Precipitation Characteristics, Mechanical Properties and Stress Corrosion Cracking of High-Strength 7XXX Al-Zn-Mg-Cu Alloys

Autor: YI-YUN LI, 李溢芸
Rok vydání: 2010
Druh dokumentu: 學位論文 ; thesis
Popis: 98
This thesis primarily study the precipitation behavior of Al-Zn-Mg-Cu alloys with different Cu/Mg ratio, high resolution characterization of the precipitation behavior of an Al-Zn-Mg-Cu alloy, and the influences of heat treatments on the mechanical properties and corrosion behavior of a high-strength Al-Zn-Mg-Cu alloy. From our experimental results, both M9F and M9G alloys, Cu/Mg: M9F < M9G, exhibit obvious precipitation hardening during natural aging, and possess good thermal stability during 120℃ artificial aging. GPI zones and η’ phases could be found at the early stage of 120℃/6h artificial aging and these precipitates were also the main strengthening particles at the peak aging condition (120℃/24h). While GPII was not found in our study. Therefore, the formation of η’ phase could be transformed from GPI or directly formed from the matrix. The Cu/Mg ratio shows less influence on the size distribution of the round precipitates presented in both alloys than the elongated ones. The elongated precipitates grow on the {111}Al, and they have greater length-to-thickness ratio in M9G than in M9F, and plate-like in M9G while lath-like in M9F. For the higher Cu/Mg would introduce greater lattice strain field during precipitation, the precipitates would prefer to grow in the form of plate-like to minimize the strain energy. High density of fine η’ phases precipitate from the matrix after 120℃/24h peak aging contributing to the high strength. The strength of the alloy can be greatly elevated to 670MPa for plate specimen and 800 MPa for the extruded specimens after T6 heat treatment. In addition, the studied Al-Zn-Mg-Cu alloy shows excellent thermal stability when aged at 120℃. Except T7 temper, when heat treated after various tempers, all of the plate specimens can achieve UTS greater than 600Mpa, and change the fracture surface from ductile to brittle. However, the strength of the Al-Zn-Mg-Cu alloy cannot be enhanced by implementing natural aging before T6 and two stage aging. The crystal structure and the composition of the Al-Zn-Mg(-Cu) alloys are widely studied but still quite disputing over the past 50 years. The metastable particles in an Al-Zn-Mg-Cu alloy were firstly examined by high-angle annular dark field (HAADF) in this study and the results are compared with the conventional high-resolution electron transmission microscope (HRTEM) images. From our HAADF images, the η’ thin platelets are composed of 7 atom layers with 5 Mg- and Zn-rich atom layers alternate in structure and an extra Zn-rich plane segregates on each facet side. However, the Z contrasts of the HAADF images do not support the η’ crystal structures proposed by previous authors. The smallest η phase is an 11-layered structure and an extra atomic plane forms in the layer center to reduce the strain energy due to the thickening. Almost no particles less than 7 layers and between 7 and 11 layers were found in our study. The conventional HRTEM images are sensitive to the defocus value which could introduce various morphologies of precipitates. The inconsistent results of the HRTEM images would lead to a misunderstanding of the structure of the precipitates. Cu stabilizes the clustering process and metastable phases can be attributed to the size effect. Stress corrosion cracking (SCC) of high-strength 7XXX-series aluminum posses a continuing issue for the aerospace industry. This research utilized SCC tensile testing and potentiodynamic polarization technique to study the corrosion behavior of the Al-Zn-Mg-Cu alloy under various tempers. To further clarify the SCC, deformation was also implemented on the specimens to investigate the influence of dislocations on the SCC behavior of Al-Zn-Mg-Cu alloy. T7，RRA and two stage exhibit better SCC resistant than T6. From the polarization testing, we found that E b2 reduces with aging time and is very sensitive to the tempers. It seems that E b2 is highly associated to the susceptibility of SCC of Al-Zn-Mg-Cu alloy but not the E corr. Eb2 is highly related to the grain boundary configuration. Although T6 exhibited lower I corr than T7, it shows greatest current inversion at E b2. This means continues dissolution at the grain boundaries and H absorption on the grain boundary precipitates are the more crucial factors that contribute to the high SCC sensitivity of T6. Plastic deformation seems to increase the corrosion current density and promote inter granular corrosion of T6 specimen, while it did not show great influence on T7 specimen. This could indirectly explain the SCC sensitivity of T6 specimens.
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