Copper-nickel alloys are alloys of copper (base metal with the largest single content) and nickel with or without other elements, but in any case the zinc content must not exceed 1 %. If other elements are present, nickel has the largest single content after copper compared to any other element.
As with other copper materials, a distinction must be made between wrought alloys, which are processed into semi-finished products, and cast alloys, from which castings are produced by various casting processes. In addition to 8.5 to 45 % Ni, the common alloys usually contain manganese, iron a nd tin to improve certain properties; the cast alloys also mainly contain additives of niobium and silicon.
Copper-nickel alloys have interesting physical properties, good strength characteristics – even under continuous stress and elevated temperatures – as well as high corrosion resistance to many media – especially seawater. The properties of the binary copper-nickel alloys are not yet sufficient for some applications. Some additives decisively improve certain properties of the copper-nickel alloys. Of the additional alloying elements, manganese, iron and tin in particular, as well as niobium and silicon, also chromium, beryllium and aluminium are technically significant. At low temperatures, copper-nickel alloys, like other copper materials, have excellent strength properties: tensile strength increases as the temperature drops, without any noticeable reduction in elongation at break or necking. These alloys therefore show no embrittlement at low temperatures. Therefore, they are very well suited for applications in cryogenics as well as for other low-temperature applications in marine and off-shore technology or in liquefied natural gas (LNG) technology and plants.
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Nickel has a decisive influence on the colour of copper-nickel alloys. The copper colour becomes brighter with increasing nickel addition. From about 15 % Ni, the alloys are almost silvery white. The lustre and purity of the colour increase with the nickel content; from about 40 % Ni, the polished surface can hardly be distinguished from that of the silver. The density of copper (8.93 kg/dm3 at 20°C) is only slightly changed by increasing nickel content (density of nickel at 20°C = 8.90 kg/dm3) and is 8.9 kg/dm3 for all copper-nickel alloys. The high thermal conductivity of pure copper of 394 W/(m × K) is strongly reduced by nickel; it reaches a minimum value of approx. 21 W/(m × K) at about 45 % Ni. The coefficient of elongation decreases with the addition of nickel, first more strongly, then more slowly. The specific heat (at 20°C) of copper is 0.385 J/(g × K) and of nickel 0.452 J/(g × K). With increasing nickel content, it initially decreases slightly and one can expect an average value of 0.377 J/(g × K). All physical properties of the two wrought copper-nickel alloys CuNi10Fe1Mn and CuNi30Mn1Fe have been studied in detail and are well known from room temperature up to 1000°C.
The electrical resistance of the copper-nickel resistance alloys is given for various temperatures in Tab. 8. It increases strongly with the nickel content, so that the copper-nickel alloys are suitable as resistance materials. A maximum occurs at approx. 45 % Ni. Approximately in the same concentration range lies the minimum of the temperature coefficient of electrical resistance. Particularly noteworthy is the high thermoelectric strength of the copper-nickel alloys in the range between 40 and 50 % Ni against other metals such as iron , copper, platinum, etc.. They are therefore particularly suitable for use in thermocouples for temperature measurements in the medium temperature range. The high thermoelectric voltage of CuNi44 precludes its use as a resistance material in low-voltage devices because the copper connections with CuNi44 form a thermocouple.
The high thermal conductivity of pure copper of 394 W/(m × K) is strongly reduced by nickel; it reaches a minimum value of approx. 21 W/(m × K) at approx. 45 % Ni. The coefficient of elongation decreases with the addition of nickel, first more strongly, then more slowly. The specific heat (at 20°C) of copper is 0.385 J/(g × K) and of nickel 0.452 J/(g × K). With increasing nickel content, it decreases slightly at first and one can expect an average value of 0.377 J/(g × K). Copper-nickel alloys still have good strength properties even at higher temperatures. Even small additions of nickel increase the high-temperature strength of the copper. The influence of the nickel content on the softening of cold-rolled copper-nickel alloys at higher temperatures is significant. The addition of iron improves the strength properties not only at room temperature but also at elevated temperatures.
Copper-nickel alloys are among the most corrosion-resistant copper materials. They are resistant to moisture, non-oxidising acids, alkalis and salt solutions, organic acids and dry gases such as oxygen, chlorine, hydrogen chloride, hydrogen fluoride, sulphur dioxide and carbon dioxide. There is also no risk of stress corrosion cracking, and the tendency to selective corrosion is also extremely low.
Copper-nickel alloys do not show ferromagnetism. Copper is diamagnetic, nickel ferromagnetic. Nickel-copper alloys pass from the diamagnetic via the paramagnetic to the ferromagnetic state with increasing nickel content. Depending on the alloy, iron has a minor influence if it is present in solid solution. If the iron is present in precipitated form, these ferromagnetic microscopic particles lead to a macroscopic increase in ferromagnetism. The precipitation-free matrix remains dia- or paramagnetic. Copper-nickel alloys with 20 to 25 % Ni and 20 % Fe or about 25 % Co are distinctly magnetic materials. Due to their high remanence and coercivity, they are also suitable for permanent magnets.
Wrought copper-nickel alloys
Strength data for sheets and strips made of wrought copper-nickel alloys are contained in DIN EN 1652. Further strength data are contained in the respective semi-finished product standards. The material condition is identified in the strength standards by adding the letter R to the alloy abbreviation followed by a number, e.g. CuNi30Mn1Fe R350. A tensile strength of at least 350 N/mm2 is guaranteed for the R350 strength condition. The strength state also determines the 0.2 % yield strength and elongation at break. By adding the letter H followed by a number, only a minimum hardness (Vickers hardness) is guaranteed, e.g. CuNi30Mn1Fe H110. Increasing tensile strength is associated with only a relatively small decrease in elongation at break and necking. In contrast, the hardness shows a strong increase with increasing nickel content. Notched impact strength is only slightly influenced by the nickel content. Iron has a favourable influence on the strength properties of copper-nickel alloys. .are achieved by increasing the iron and manganese contents to 2% each, e.g. strips and sheets made of the alloy CuNi30Fe2Mn2 have a tensile strength of 440 n/mm2 and a 0.2% yield strength of 145 n/mm2. A further increase in the strength values is achieved, for example, by adding aluminium or chromium. In addition, as with all metallic materials, the tensile strength, the 0.2 % yield strength and the hardness of wrought copper-nickel alloys increase with increasing cold forming, while the elongation at break decreases.
Copper-nickel casting alloys
Three age-hardenable copper-nickel casting alloys with additions of aluminium, chromium or beryllium should be mentioned. The alloy with about 2 % Al can be used in the as-cast state or in a hardened state. The greatest increase in strength is achieved by adding beryllium – after age hardening. Such an alloy is already used in marine engineering in the USA. Age-hardenable copper-nickel casting alloys of high strength with tin contents of up to 6 %, which usually contain other additives such as lead and zinc, are standardised in ASTMN 584.
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