Amongst commercial metals, copper and aluminium have the lowest electrical resistance and thus make good electrical conductors. In fact, all metals are benchmarked on copper’s electrical conductivity – in 1913 the International Electrotechnical Commission defined the International Annealed Copper Standard (IACS) in terms of copper’s electrical conductivity, setting it at 100 % IACS (58.00 MS/m). This does not mean copper has no resistance, but rather that it is the standard by which other materials are measured. An aluminium wire must be larger than a copper wire to achieve the same efficiency. As copper is twice as strong as aluminium, the cross-section of an aluminium wire must be twice as large as the copper one to get the same load-bearing capacity. Generally, copper is the first choice for electronic and telecom wires, whereas aluminium, a lighter-weight and lower-cost material, is suitable for overhead power lines.
Combining materials in new ways
Material design strategies inspired by natural biological systems can introduce shapes and arrangements of the building blocks that enable new degrees of freedom for hybrids. The EU-funded HybridMat project demonstrated hybrid materials with spiral inner architecture that possess exceptional physical and mechanical properties. The hybrid combines copper’s high strength and high conductivity with aluminium’s light weight and low cost. Researchers investigated many different helix parameters to identify which design leads to enhanced strength and conductivity in hybrid conductors. Special attention was dedicated to the evolution of intermetallic phases formed at the aluminium-copper interface during processing. This type of intermetallic alloy is known to exhibit lower conductivity than the respective pure components, which, in addition to their intrinsic brittleness, can compromise the beneficial effects of the hybrid material. The project team also developed a new analytical model to predict the effective electrical conductivity of hybrid samples. The model takes into account the presence of an intermetallic layer. Contrary to the rule of mixtures, the model depends on two parameters – copper’s volume fraction and the geometry of the helical constituent. “The model proved instrumental in assessing how the helix parameters and the interface width affect the effective conductivity of the hybrid samples and is thus suitable for the optimal design of hybrid conductors,” explains Marie-Curie Fellow Rimma Lapovok.
Metal deformation at the nanoscale
Significant effort was put in developing severe plastic deformation (SPD) techniques – metal-forming processes which impart shear strain on bulk metal materials. The shear strain results in extreme ultrafine grain sizes down to the nanometre range. “SPD opens up new opportunities for producing hybrid materials with enhanced physical and mechanical properties, e.g. high strength and high electrical conductivity, for targeted industrial applications. We showed that hybrid samples with helical reinforcement exhibit higher strength, increased load-bearing capacity and higher strain hardening during deformation compared to samples with straight parallel reinforcement,” concludes Lapovok.
HybridMat, copper, hybrid material, aluminium, conductivity, strength, load-bearing capacity, severe plastic deformation (SPD)