The Effect of an External applied Far Field Tensile Stress on the Early Stages of Ageing in a 7475 Aerospace Alloy

The effect of aging under an externally applied stress on the microstructure development of strengthening phases in Al-alloys is of considerable practical interest to the age, or creep - forming process. Although considerable research has been carried out on the effects of an applied stress during ageing Al-Cu and 2xxx series alloys, there are hardly any results reported in the literature of similar effects in 7xxxx series alloys, which are of more interest to the civil aerospace industry for creep forming applications.The main aim of the work presented was to investigate if similar phenomena, to those that are known to occur under the application of an external stress during ageing 2xxx series alloys, can be observed in a typical 7xxx aerospace alloy. The specific focus of the work was on the early stages of ageing, where ~70% of the creep relaxation takes place. In addition, any stress-related interactions during the early stages of GP zone formation could potentially affect subsequent precipitation of the important ?????? phase in this system. To this end, a 7475 alloy was aged under different levels of constant tensile stress. The microstructures of the stress aged samples were thoroughly investigated by means of 3DAP, HRTEM, Small Angle X-ray Scattering (SAXS) and conventional TEM techniques. The results show that during the early stages of ageing, under an applied stress, observable interactions can take place that preferentially align and promote plate like GP-II zones, as well as affecting their size, stoichiometry, and volume fraction. However, these effects were not found to carry through to greatly influence the subsequent artificial ageing behaviour.