The binding between vacancies and Mg atoms in an aluminum solid solution is not fully understood but essential for understanding its role in age hardening of many Al alloys. After annealing and quenching, Mg prevents the loss of excess vacancies during natural ageing and forms complexes containing one, possibly two, vacancies, and various Mg atoms. By heating the alloy after natural ageing, these complexes are dissolved, i.e., natural ageing is reverted. This reversion process is studied by in situ positron annihilation lifetime spectroscopy utilizing the very high count rate at the accelerator driven facility ELBE. Positron spectra are continuously acquired during heating at rates between 3 and 50 K min −1 . After correcting for the contributions of the oxidized surface and decomposing spectra into components, the process can be followed in detail and is found to take place in distinct stages: first, the number of vacancy–Mg complexes is reduced and then the liberated vacancies agglomerate into clusters that eventually dissolve at even higher temperatures.

Studies of the kinetics of precipitation in aluminium alloys help to understand the role of quenched-in excess vacancies in the ageing process. Investigations of ageing in nanometre-sized samples are valuable because excess vacancies are believed to anneal out quickly while the alloy is still supersaturated with solute atoms. We prepare samples suitable for atom probe tomography (APT) with a sharp tip of some tens of nm radius and verify that they can be solutionised and quenched, after which they are still suitable for APT. The investigated Al-Zn alloy is known to show extremely strong clustering after quenching of the bulk material but we find that this clustering is completely suppressed in solutionised and quenched samples. Precipitation calculations explain this by a fast loss of vacancies, within seconds, during and after quenching. As such calculations also suggest that at elevated temperatures clustering could occur in the thermal equilibrium, we carry out exploratory ageing experiments on APT samples at 100°C and 135°C. No clusters are found in such samples, which could be related to experimental restrictions (Zn losses) or unfavourable nucleation conditions in nm-sized samples.

Al-Mg-Si alloys with total solute contents ranging from 0.8 to 1.4 wt % were solutionized, quenched, and then artificially aged (AA) at 180 degrees C, after which positron annihilation lifetime spectroscopy was applied to obtain information about precipitation and vacancy evolution during the preceding ageing step. Hardness and electrical resistivity measurements were carried out to complement these measurements. AA was carried out in four different heating media, which allowed for varying the average heating rate from 2.4 to 170 K s(-1). The main result of the study is that there is a competition between vacancy losses and precipitation. Any precipitation taking place during quenching or during heating to the AA temperature helps to prevent vacancies from going to sinks and allows them to assist in solute clustering. Higher solute content, slower heating to 180 degrees C, and natural preageing before AA were found to have a comparable effect.

Precipitation hardening involves solutionising, quenching and annealing steps, the latter often at various temperatures. The phenomena observed in Al-Mg-Si alloys are very complicated and partially not well understood. During and after quenching, solute atoms diffuse through the lattice assisted by vacancies and form atom clusters that gradually grow. These act back onto vacancies, which complicates the situation. We apply positron annihilation techniques in addition to traditional hardness, resistivity and thermal measurements to clarify what happens in various stages of thermal treatment: The quenching process can be divided into a stage of vacancy loss and of precipitation. Very short artificial ageing treatments after heating at different rates show that there is a competition between vacancy losses and cluster formation as the temperature increases. The difference between natural ageing and artificial ageing can be defined based on the importance of excess vacancies. Based on such results the behaviour of “invisible” objects such as vacancies and small clusters can be better understood but some open question remain such as the kinetics of secondary ageing or the details of the negative effect of natural ageing on artificial ageing.

Vacancy and solute atom clusters of various sizes and number densities may be formed in Al alloys during natural ageing (NA) after solutionising and quenching. We study the clustering characteristics of a series of Al–Mg–Si alloys and discuss ageing mechanisms as a function of solute content: in alloys low in Mg (i.e. <0.1 at.%), vacancies tend to aggregate during quenching and subsequently assist the formation of Si-rich clusters in the course of NA. For alloys with higher Mg contents (>0.1 at.%), clustering is initially governed by Si atoms and then by Mg as NA proceeds.

Al-Mg-Si alloys with total solute contents ranging from 0.8 to 1.4 wt.% were solutionised, quenched and then artificially aged (AA) at 180 {\deg}C, after which positron annihilation lifetime spectroscopy was applied to obtain information about precipitation and vacancy evolution during preceding ageing. Hardness and electrical resistivity measurements were carried out to complement these measurements. AA was carried out in four different heating media, which allowed for varying the heating rate from 2.4 K/s to 170 K/s. The main result of the study is that there is a competition between vacancy losses and precipitation. Any precipitation taking place during quenching or during heating to the AA temperature helps to prevent vacancies from going to sinks and allows them to assist in solute clustering. Higher solute content, slower heating to 180 {\deg}C and natural pre-ageing before AA were found to have a comparable effect.

Alloy 5182 has been extensively used in the O temper for automotive sheet parts requiring high formability and moderate strength. The grain size of the sheet has been shown to impact strength, formability, and Lüdering behavior during forming. The present study examined the effect of heating rate on the recrystallization behavior of the alloy. Various heating rates, recovery treatments and annealing temperatures were used to manipulate the final grain size. Metallographic observations, EBSD and hardness tests were used during the research. The results are discussed in terms of operative recrystallization mechanisms for this alloy.