High-performance copper alloys for modern technology applications

Recent studies show that high-strength copper alloys offer excellent stability in hydrogen-rich environments, making them a promising alternative to other materials in the promising hydrogen technology of the future. In contrast to competing materials such as high-strength steels, CuBe alloys show no susceptibility to hydrogen embrittlement, despite their high strength. In addition, they provide an effective barrier against the penetration and diffusion of hydrogen. These alloys and the latest findings on them are discussed below.

Alloys and property profiles  

Copper-beryllium alloys, also known as CuBe alloys, are an established standard in materials technology and have become indispensable in many areas of application. In general, these alloys can be divided into two main groups: those that are designed for extremely high strength and those that offer high electrical conductivity.

High-strength alloys include Alloy 25 (C17200) and Alloy M25 (C17300), for example. These contain beryllium in a range of 1.80 to 2.00% and offer exceptional mechanical properties. Alloy M25 is additionally alloyed with lead, which improves the machining properties in turning, drilling and milling processes. In the hardened state, Alloy 25 and Alloy M25 achieve the highest strength and hardness levels of all commercially available copper-based alloys, making them particularly valuable for demanding technical applications.

The CuBe alloys, which achieve lower strengths but higher conductivities in comparison, are generally alloyed with lower beryllium contents and contain additional elements such as cobalt and/or nickel. For example, the beryllium content of alloys Alloy 3 (C17510) and Alloy 10 (C17500) is 0.4 to 0.7 %. To avoid a drop in strength at higher temperatures, the alloy Alloy 10X was developed, which, in comparison to Alloy 10, is additionally alloyed with low levels of zirconium and thus achieves high strength and ductility in a temperature range up to approx. 430°C. The following table shows the most important characteristic values for the alloys mentioned. 

To achieve the high strength values of CuBe alloys, a specific precipitation hardening process is required. This process takes place in a precisely defined temperature range. Temperature hardening is often combined with cold forming to further optimize the mechanical properties, particularly in the production of strips.

Semi-finished products such as strips, which are used to manufacture the final components, are often subject to further forming processes such as crimping or extrusion. In such cases, heat treatment can also take place after the components have been manufactured. This means that the components are initially produced in a softer, more easily formable state, which enables the realization of complex geometries. The subsequent precipitation hardening takes place directly on the component and produces the desired high strength.

This method is very similar to piece tempering, which is used for steel components, where the final mechanical properties of the component are also set by subsequent heat treatment. This technique enables efficient and material-saving production of complex components with high material performance. 

Due to their resistance to stress relaxation - even at higher temperatures - these high-strength alloys are often used in connectors, contacts, spring elements and switches in the electronics industry.4) Even at cryogenic temperatures, the strength and elongation properties are very stable; there is no embrittlement whatsoever. Furthermore, the alloys are used as electromagnetic shielding seals (EMI/RFI) due to their electrical and magnetic properties and, usually at the same time, as heat sinks to protect sensitive electronic components due to their thermal conductivity. 

Another important area of application for CuBe alloys is friction wear applications. The combination of a relatively low modulus of elasticity and high yield strength ensures a high resilience value, which, for example, leads to lower local stresses in plain bearings and thus to longer service lives compared to other bearing materials, which in turn results in longer maintenance intervals, e.g. in construction machinery or machines in underground operation. In addition, these alloys are inherently resistant to abrasion and wear and enable material pairings that are characterized by minimal friction and surface damage. Threaded connections made from alloy 25 with stainless steels showed resistance to abrasion and cold galling. (4)

The special chemical composition of copper-beryllium alloys leads to the formation of beryllium-containing oxide layers, which give these alloys a higher corrosion resistance than most other copper-based alloys. In particular, CuBe alloys exhibit high resistance to many aqueous solutions of most alkali hydroxides, both at low and high temperatures.

However, like other copper-based alloys, certain media can be problematic. In particular, ammonium hydroxide and ammonia, if not dry or oxygenated, pose challenges. However, CuBe alloys are stable in cold concentrated sulphuric acid, both cold and warm dilute sulphuric acid, hydrofluoric acid and cold dilute hydrochloric acid.

A significant advantage of these alloys over stainless steels is their resistance to stress corrosion cracking, which is typically triggered by chlorine ions. In addition, CuBe alloys offer high resistance to biofouling, which makes them particularly suitable for special applications such as interference pass port seals in underwater telecommunications. These properties make CuBe alloys a valuable choice for applications that require high corrosion resistance and reliability. (4)

Behavior in hydrogen-containing environments 

The best-known problem when using materials in hydrogen-containing atmospheres is that of hydrogen embrittlement. The susceptibility of the material used is determined by a critical interaction between the material (alloy composition, microstructure, strength condition), the hydrogen environment and the external or internal stresses in the component. This combination then results in a deterioration of the strength and toughness properties of the material, which in turn can lead to catastrophic component damage and corresponding failures. The term hydrogen embrittlement covers various phenomena (5), which in most cases affect high-strength metallic materials. The problem with hydrogen embrittlement is that it can occur during the production of the materials, but also later during component production or - in the worst case - during operation. In contrast to other material defects, not every component or individual material batches have to be affected, but individual components from individual batches can also show this damage pattern. In addition, measuring hydrogen, especially in components, is quite difficult and usually not feasible in a series-accompanying test.

Comparative tests are crucial to ensure the stability of materials before they are used in hydrogen environments. In an extensive research program carried out in collaboration with the Ruhr University Bochum and the DECHEMA Research Institute, the stability of various high-strength copper-based alloys was investigated. The results for the CuBe alloy Alloy 25 were particularly positive.

The investigations showed that no significant hydrogen content could be detected in the material after longer loading times under gaseous and electrochemical conditions. This indicates that Alloy 25 does not show any significant hydrogen absorption. Furthermore, the hydrogen loading had no negative influence on the mechanical properties of the alloy in the quasi-static tensile test. This result underlines the resistance of Alloy 25 to hydrogen embrittlement, a common problem with many other metallic materials under similar conditions.

Such findings are of great importance as they confirm the suitability of Alloy 25 for applications in hydrogen-rich environments and thus ensure the reliability and safety of components under extreme conditions.

Diffusion measurements using a permeation cell also showed that not only does hardly any hydrogen penetrate the material, but that there is also no measurable diffusion through the material. The alloy can therefore be seen as a barrier to hydrogen ingress or passage and can therefore be considered for sealing components in the field of hydrogen technology. Material combinations that can be realized via cladding processes are also conceivable. (6, 7) 

Safety and environment 

In connection with beryllium-containing alloys, the question of safety or environmental aspects or restrictions is repeatedly raised, which will be briefly discussed here. Basically, the use of copper-beryllium alloys is not prohibited or restricted by any country worldwide. 

Copper-beryllium alloys were not included or even restricted in the final EU directives on end-of-life vehicles (ELV), on the restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS) or on waste electrical and electronic equipment (WEEE). An evaluation commissioned by the European Commission concluded that beryllium should not be included in the RoHS list and that the corresponding alloys should continue to be permitted in electrical and electronic equipment (EEE). 

Beryllium and beryllium oxide have been registered in REACH. However, beryllium is not on the list of substances of very high concern (SVHC) that are eligible for authorization. Beryllium was reviewed in 2011/2012 within the framework of REACH. The German BAuA carried out this review and concluded that there is no risk in the end use of beryllium, but only in industrial processes. BAuA and the EU concluded that a binding occupational exposure limit value is sufficient to reduce the risk. 

This binding occupational exposure limit (OEL) has been laid down in the CAD/CMRD Directive on Chemical Agents and the Directive on Carcinogens and Mutagens since 2019. Corresponding recommendations regarding technology and implementation also exist for various manufacturing processes. (8) 

At this point, it should also be mentioned that beryllium is one of the raw materials currently classified as critical by the European Commission. Materials are classified as “critical” (critical raw materials) if the risks of supply bottlenecks and/or their impact on the economy are high compared to most other materials, as these materials cannot simply be substituted by other materials.

Summary  

Copper-beryllium alloys are characterized by an exceptional property profile that makes them particularly versatile. These alloys combine extremely high strength over a wide temperature range with special friction properties, high corrosion resistance and excellent stress relaxation and wear resistance. At the same time, they retain the typically good conductivity properties of copper-based alloys.

Thanks to compliance with REACH and RoHS guidelines during production, these materials are available worldwide and can be used without restriction. Extended test programs have also shown that copper-beryllium alloys have a high resistance to hydrogen embrittlement and can effectively serve as a barrier against the entry and diffusion of hydrogen. These properties make them a safe choice for applications in hydrogen-rich environments.

Due to this extensive and advantageous property profile, copper-beryllium alloys are already widely used in numerous industrial sectors, including sealing technology. Their resistance and barrier effect in hydrogen-containing atmospheres also offer considerable potential for new applications in hydrogen technology, which is increasingly being promoted by politicians. This is reflected in the constant development of new applications, which underlines the importance and value of these alloys in current and future technological applications.