Aj Ardell

Bio: Aj Ardell is an academic researcher from University of California. The author has contributed to research in topics: Grain boundary & Dispersion (chemistry). The author has an hindex of 1, co-authored 1 publications receiving 159 citations.

Effect of Retrogression and Reaging Treatments on the Microstructure of AI-7075-T651

Abstract: The effect of the retrogression and reaging treatments (RRA) on the microstructure of Al-7075 in the T651 temper, both in the matrix and on grain boundaries, was studied using transmission electron microscopy. The processes occurring in the matrix during the retrogression treatment are principally the dissolution of small particles of the η’ transition phase, transformation to η of the larger particles of η’, coarsening of the three commonly observed variants of the η phase precipitates (η1, η2, and η4), and precipitation of new η phase particles, particularly the η1 variant. The main process occurring during the reaging treatment is either growth of partially dissolved η’ particles or precipitation of the η’ phase. These lead to a microstructure containing many fine η’ precipitates and some larger η1 and η2 particles with a smaller amount of coarse η4 particles, resulting in a broad particle size distribution. The high strength of the 7075 alloy in the RRA temper is believed to arise from the relatively high overall concentration of particles in this dispersion. The retrogression treatment produces rapid initial coarsening of the grain boundary particles, which are primarily η phase precipitates, resulting in an increase in their volume per unit grain boundary area,V A . The beneficial effect of the RRA treatment on the susceptibility of 7075-T651 to SCC is believed to be due, at least partially, to the increased value ofVA produced by the RRA treatment.

Effect of Cu Content on Corrosion Behavior of 7xxx Series Aluminum Alloys

Abstract: The corrosion behavior of 7xxx aluminum alloys with various Cu content was investigated using polarization and electrochemical impedance spectroscopy (EIS) techniques. Two breakdown potentials were found in Cu-containing alloys in deaerated chloride solution, and the values increased logarithmically with increasing Cu content. However, in aerated chloride solution, polarization resistance as determined by EIS decreased with increasing Cu content. The first breakdown potential corresponded to transient dissolution associated with attack of the fine hardening particles and the surrounding solid solution in a thin surface layer. The second breakdown potential was associated with combined intergranular and selective grain attack. The correlation between the microstructure, especially Cu content, and corrosion behavior was made by composition analysis of the grain boundary regions including precipitate free zone and grain boundary precipitates as well as the matrix by scanning transmission electron microscopy. The corrosion potential increased with increasing alloy Cu content in aerated chloride solutions because of enhanced rates of oxygen reduction. This degraded the corrosion resistance despite the increase in the breakdown potentials with Cu content.

The effect of RRA on the strength and SCC resistance on AA7050 and AA7150 aluminium alloys

Abstract: The effect of retrogression and re-aging (RRA) on the strength and stress corrosion cracking (SCC) performance was evaluated in AA7050 and AA7150 aluminum alloys. Samples from extruded profiles, at their original condition (T77511 for AA7150 and T76511 for AA7050), were solution heat treated and aged at 120 °C for 1440 min (T6 condition). After that, retrogression was performed at 200 °C for times of 5, 10, 20 and 40 min, followed by a re-aging at 120 °C for 1440 min. Hardness and electrical conductivity measurements were used to characterize the samples after different retrogression times and during re-aging. Maximum hardness after re-aging was obtained in the samples retrogressed for 20 and 40 min. Tensile and SCC testing were performed on samples retrogressed for 20 and 40 min and re-aged, and the results were compared with the alloys at their original conditions. After RRA treatment it was observed an increase of 30% in the yield strength of the AA7050 alloy, while for the AA7150 alloy the mechanical properties were similar to the original condition. Both alloys, when submitted to RRA for 40 min retrogression, had a SCC performance similar to their original condition. However, retrogression time of 20 min decreased significantly the SCC performance.