Ni-Cu interdiffusion and its implication for ageing in Ni-coated Cu conductors

After heat treatment at 400 °C the effective resistivities of typical Ni-coated Cu conductor wires increased by up to 6.9% as a result of Ni-Cu interdiffusion. Direct Ni-Cu interdiffusion experiments were performed between metal foils at temperatures of 400-600 °C for times up to 192 h. Calculated activation energies were in range 80-90 kJ mol−1, consistent with a grain boundary diffusion mechanism. Analysis of published Ni-Cu interdiffusion coefficients suggested a clear dependence on grain size and grain shape. A concentric circle model was developed to simulate changes in composition and effective resistivity in the Ni-Cu wires as a function of time. It was predicted that it would take 1.4 × 105 h at 400 °C for 10% increase in the effective resistivity of an AWG18-Class27 conductor wire. Good agreement between simulated and experimental data for effective resistivity was only achieved by employing effective diffusion coefficients corrected for microstructural effects.