temperature.

The nickel-copper alloys showcase strength and toughness, offering corrosion resistance across various settings, including brine, sulfuric, and other acids, and demonstrating immunity to chloride-ion stress corrosion. Such alloys find applications in chemical processing and pollution control equipment. Alloy K-500 incorporates an age-hardening component to enhance the solution strengthening and work-hardening traits already present, leveraging a nominal 30% Copper base in alloy 400. The compositions can be tailored to decrease the Curie temperature below room temperature.

Nickel-chromium-iron (-molybdenum) alloys may essentially be perceived as nickel-based counterparts to iron-based austenitic stainless steel alloys, essentially interchanging iron and nickel content. In this substantial group of alloys, chromium levels generally range from 14% to 30%, and iron constitutes between 3% and 20%. With a stable Cr2O3 surface film, these alloys demonstrate excellent corrosion resistance across numerous severe surroundings, exhibiting immunity to chloride-ion stress-corrosion cracking. Additionally, they provide good oxidation and sulfidation resistance, with strength sustained at elevated temperatures, with maximum operational temperatures typically at around 540 °C.

The Ni-Cr-(Fe)-Mo alloys comprise a significant family of alloys utilized in the chemical processing, pollution control, and waste treatment sectors, exploiting their remarkable heat and corrosion resistance. Alloys like C-276 and 625 are enhanced by superb weldability, ensuring the corrosion resistance of welded structures.

Molybdenum additions to these alloys bolster resistance to pitting and crevice corrosion. Aluminum enhances the protective surface oxide film, while carbide forming elements like titanium and niobium stabilize the alloys against chromium-carbide sensitization. Even modest additions of aluminum and titanium in alloy 800, for instance, can initiate the formation of small amounts of γ during service exposure to high temperatures. The significant molybdenum and silicon additions in Hastelloy B and D promote resistance to hydrochloric and sulfuric acids.

Heat-Resistant Nickel Alloys. Heat-resistant nickel-containing materials encompass nickel-, iron-nickel-, or cobalt-base alloys. These can be produced via wrought and P/M methods, as well as through casting under tightly regulated conditions to fabricate the desired polycrystal structure, elongated (directionally solidified), or single-crystal grain structures that enhance mechanical properties at elevated temperatures. The majority of nickel-base superalloys leverage the combined fortification of a solution-hardened austenite matrix with γ precipitation.

The iron-base Fe-Ni-Cr heat-resistant alloys build upon the foundations of iron-base stainless steels, incorporating heightened nickel and a blend of other alloying components. Retaining the fcc iron-nickel austenite matrix, these alloys (examples include alloys A-286 and 901) can be wrought into various forms and exhibit precipitation hardening with γ.

Alloys 903 and 909 are controlled thermal expansion Fe-Ni-Co-base alloys that can age harden with Ni3(Nb, Ti) precipitation, designed to achieve high strength and low thermal expansion coefficients for applications in gas turbine rings and seals at temperatures nearing 650 °C.

These alloys undergo hot working at approximately 870 °C, followed by solution heat treatments at 815 to 980 °C. Typical aging procedures involve treating at 720 °C for 8 hours, furnace cooling at a rate of 55 °C/h down to 620 °C for 8 hours, followed by air cooling. For instance, alloy 909 retains a substantial portion of its yield strength (around 895 MPa) at a temperature of 540 °C.

Specialty Nickel Alloys. Unique amalgamations of properties emerge with other nickel-base alloys for specialized applications. While some of these traits can be found to some extent within previously mentioned alloys, the following alloys were specifically engineered to accentuate their distinctive properties.

A variety of electrical resistance alloys are utilized for heating elements. These generally contain between 35% to 85% Nickel and invariably surpass 15% Chromium to form a protective adherent surface oxide against oxidation and carburization at temperatures exceeding 1200 °C in air.

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