Long-term Natural Aging Research Articles

Pre-precipitate clusters and precipitation processes in Al–Mg–Si alloys

The solute clusters and the metastable precipitates in aged Al–Mg–Si alloys have been characterized by a three-dimensional atom probe (3DAP) and transmission electron microscopy (TEM). After long-term natural aging, Mg–Si co-clusters have been detected in addition to separate Si and Mg atom clusters. The particle density of β″ after 10 h artificial aging at 175°C varies depending on pre-aging conditions, i.e. pre-aging at 70°C increases the number density of the β″ precipitates, whereas natural aging reduces it. This suggests that the spherical GP zones formed at 70°C serve as nucleation sites for the β″ in the subsequent artificial aging, whereas co-clusters formed at room temperature do not. Atom probe analysis results have revealed that the Mg:Si ratios of the GP zones and the β″ precipitates in the alloy with excess amount of Si are 1:1, whereas those in the Al–Mg 2Si quasi-binary alloy are 2:1. Based on these results, the characteristic two-step age-hardening behavior in Al–Mg–Si alloys is discussed.

The Effect of Grain-Boundary Structure Formation on β-Precipitation in Aged Al-Mg Alloys

Microstructure and mechanical properties evolution during long-term natural aging has been studied by means of optical and transmission electron microscopy as well as tensile tests and corrosion resistance measurements. It was found that the sharp decrease of plasticity and corrosion resistance during aging results from film like grain - boundary β' - and β - phases precipitation. Special thermomechanical and thermocyclic treatments have been worked out to produce a desired grain - boundary structure that affect the grain - boundary precipitation and alloys properties. The results have been demonstrated that grain - boundary precipitation of Al - Mg alloys can be controlled by prior deformation and recrystallization. The creation of special boundaries with a high density of coincident sites or low - defect faceted boundaries remain free of precipitation during subsequent aging. Pure grain boundaries provide stable ductility and corrosion resistance.

GP zones and precipitate morphology in aged Al-Mg alloys

Abstract Ageing behaviour of binary Al-Mg alloys has been examined extensively using differential scanning calorimetry and scanning, optical and electron microscopy. From the results of this study, one can conclude that GP zones can form only after a long-term natural ageing of Al-8 wt% Mg alloy and β (Al3Mg2) phase at higher-temperature ageing. Particles of β phase can have different morphology and exceptionally nucleate at the β′/matrix interface.

Decrease of Ductility in Al-10 Pct Mg Alloys During Long-Term Natural Aging

The changes in mechanical properties of Al-10 pct Mg casting alloys during natural aging up to about 13 years were investigated. An elongation of more than 20 pct in specimens naturally aged for less than 3 months fell to only 1 ∼ 2 pct due to natural aging over ten years. By reversion experiments and electron microscopy, it was shown that this large decrease in ductility was caused by the formation of spherical coherent GP zones during the process of natural aging. TEM investigations also indicated that the structure of GP zone appears to be the Ll2 structure, in which Al and Mg atoms alternately align in three-dimensional periodicity along the [100] direction.