Nickel-Copper (Ni-Cu) alloys exhibit simultaneously high strength and toughness (particularly, at cryogenic temperatures), excellent corrosion resistance, and may show good wear resistance. Therefore, they are widely used for manufacturing of (i) structural components of equipment in the chemical, oil, and marine industries, (ii) resistors and contacts in electrical and electronic equipment, (iii) corrosion resistant coatings, and (iv) fuel cells. Processing technologies includes bar forging, plate and tube rolling, wire drawing, heat treatment (for certain alloy compositions), powder and wire arc additive manufacturing, electrodeposition.
The Ni-Cu system forms the basis for the Monel alloy family (Table 1). Monel was discovered by Robert Crooks Stanley who was employed by the International Nickel Company (INCO) in 1901. The new alloy was named in honour of the company president, Ambrose Monell. The name is now a trademark of Special Metals Corporation .
Table 1. Chemical compositions of Monel alloys (wt.%).
Ni and Cu exhibit very similar atomic characteristics. They both have face centred cubic (fcc) crystal structure type, less than three percent difference in atomic radii, and exhibit similar electronegativity and valence state. The Ni-Cu system has complete solid solubility, which allows production of single phase alloys over the entire composition range . Although the Ni-Cu system exhibits complete solid solubility , the large differences in melting points between Ni (1455 °C) and Cu (1085 °C) can result in Cu segregation. Following equilibrium solidification at slow cooling rates, dendrites become enriched in Ni and interdendritic regions get enriched in Cu . However, with an increase in cooling rate during solidification the compositional gradient decreases and the microstructure morphology changes from dendritic to cellular . Higher undercoolings during solidification also lead to finer and more equiaxed grain sizes after annealing .
Monel alloys can be easily fabricated by hot and cold metal forming processes and machining. Recrystallisation studies determined the optimum hot deformation temperatures to be 950–1150 °C , which are quite similar to other Ni-base alloys and steels. However, heat treatment schedule requires rigorous development: usually a two-step age-hardening heat treatment in the temperature range of 650–480 °C is used for Ni-Cu alloys . The higher temperature stage helps to quickly nucleate precipitates of alloying elements, and the lower temperature stage provides a superior distribution of higher number density of smaller-sized particles. Ni-Cu alloys are usually quite weldable to each other and to other Ni alloys and stainless steels . Lower heat inputs produce finer grain microstructures with random texture and higher strength and ductility . Monel alloys are expensive, with their cost reaching up to 3 times that of Ni and 7 times that of Cu . Hence their use is limited to those applications where they cannot be replaced with a cheaper alternative.
Major additions of copper (28–40 wt.%) improve corrosion resistance of Ni in many agents, in particular nonoxidizing acids, nonaerated sulphuric and hydrofluoric acids . This determines areas of application of Ni-Cu alloys. They are widely used for manufacturing various components of equipment in chemical, oil and marine industries (such as drill collars, pumps, valves, fixtures, piping, fasteners, screws, propeller shafts, steam generators, turbines ), for protective coating , for manufacturing electrical and electronic equipment (resistors, bimetal contacts, capsules for transistors and ceramic-to-metal sealing ), and in fuel cells .
2. Mechanical Properties
Room temperature tensile properties of Monel 400 and K500 are shown in Table 2 . In hot-rolled and annealed conditions Monel K500 shows a 100–200 MPa higher yield stress (YS) and ultimate tensile strength (UTS) than Monel 400, due to solid solution and precipitation strengthening. In both alloys YS and UTS are slightly higher for cold formed products, due to work hardening. In hot finished Monel K500, annealing may lead to strength decrease by about 30%, following dissolution of precipitates and dislocation annihilation. In contrast, age-hardening may result in 1.3–2.5 times increase in strength due to precipitation. For the cold formed products, annealing decreases strength by about 40–50%; although, age-hardening may increase strength by up to 1.3 times. As seen, the age-hardening heat treatment is more effective in increasing strength of the hot finished products, compared to the cold finished.
Table 2. Mechanical properties of Monel 400 and K500 .
|Processing Condition||Yield Strength, MPa||Tensile Strength, MPa||Elongation, %||HB|
|Hot-Finished, Annealed and Aged||586–830||896–1140||35–20||250–315|
|Cold-Drawn, Annealed and Aged||586–830||896–1310||30–20||250–315|
High and low temperature tensile properties of Monel K500 are shown in Figure 1. For hot rolled product, with an increase in temperature the YS and UTS do not vary significantly until 650 °C (1200 °F) and 150 °C (300 °F), respectively. Age-hardening increases YS and UTS by about 1.5 and 2 times, respectively. However, the YS stability at high temperature decreases, YS starts going down at a lower temperature of 430 °C (800 °F) compared to the non-aged material. In contrast, the stability of UTS increases, UTS starts decreasing at a higher temperature of 300 °C (550 °F) compared to the non-aged material. The decrease in YS stability after ageing can be associated with destruction of the dislocation sub-structure during ageing, and the increase in UTS stability follows particle precipitation. Annealing prior age-hardening does not provide any strength gain compared to the hot-rolled and age-hardened product, however the stability of elongation increase. This is a consequence of significantly reduced dislocation density after annealing and increased distance of dislocations free pass. With a decrease in temperature, the tensile strength and yield stress both increase while ductility and toughness remain virtually the same (Figure 1d). No ductile-to-brittle transition occurs even at temperatures as low as that of liquid hydrogen. Thus, this alloy is suitable for many cryogenic applications.
Figure 1. High temperature tensile properties of Monel K500: (a) hot-rolled, (b) hot-rolled and age-hardened, and (c) annealed and age-hardened; (d) low temperature tensile properties of Monel K500  (reproduced with a permission from the Special Metals Corporation).
In view of excellent corrosion resistance coupled with high strength and toughness at cryogenic temperatures  Monel alloys are good candidates for structural components of machinery and storage of liquefied gases in aerospace and chemical industry. High temperature application of Monel is limited by low melting temperature of Cu. However, the precipitation strengthening capacity in the Ni-Cu alloy system can be improved with appropriate alloying element additions and heat treatment. Ni-Cu alloys are easily weldable  and were successfully used as an input material in powder based  and wire arc additive manufacturing . Development of these modern technologies allows to apply the Ni-Cu alloys as a surface cladding material for protection of less corrosion resistant core or in components with mechanical properties gradient.
3. Strengthening Mechanisms
Due to Ni and Cu exhibiting complete solid solubility, their alloys are single phase. Thus, in Ni-Cu alloy four strengthening mechanisms operate: grain refinement, solid solution, precipitation, and dislocation strengthening (work hardening). Composition of an alloy determines whether the solid solution or precipitation strengthening dominates. Mo, Ti, Cr, and Mn are the most frequently used solid solution strengthening elements, due to their significant difference in atomic sizes from Ni. Such elements as Fe, Co, and Cu are the second order solid solution strengtheners, due to their high solubility in Ni. Al and Ti are the most effective precipitation strengthening elements in Ni-Cu alloys, as they tend to form NiAlTi-rich intermetallic particles. Sometimes Mn-rich M23C6 or Ti-rich MC carbides can also precipitate. The dislocation strengthening capacity of Ni-Cu alloys is substantial, which is determined by their fcc crystal structure. However, cold deformation above 20% would decrease ductility below the practically reasonable limits of 30% elongation. In presence of particle forming elements in alloy composition, age hardening heat treatment is frequently used as the final strengthening operation.
The entry is from 10.3390/met10101358
- Waite, J.G.; Look, D.; Gayle, M. Metals in America’s Historic Buildings: Uses and Preservation Treatments; US Department of the Interior, National Park Service, Cultural Resources: Washington, DC, USA, 1995.
- Lippold, J.C.; DuPont, J.; Kiser, S.D. Alloying Additions, Phase Diagrams and Phase Stability. In Welding Metallurgy and Weldability of Nickel-Base Alloys; John Wiley & Sons: Hoboken, NJ, USA, 2009; Chapter 2; pp. 15–45.
- Czajkowski, C.; Butters, M. Investigation in Hardsurfacing a Nickel-Copper Alloy (Monel400); Report BNL-52651; Brookhaven National Laboratory: Upton, NY, USA, 2001.
- Heckel, R.W.; Rickets, J.H.; Buchwald, J. Measurement of the degree of segregation in Monel 400 weld metal by X-ray line broadening. Weld. J. 1965, 34, 332–336.
- Doherty, R.D.; Feest, E.A.; Holm, K. Dendritic solidification of Cu-Ni alloys: Part I. Initial growth of dendrite structure. Metall. Mater. Trans. A 1973, 4, 115–124.
- Dundar, S. Dendritic solidication in a copper nickel alloy. Turkish J. Eng. Env. Sci. 2004, 28, 129–134.
- Algoso, P.; Hofmeister, W.; Bayuzick, R. Solidification velocity of undercooled Ni–Cu alloys. Acta Mater. 2003, 51, 4307–4318.
- Hou, H.; Li, Y.; Xu, X.; Zhao, Y.; Liu, F. Non-equilibrium effects on solid transition of solidification microstructure of deeply undercooled alloys. Mater. Sci. Technol. 2017, 34, 402–407.
- Arjmand, M.; Abbasi, S.; Taheri, A.K.; Momeni, A. Hot workability of cast and wrought Ni–42Cu alloy through hot tensile and compression tests. Trans. Nonferrous Met. Soc. China 2016, 26, 1589–1597.
- Ebrahimi, G.R.; Momeni, A.; Ezatpour, H.; Jahazi, M.; Bocher, P. Dynamic recrystallization in Monel400 Ni-Cu alloy: Mechanism and role of twinning. Mater. Sci. Eng. A 2019, 744, 376–385.
- Es-Said, O.; Zakharia, K.; Ventura, C.; Pfost, D.; Crawford, P.; Ward, T.; Raizk, D.; Foyos, J.; Marloth, R.; Zakharia, Z. Failure analysis of K-monel 500 (Ni–Cu–Al alloy) bolts. Eng. Fail. Anal. 2000, 7, 323–332.
- Harris, Z.D.; Burns, J.T. The effect of isothermal heat treatment on hydrogen environment-assisted cracking susceptibility in Monel K-500. Mater. Sci. Eng. A 2019, 764, 138249.
- Ramkumar, K.D.; Joshi, V.; Pandit, S.; Agrawal, M.; Kumar, O.S.; Periwal, S.; Manikandan, M.; Arivazhagan, N. Investigations on the microstructure and mechanical properties of multi-pass pulsed current gas tungsten arc weldments of Monel 400 and Hastelloy C276. Mater. Des. 2014, 64, 775–782.
- Mani, C.; Karthikeyan, R.; Kannan, S.; Periasamy, C. Optimization of tensile properties of 316L stainless steel and Monel 400 weld joints using genetic algorithm. Mater. Today: Proc. 2020, 27, 2846–2851.
- Heidarzadeh, A.; Chabok, A.; Pei, Y. Friction stir welding of Monel alloy at different heat input conditions: Microstructural mechanisms and tensile behavior. Mater. Lett. 2019, 245, 94–97.
- Available online: https://markets.businessinsider.com/commodities/nickel-price (accessed on 21 August 2020).
- Available online: https://www.fastmarkets.com/commodities/exchange-data/lme-base-metal-prices-and-charts (accessed on 21 August 2020).
- Available online: https://www.superiorsteel.in/blog/monel-alloy-400-k500-price-list (accessed on 21 August 2020).
- Davis, J.R. Introduction to Nickel and Nickel Alloys. In Nickel, Cobalt, and Their Alloys; ASM Specialty Handbook; ASM International: Materials Park, OH, USA, 2000; Chapter 1.
- High-Performance Alloys for Resistance to Aqueous Corrosion, Special Metals Corporation, Publication Number SMC-026. 2000. Available online: https://www.scribd.com/document/55871734/Nickel-Base (accessed on 27 August 2020).
- Stoloff, N.S. Wrought and P/M superalloys. In Properties and Selection: Irons, Steels, and High-Performance Alloys; ASM Handbook; ASM International: Materials Park, OH, USA, 1990; Volume 1, pp. 950–980.
- Lippold, J.C.; DuPont, J.; Kiser, S.D. Precipitation-Strengthened Nickel-Base Alloys. In Welding Metallurgy and Weldability of Nickel-Base Alloys; John Wiley & Sons: Hoboken, NJ, USA, 2009; Chapter 4; pp. 157–254.
- Monel Alloy 400, High Performance Alloys. Available online: http://www.hpalloy.com (accessed on 27 August 2020).
- Health Hazard Evaluation Report: HETA-97-0141-2819; Special Metals Corporation, Princeton Powder Division: Princeton, KY, USA, 2001.
- Monel Alloy R-405, High Performance Alloys. Available online: http://www.hpalloy.com (accessed on 27 August 2020).
- Dutta, R. Corrosion aspects of Ni–Cr–Fe based and Ni–Cu based steam generator tube materials. J. Nucl. Mater. 2009, 393, 343–349.
- Prabhu, A.G.; Sathishkumar, N.; Pravinkumar, K.; Kumar, P.M.; Balasubramanian, T.; Sudharsan, P. Heat treatment and analysis of nickel super alloy for gas turbine applications. Mater. Today Proc. 2020.
- Ghosh, S.; Dey, G.; Dusane, R.; Grover, A. Improved pitting corrosion behaviour of electrodeposited nanocrystalline Ni–Cu alloys in 3.0wt.% NaCl solution. J. Alloy. Compd. 2006, 426, 235–243.
- Silaimani, S.M.; Vivekanandan, G.; Veeramani, P. Nano-nickel–copper alloy deposit for improved corrosion resistance in marine environment. Int. J. Environ. Sci. Technol. 2014, 12, 2299–2306.
- Monel Alloy 401; Publication SMC-084; Special Metals Corporation: New Hartford, NY, USA, 2004.
- Monel Alloy 404; Publication SMC-059; Special Metals Corporation: New Hartford, NY, USA, 2004.
- Nady, H.; Negem, M. Ni–Cu nano-crystalline alloys for efficient electrochemical hydrogen production in acid water. RSC Adv. 2016, 6, 51111–51119.
- Wang, G.; Li, W.; Huang, B.; Xiao, L.; Lu, J.; Zhuang, L. Exploring the composition—Activity relation of Ni–Cu binary alloy electrocatalysts for hydrogen oxidation reaction in alkaline media. ACS Appl. Energy Mater. 2019, 2, 3160–3165.
- Monel Alloy K-500; Publication SMC-062; Special Metals Corporation: New Hartford, NY, USA, 2004.
- Watson, J.F.; Christian, J.L. Low-Temperature Properties of K-monel, inconel-X, Rene´ 41, Haynes 25, and Hastelloy B Sheet alloys. J. Basic Eng. 1962, 84, 265–277.
- Hurlich, A. Low Temperature Materials. In Chemical Engineering; McGraw-Hill Publishing Go. Inc.: New York, NY, USA, 1963; pp. 311–325.
- Ramkumar, K.D.; Arivazhagan, N.; Narayanan, S.; Mishra, D. Hot corrosion behavior of monel 400 and AISI 304 dissimilar weldments exposed in the molten salt environment containing Na2SO4 + 60% V2O5 at 600 °C. Mater. Res. 2014, 17, 1273–1284.
- Raffeis, I.; Adjei-Kyeremeh, F.; Vroomen, U.; Westhoff, E.; Bremen, S.; Hohoi, A.; Bührig-Polaczek, A. Qualification of a Ni–Cu Alloy for the Laser Powder Bed Fusion Process (LPBF): Its Microstructure and Mechanical Properties. Appl. Sci. 2020, 10, 3401.
- Marenych, O.; Ding, D.; Pan, Z.; Kostryzhev, A.G.; Li, H.; Van Duin, S. Effect of chemical composition on microstructure, strength and wear resistance of wire deposited Ni-Cu alloys. Addit. Manuf. 2018, 24, 30–36.