The microstructure and localized corrosion behavior of the 7050‐T6 Al alloys treated with different quench transfer time were investigated. Optical microscope observations show that the volume fraction of the recrystallized grains increases slightly with prolonging quench transfer time. Scanning electron microscope observations reveal that the stable η (MgZn2) phase nucleates and precipitates on grain boundaries in the process of transferring to quench. Further observations, using transmission electron microscope, found that the size, nearest neighbor distance, and copper content of the grain‐boundary precipitates increase with quench delay. As a result, the open‐circuit potentials and charge transfer resistance (Rt) of the alloys reduce with increasing transfer time, while the susceptibility to intergranular corrosion (IGC) and strength loss after exfoliation corrosion tests increase sharply. In addition, the IGC network appearance changes from large network to fine network structure, due to the different temperature range where very rapid η phase precipitation takes place between on grain boundaries and on sub‐grain boundaries.

No abstract

No abstract

A two-step overaging treatment was exploited in thermomechanical processing for producing fine grained Al–Mg–Li alloy sheets. The effects of each step and temperatures of overaging on the distribution of precipitates and recrystallised grain structure were investigated. The influence of preheating treatment before rolling on precipitate coarsening or dissolution was also considered. The results showed that, in contrast to rod shaped precipitates produced by single overaging at 300°C, precipitates produced by a two-step overaging tended to have globular morphologies. The globular precipitates were not deformed during the following large rolling reductions. Overaging at 200°C for 24 h then at 300°C for 24 h resulted in a distribution of globular precipitates with sizes of 1·0–1·2 μm. Preheating at 400°C or raising the overaging temperatures of each step to 300 and 400°C led to a reduction in the precipitate size. Subsequent rolling and recrystallisation resulted in a fine grain structure with an average grain diameter of approximately 8·1 μm for 200°C/24 h+300°C/24 h and 9·2 μm for 300°C/24 h+400°C/24 h overaging treated material respectively in the surface layers of the sheet.

The single-crystal yield surfaces of bcc crystals for slip on {123}͗111͘ systems have been analyzed systematically and derived based on the Taylor/Bishop-Hill theory. It is found that there are 338 stress states altogether. All the stress states are classified according to the crystal symmetry. The results demonstrated that there are 14 groups of yield vertices, which activate five, six, or eight slip systems, depending on the crystallographically nonequivalent groups. In addition, the four groups of yield vertices that Chin and Wonsiewicz [15] did not consider have been obtained (there are 126 stress states). Among all these vertices, the fraction of vertices for which there is slip ambiguity (more than five active slip systems) is reduced considerably to 146/338 (Ϸ43 pct). By means of the stress states obtained by Chin et al. and by the present authors, respectively, the plastic flow for axially symmetric deformation and three-axial deformation have been analyzed. In the case of axially symmetric deformation, the results are the same using the two different kinds of stress states. The obtained new stress states are necessary in order to analyze the three-axial deformation.