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 B-L), 373 K for 7200 h (sample B-H) and 323 K for 14688 h (sample C). Values of Vickers hardness of samples A, B-L and C were about 75 H-V. However, characters of precipitates were different among the samples: needle-like beta '' 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 B-L and C by three-dimensional atom probe (3DAP). In sample B-H showing the maximum hardness (103H(V)) after 373 K aging, fine beta '' particles about 5 nm in length were observed. Samples A and B-H with the beta '' precipitation exhibited higher values of yield stress than those for samples B-L and C, and the beta '' precipitates pinned dislocations. On the other hand, samples B-L 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 B-L 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 beta '' precipitates.