Quantitative measurements of dynamic precipitation during fatigue of an Al-Zn-Mg-(Cu) alloy using small-angle X-ray scattering

The room temperature cyclic deformation behaviour of an Al-Zn-Mg-(Cu) alloy was examined in the under-aged and peak-aged states. The tests were performed under constant plastic strain amplitude loading in tension-compression mode, and the effect of cumulative strain (number of cycles), strain rate and plastic strain amplitude was examined. Pronounced cyclic hardening was observed in the under-aged states, and characterization of the microstructure using small-angle X-ray scattering illustrated that this was due to profuse dynamic precipitation at room temperature during cycling. Regardless of deformation conditions, the radius of the dynamically formed GP zones remained constant at 4-5 Å, and only the volume fraction of the zones increased during cycling. The strain, strain rate and plastic strain amplitude dependence of the evolution of the volume fraction of zones forming dynamically was interpreted in terms of a nucleation-dominated reaction, and it is shown that the behaviour can be satisfactorily rationalized when the role of strain-induced vacancies is considered.