The 7XXX series age-hardenable high-strength aluminum alloys find useful applications in the field of aerospace engineering. Constant efforts are being made to tailor the mechanical and corrosion properties of these alloys as per requirements for a particular application. These properties are a function of factors like microstructure, chemical composition and processing parameters. An effort has been made to collate the information available from different studies conducted on alloys Al 7050 and Al 7055. Databases were created to consolidate the information about microstructure, mechanical properties and corrosion behavior for the two alloys. Existing models were utilized to predict strength and fracture toughness for these alloys and these models were validated using experimental values and a qualitative evaluation was made for the corrosion behavior of these alloys. Available data were utilized to prepare maps that are intended to serve as guides to design aluminum alloys with desired combination of properties.

Structure-property correlations in Al 7050 and Al 7055 high-strength aluminum alloys

Aluminium alloys used in the aeronautical industry, and in particular the high strength Al-Zn-Mg(-Cu) alloys of 7XXX series, can be susceptible to stress corrosion cracking (SCC). In the numerous studies reported in the scientific literature on this subject, two basic mechanisms have been proposed to model SCC: anodic dissolution and hydrogen embrittlement. However, there is currently no consensus on the precise mechanism. By coupling discriminating slow strain rate tests and scanning electron microscopy (SEM) examinations, it is shown that: (1) both anodic dissolution and hydrogen embrittlement operate during the SCC process of a 7050 aluminium alloy stressed in a chloride solution (NaCl 3%); (2) the main role of anodic dissolution is to produce critical defects which promote subsequently localized hydrogen discharge, entry and embrittlement; and (3) the relative influence of these two mechanisms depends on the main parameters that govern cracking, i.e. the microstructure, the electrochemical potential and the strain rate.

Influence of critical surface defects and localized competition between anodic dissolution and hydrogen effects during stress corrosion cracking of a 7050 aluminium alloy

Microstructural processes of plastic instabilities in strengthened metals

Fatigue of particle-and whisker-reinforced metal-matrix composites

The precipitation behaviors and aging reactions of the pseudobinary Al-Cu-Mg alloy and the commercial 2024 alloy under unstretched and stretched conditions have been investigated in this study by means of conductivity and hardness measurements, differential scanning calorimetry, and transmission electron microscopy (TEM). The morphologies and growth modes of various defects and transition phases as well as the interaction among them were widely discussed. In particular, an electron diffraction ring pattern was found to correspond to the axial growth of the GPB2 zone. This suggested that the atom groups constructing this cylindrical zone are statistically, uniformly arranged in the adjacent {100}A1 planes and the GPB2 zone is only a partially ordered version of the GPB zone in Al, directions. Moreover, few GPB2 zones can survive long time overaging due to the Gibbs-Thomson effect. As for the S' precipitates, they preferentially nucleate on dislocations. During subsequent growth, they can further coalesce into two morphologies (corrugated sheets and wide plates) for the unstretched specimens. However, for the stretched specimens, this coalescing process does not occur until a long time of overaging due to more introduced dislocations. Therefore, the rate of Ostward ripening decreases and the peak hardness becomes flattened. Finally, based on the present analyses, the aging sequence of the two alloys studied could be revised with respect to previous investigations and their isothermal aging reactions can be subdivided into five main stages. These stages correspond to (1) GPB zone precipitation, (2) fast in situ precipitation of GPB2 zones from GPB zones and their subsequent growth, (3) fast nucleation and accelerating growth of the S' phase, (4) decelerating growth of the S' phase, and (5) Ostward ripening of the S' and S phases, respectively.

Precipitation behaviors in Al-Cu-Mg and 2024 aluminum alloys

The effect of microstructure on the monotonic and low cycle fatigue properties of a high purity, large grain, ternary aluminum-zinc, magnesium (Al-Zn-Mg) alloy and a high strength 7050 aluminum alloy was investigated. The best combination of fatigue life, strength, and ductility for the ternary alloy resulted when aged to produce a microstructure containing predominately η′ having a Guinier radius of approximately 65a and a small amount of incoherent η (MgZn2). Superior fatigue life, strength, and ductility were found when the 7050 alloy was aged to produce the maximum number of partially coherent η′ precipitates having a Guinier radius of approximately 35a. Aging the 7050 alloy to produce particles larger than 50a gave a microstructure that had lower fatigue properties at the low plastic strain amplitudes, δep/2 <1.0 pct. The empirical CoffinManson relationship was found to hold for a given deformation process, however changes in deformation character resulted in changes in the Coffin-Manson parameters.

T. H. Sanders

Papers

The relationship of microstructure to monotonic and cyclic straining of two age hardening aluminum alloys