Recent advances in ageing of 7xxx series aluminum alloys: A physical metallurgy perspective

Influence of retrogression and re-aging treatment on corrosion behaviour of an Al–Zn–Mg–Cu alloy

Influence of retrogression temperature and time on the mechanical properties and exfoliation corrosion behavior of aluminium alloy AA7150

Influence of repetitious-retrogression and reaging (repetitious-RRA) treatment on the strength and SCC resistance of Al–Zn–Mg–Cu alloy was investigated by the slow strain rate technique (SSRT) and electrical conductivity test, using TEM, EDS and SEM. Additionally, the strength of alloy was characterized by Vickers hardness. The results show that within 3 times RRA treatments, SCC resistance increases with the increase of time, while maintaining strength as compared to RRA treatment, TEM observation along with EDS analysis indicates that within 3 times, matrix precipitates (MPs) in repetitious-RRA treatment are similar to those by RRA treatment and grain boundary precipitates (GBPs) become more discrete and coarser with the time. However, RRA treatment reaches 4 times, the SCC resistance gets worse because MPs are also coarse. Besides, the Cu content of GBPs increases with the time, inversely for Zn content, which is another reason for improving the SCC resistance.

Influence of repetitious-RRA treatment on the strength and SCC resistance of Al―Zn―Mg―Cu alloy

The effects of Zr, Er and Cr additions on the microstructures, mechanical properties and corrosion resistance of Al–Zn–Mg–Cu alloy were investigated. The combined additions of Zr, Er and Cr to Al–Zn–Mg–Cu alloy led to the formation of coherent Zn, Mg, Cu, Cr-containing Al 3 (Zr, Er). These 15–25 nm secondary precipitates remarkably inhibited the recrystallization of Al matrix, and numerous fine subgrain boundaries were retained. Compared to the partial recrystallized Al–Zn–Mg–Cu–Zr alloy, the unrecrystallized Al–Zn–Mg–Cu–Zr–Er–Cr alloy exhibited higher resistance to intergranular corrosion and stress corrosion with the improved mechanical properties and fracture toughness. The synergetic effects of Zr, Er and Cr on the precipitates, recrystallization and corrosion resistance of ultra-high strength Al–Zn–Mg–Cu alloys have been discussed.

Effect of Zr, Er and Cr additions on microstructures and properties of Al–Zn–Mg–Cu alloys

Retrogression and re-aging treatment of Al-9.99%Zn-1.72%Cu-2.5%Mg-0.13%Zr aluminum alloy

The effects of heat treatment on the microstructure and mechanical properties of two alloys, namely Al-12.2% Zn-2.48% Cu-2.0% Mg-0.15% Zr-0.166% Ag (alloy 1), and Al-9.99% Zn-1.72% Cu-2.5% Mg-0.13% Zr (alloy 2) were investigated. The results show that low temperature aging after promotive solution treatment can increase elongation without the loss of strength for the studied alloys. The optimum aging treatment (T6) for alloy 1 and alloy 2 is 100 °C/80 h and 100 °C/48 h, respectively. Compared with other heat treatment alloys, alloy 1 and alloy 2 show super-high tensile strength up to 753 MPa and 788 MPa, remaining 9.3% and 9.7% elongation under T6 condition, respectively. During aging, trace addition of Ag enhances the formations of GP zone and metastable phase, and stabilizes GP zone and metastable phase to a higher temperature. Trace addition of Ag prolongs the aging time of reaching the peak strength and delays over-aging condition of the alloy. However, trace addition of Ag promotes the formation of coarse constituent in the alloy and consumes hardening alloying elements of Zn and Mg. Morecover, the addition of the transition element Zr in 7000 series super-high alloy forms incoherent Al3Zr dispersoid which can serve as nucleation sites for nonuniform precipitation of η phase during aging process. The higher the aging temperature, the greater the tendency for nonuniform precipitation of η phase.

Effect of Low Temperature Aging on Microstructure and Mechanical Properties of super-high Strength Aluminum Alloy

The evolution of second phases and its effect on tensile mechanical property in artificial ageing and RRA process of super-high strength aluminum alloy is investigated.The result of tensile test shows that the samples of Al-Zn-Mg-Cu-Ag alloy aged at lower temperature(100°C) not only have higher tensile strength up to 753Mpa but also relatively higher tensile elongation above 9% than normal temperature(120°C) in artificial ageing. The sample of Al-Zn-Mg-Cu alloy has the highest tension strength upto 788Mpa when aged at 100°C for 48 hours in single step of artificial ageing. Further more in RRA process samples of Al-Zn-Mg-Cu alloy preaged at 100°C for 24 hours retrogressed at 200°C for 7min and reaged at 100°Cfor 24 hours present the best tensile strength of 795Mpa than others. The tensile strength of Al-Zn-Mg-Cu alloy after RRA treated decreases with prolonging of retrogression time and reageing time starting from 7mins and 24 houres respectively. SEM observation shows that crack of the samples in tensile test is created at large particles in the fracture while there are more particles of undissolved phase in Al-Zn-Mg-Cu alloy containing Ag. TEM observation shows that the dominant strengthening particle corresponding to the peak strength of Al-Zn-Mg-Cu alloy containing Ag when aged at 120°C for 8 hours is η’ phase while dominant strengthening particle is G.P zone when aged at 100°C for 80 hours. However, η’ phase as the dominant strengthening particles corresponds to the peak strength of Al-Zn-Mg-Cu alloy without content of Ag when aged at 100°C for 48hours. TEM observation also shows that G.P zone as strengthening particle is dominant in the samples of Al-Zn-Mg-Cu alloy reaged at 100°C for 24 hours, and strengthening particles is coarsened when the sample is retrogressed at 200°C and reaged for a longer time. It is suggested that whether at the presence of coarse particles of undissolved phases or when G.P zone and η’ particle grow up in the retrogression ,sample needs deformable G.P zone instead of undeformable η’ in subsequent artificial ageing and reageing as dominant strengthening particle, in order to present a larger freedom spacing for dislocation to slip and let the sample not to behave too brittle to display high resistance to imposed plastic deformation or high tensile strength.

Chun Feng

Papers

Retrogression and re-aging treatment of Al-9.99%Zn-1.72%Cu-2.5%Mg-0.13%Zr aluminum alloy

Effect of Low Temperature Aging on Microstructure and Mechanical Properties of super-high Strength Aluminum Alloy

Effect of Second Phases on Tensile Property in Artificial Ageing and RRA Process of Super-High Strength Aluminum Alloy