Positron lifetime evolution during room temperature ageing in Al-Zn-Mg-(Cu)

1.7Zn-1.9Mg, Al-1.7Zn-1.9Mg-0.2Cu , Al-1.7Zn-1.9Mg- 0.7Cu (at.%) and is the third in a sequence of studies aim- ing at clarifying the role of solute clusters in hardening phenomena and, more specifically, the action of Cu in the formation of the clusters. The first work of this sequence (2), based on atom-probe tomography (APT), show statis- tically significant tendency to separate clustering of the solute elements in as-quenched (AQ) specimens, whereas co-clustering of different atomic species is observed after ageing. In the second work (5), coincidence Doppler broadening (CDB) spectroscopy was used to investigate on the vacancy association with the solute clusters. It was found that Cu helps in retaining quenched-in vacancies, as demonstrated by a strong increase of the trapping fraction (percentage of positron annihilated after trapping at open- volume defects). The environment of the vacancies in AQ samples, and even more in artificially aged samples, is ri- cher in solute than in Al; however, the increased trapping due to Cu is also correlated with a moderate decrease of the solute fraction in contact with the vacancies. In the pre- sent work, positron annihilation lifetime spectroscopy (PALS) is applied to study the evolution of the vacancy- solute aggregates at room temperature (RT), both for AQ samples and in condition of secondary ageing (6). In most previous PALS studies of RT ageing in Al alloys, the posi- tron lifetime is shown to decrease monotonically during RT ageing, asymptotically converging toward a plateau. However, recent studies (4, 6) have revealed a non- monotonic evolution of the positron lifetime. In these al- loys, before the "normal" asymptotic decline, the average positron lifetime displays a rising branch leading to a maximum. This result has been attributed to initial positron trapping in almost pure Zn clusters, followed by trapping in vacancy-Mg-Zn co-clusters (4). The decreasing branch The effect on the formation of vacancy-solute clusters by the addition of small percentages of Cu to Al-Zn-Mg has been studied by positron annihilation lifetime spectroscopy. The results document the ability of Cu to increase the number of vacancies retained after quenching, by forming an additional population of vacancy-solute clusters in competition with pure Zn clusters. It has also been shown that Cu accelerates the decomposition of the alloy at 150°, thereby leaving a re- duced supersaturation of the matrix after an early interrup- tion. An unexpected result is that the curve positron lifetime vs, ageing time at RT displays the symptoms of a change in the nature of the coherent aggregates that occurs in about 10 hours of RT exposure.