Papers by Keyword: Al-Zn-Mg

Abstract: In the present study the influence of strain rate and temperature on the behaviour of two commercial aluminium alloys, 6063-T6 and 7030-T6, was investigated. Both alloys are high strength precipitation hardened alloys that are expected to have low strain rate and temperature sensitivity. Tensile tests were performed at room temperature at strain rates ranging from 10-4 to 102 s-1, and at -40°C and +60°C at strain rates of 10-4 and 10-1 s-1, due to equipment limitations. Both alloys showed low but positive strain rate sensitivity at all temperatures. Also the temperature sensitivity was low, showing negative values in all cases. The dependence of the flow stress on temperature was more pronounced than the strain rate dependence. The area reduction at fracture was higher in 6063 than 7030, although the uniform elongation was larger in 7030. 6063 showed almost no strain rate dependence of the ductility and a limited reduction with increased temperature. 7030 showed markedly increasing area reduction with increasing temperature and decreasing values with increasing strain rate. The energy absorption was higher in 7030 by a factor of approximately three.

Abstract: The quantitative correlation between strength, ductility and precipitate microstructures in the vicinity of grain boundaries with precipitate free zones (PFZ) was evaluated for Al-Zn-Mg(-Ag, Cu) alloys using transmission electron microscopy (TEM), three-dimensional atom probe (3DAP) and tensile test. In the Al-Zn-Mg ternary and Cu-added alloys aged at 433K, larger widths of PFZ were observed by TEM and resulted in lower elongations to fracture, independent of the size of grain boundary precipitates. On the other hand, the elongation of the Ag-added alloy was higher, if compared at the same levels of proof stress, due to the much smaller width of PFZ. This strongly suggests that PFZ is harmful to fracture of the investigated alloys. From a 3DAP analysis, furthermore, it was revealed that Ag and Cu atoms are incorporated in the nanoclusters from the initial stage of aging. In this work, the elongation was well correlated to the width of PFZ, size of grain boundary precipitates and the level of proof stress, enabling to predict ductility of the alloys from known microstructural factors.

Abstract: The commercial 7xxx series Al alloys are based on medium strength Al-Zn-Mg and high strength Al-Zn-Mg-Cu systems. The medium strength alloys are weldable, whilst the high strength alloys are nonweldable. On the other hand, the Cu-free, weldable alloys suffer from poor SCC resistance. It is the purpose of this article to provide quantitative data and microstructural analysis to demonstrate that small additions of either Ag or Sc to Al-Zn-Mg and Al-Zn-Mg-Cu alloys bring about very significant improvement in SCC resistance and weldability, respectively. The improvement in SCC resistance of the Cu-bearing alloys due to over aging and retrogression and reaging (RRA) is further discussed in light of a similar improvement in the SCC resistance of these alloys, when peak aged, due to Ag and Sc additions.

Abstract: Positron annihilation spectroscopy in two versions (lifetime and coincidence Doppler broadening) has been applied to investigate solute/vacancy interactions when minor amounts (<1wt.%) of Ag or Cu are added to the alloy Al-4Zn-3Mg (wt.%) during ageing at 150°C. The results show early clustering of vacancies with Zn (and with Cu, if present). Ag displays a strong interaction with vacancies in competition with Mg and forms clusters that may help further aggregation of the other alloying elements during artificial ageing. High Mg concentration is observed at the misfit interfaces of semi-coherent or incoherent precipitates.

Abstract: The nano-scale precipitate microstructures and properties of age-hardenable aluminum alloys such as Al-Cu, Al-Mg-Si and Al-Zn-Mg alloys were investigated using conventional electrical resistivity and hardness measurements, TEM and 3D-AP techniques. To increase mechanical strength and ductility of the alloys nano-scale precipitates were effectively controlled by applying new type heat treatments and microalloying elements. In the initial stage of phase decomposition of the alloys containing microalloying elements several types of nanoclusters were formed and distinctly detected by the 3D-AP technique. These nanoclusters greatly affect the nucleation of the subsequent precipitates and resultantly mechanical properties. In Al-Mg-Si and Al-Zn-Mg alloys complicated two-step aging behaviors were found to be originated in the positive or negative effect of the nanoclusters with different structures.