Ken Takata

Precipitation Behaviour of Mg and Si at RT in Pre-Aged Al-Mg-Si Alloys

The precipitation behavior of Mg and Si during storage at RT in Al-Mg-Si alloys pre-aged at 90°C was studied using a tensile test and differential scanning calorimetry (DSC) measurement. Specimens were solutionized at 530°C, water-quenched and then pre-aged for 2, 6 and 12 hours at 90°C during which small precipitates were formed. In the pre-aged alloy, the strengthening rate at RT has two stages. In the initial stage, the yield strength increases slowly with the aging time and in the final stage, it increases rapidly. In the initial stage, the strength in the pre-aged alloy is smaller than that in the non pre-aged alloy, while in the final stage, the strength in the pre-aged alloy is larger than that in the non pre-aged alloy. Furthermore, the period of the initial stage is dependent on the pre-aging period at 90°C. The DSC curves of alloys in the initial stage do not show the presence of clusters, while those in the final stage do. It seems that in the initial stage Mg and Si atoms accumulate around the small precipitates that have been formed in pre-aging at 90°C while in the final stage, the clusters of these atoms are formed.

Precipitation Morphology and Tensile Properties of Al‐0.62Mg‐0.32Si Alloy Aged at Low Temperature

The influence of aging precipitates on tensile properties and dislocation substructure in Al-0.62Mg-0.32Si (mass %) alloy has been investigated by tensile tests and microstructural observations. Cold-rolled sheets of the alloy were aged under the following conditions: 453 K for 20 h (sample A), 373 K for 120 h (sample Bl), 373 K for 7200 h (sample Bh) and 323 K for 14688 h (sample C). Values of Vickers hardness of samples A, Bl and C were about 75 HV. However, characters of precipitates were different among the samples: needle-like β′′ precipitates about 50 nm in length and 4 nm in diameter were observed in sample A using transmission electron microscopy (TEM), and Mg-Si clusters below 2 nm in size were observed in samples Bl and C by three-dimensional atom probe (3DAP). In sample Bh showing the maximum hardness (103HV) after 373 K aging, fine β′′ particles about 5 nm in length were observed. Samples A and Bh with the β′′ precipitation exhibited higher values of yield stress than those for samples Bl and C, and the β′′ precipitates pinned dislocations. On the other hand, samples Bl and C with the cluster formation had excellent ductility and showed different dislocation substructure from each other. Electron tomography observations revealed that dislocations in sample Bl was wavy in three dimensions, while dislocations in sample C tended to be linear in shape. These results suggested that the tensile properties and dislocation motion were dependent on the aging products, such as clusters and β′′ precipitates.