Lattice and grain boundary diffusion of copper into cladding on Alclad AI–Cu-Mg alloy sheet

The diffusion behaviour of copper into the relatively pure aluminium cladding layer on commercial Al-Cu-Mg alloys 2014 and 2024 at 400–500°C was characterised by measurements of etch marker movement and concentration profiles. The latter method gave lattice interdiffusion coefficients that agreed well with published data; the former provided information on grain boundary diffusion: an activation energy of 77.5–90 kJ mol⁻¹ and diffusion coefficients (1–3) × 10⁵ times greater than those for lattice diffusion. For grain sizes of 25–50 μm, boundary diffusion was of primary importance at long diffusion times, with bulk diffusion amenable to analysis by the Hart equation. Clad layers that are completely ‘penetrated’ according to marker movement continue to provide galvanic protection to the core alloys in the T4 temper; however, the T6 temper is susceptible to core alloy corrosion.