Abstract The mechanical properties of age-hardenable aluminum alloys are strongly influenced by the volume fraction, size and spacing of hardening precipitates. In addition, researches have shown that interrupted ageing (T6I4) could benefit the hardening response of these alloys. Interrupted ageing is a term generally used for the microstructural development process of an alloy aged at a reduced temperature after partially aged at a higher temperature. Thus, this process produces a change in the mechanical properties of the alloy. In this study, the precipitate structures and mechanical properties of AA6351 and AA7050 alloys were investigated. Transmission electron microscopy analyses of the hardening precipitates were carried out. Hardness and tensile properties were performed to compare the effect of disrupted ageing on AA6351-T6 / AA6351-T6I4 and AA7050-T7451 / AA7050-T6I4 alloys. AA6351-T6I4 showed a higher volumetric fraction of hardening precipitates with heterogeneous size and resistance lower than AA6351-T6. In addition, AA7050-T6I4 showed a higher volumetric fraction of hardening precipitates with smaller size and similar resistance to AA7050-T7451. However, in both cases, interrupted ageing contributed to increase ductility and toughness.

The influences of interrupted ageing and retrogression and re-ageing heat treatments on fatigue crack growth with a single applied overload in 7050 aluminum alloy were investigated. The main aims were: to evaluate the influence of these heat treatment conditions on tensile properties, mainly fatigue crack growth rate and fracture process in the low-moderate ΔK region; to determine how both ageing treatments generate bimodal microstructure features as a contribution of the two conditions on the crack closure phenomenon; and the influence of the single applied overload to fatigue crack growth retardation. The results show that by using the RRA heat treatment condition it is possible to enhance the ductility without detrimental effects on the yield stress. T6I4-65 and RRA conditions contributed to enhancement of the crack closure phenomenon in the low-moderate ΔK region, and, accordingly, produced a lower fatigue crack growth rate in relation to the traditional T7451 condition.

Abstract The quantification of nanometric precipitates in Al-Zn-Mg-Cu alloys has been performed by a series of experimental techniques. Especially in the AA7050 alloy, after ageing heat treatment, the particles responsible for the hardening become very thin. Typically, these precipitates of nanometric sizes are mainly characterized by transmission electron microscopy (TEM), which in this particular case, requires a very meticulous preparation. This study investigated a possible alternative quantification of the precipitates by atomic force microscopy (AFM) to complement the technique by TEM. For this, three conditions for heat treatment of an aged aluminium alloy AA7050 were therefore chosen to produce different density and sizes of precipitates. The experimental results showed that the AFM technique proved to be a valid qualitative tool and may complement the results obtained by TEM an exploratory analysis for the microstructures.