The presence of large constitutive particles formed during solidification decreases the strength and hot workability of aluminum alloys especially when they are located in the grain boundary (GB) regions. Therefore, the evolution of these phases is a major issue in the homogenization process of these alloys. There is a lack of information on the behavior of the GB phases during homogenization, which constitute more than 70 pct of all the secondary phases present in the microstructure of AA7020 aluminum alloys. The dominant GB phase is identified to be Al 17 (Fe 3.2 Mn 0.8 )Si 2 . In the present research, a comprehensive study on the effect of the homogenization treatment on the evolution of the GB phases during homogenization was conducted. The analysis shows that the evolution of this phase is largely dependent on temperature, which ranges from spheroidization with insignificant dissolution at low temperatures to full dissolution during homogenization at high temperatures. A new mechanism for the dissolution of these phases called thinning, discontinuation, and full dissolution (TDFD) is proposed based on the findings of the field emission gun-scanning electron microscope (FEG-SEM) analysis.

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This research concerns the effect of homogenization treatment on the electrical resistivity of AA7020 aluminum alloy variants with different Zr and Cr contents. Small changes in the Zr and Cr contents of the as-cast alloy increase the electrical resistivity significantly. After employing various homogenization treatments, the electrical resistivity decreases, which is due to the depletion of Zr, Cr, and Mn in the matrix, by forming small dispersoids. The optimum treatment proposed in order to obtain the smallest recrystallized grains is to hold the material at 550°C for 24 hours, which results in the lowest electrical resistivity. The viability of the proposed treatment was tested through hot compression tests and static annealing. Indeed, the samples homogenized at 550°C for 24 hours showed the smallest recrystallized grains compared to those homogenized at other temperatures.

In this article, a new relationship for the calculation of the Zener drag pressure is described in which the effect of the size distribution of nanoscale dispersed particles is taken into account, in addition to particle radius and volume fraction, which have been incorporated in the existing relationships. Microstructural observations indicated a clear correlation between the size distribution of dispersed particles and recrystallized grain sizes in the AA7020 aluminum alloy. However, the existing relationship to calculate the Zener drag pressure yielded a negligible difference of 0.016 pct between the two structures homogenized at different conditions resulting in totally different size distributions of nanoscale dispersed particles and, consequently, recrystallized grain sizes. The difference in the Zener drag pressure calculated by the application of the new relationship was 5.1 pct, being in line with the experimental observations of the recrystallized grain sizes. Mathematical investigations showed that the ratio of the Zener drag pressure from the new equation to that from the existing equation is maximized when the number densities of all the particles with different sizes are equal. This finding indicates that in the two structures with identical parameters except the size distribution of nanoscale dispersed particles, the one that possesses a broader size distribution of particles, i.e., the number densities of particles with different sizes being equal, gives rise to a larger Zener drag pressure than that having a narrow size distribution of nanoscale dispersed particles, i.e., most of the particles being in the same size range.