An analytical electron microscopy study of constituent particles in commercial 7075-T6 and 2024-T3 alloys

Abstract

To better understand the role of constituent particles in pitting corrosion, analytical electron microscopic studies were performed on the constituent particles in commercial 7075-T6 and 2024-T3 alloys. Five phases, namely, Al 23 CuFe 4 and amorphous SiO 2 in 7075-T6 and Al 2 CuMg, Al 2 Cu, and (Fe,Mn) x Si(Al,Cu) y in 2024-T3, were identified. The crystal structure and chemistry of the Al 23 CuFe 4 , Al 2 CuMg, and Al 2 Cu phases in these alloys are in good agreement with the published data. Small deviations from their stoichiometric compositions were observed and are attributed to the influence of alloy composition on the phase chemistry. For the (Fe,Mn) x Si(Al,Cu) y (approximately, x ϭ 3 and y ϭ 11) phase, a rhombohedral structure, with lattice parameter a ϭ b ϭ c ϭ 1.598 nm and ␣ ϭ ␤ ϭ ␥ ϭ 75 deg, was identified and is believed to be a modified form of either Al 8 Fe 2 Si or Al 10 Mn 3 Si. Information from this study provided technical support for studying the electrochemical interactions between the individual particles (or phases) and the matrix. The corrosion results are reported in a companion article.

Black anodic coatings for space applications: Study of the process parameters, characteristics and mechanical properties

“…2a). The EDX analysis revealed that particles contained Al, Fe, Cu, Zn, Mg, in accordance with previous works (Savas and Hearthman, 2008;Gao et al, 1998) that identified for example Al 23 CuFe phases (Gao et al, 1998) in similar aluminium alloys. The sample showed a metallic appearance, while the arithmetic roughness (R a ) was (0.9±0.1) µm.…”

“…2b) with long spots including increasing contents of Zn, Mg and P. The high pH of the etching solution (pH = 11) favours both the chemical dissolution of the "type A" particles (Gao et al, 1998;Buchheit, 1995) and the immediate re-precipitation of their corresponding hydroxides and/or phosphates at the metal surface.…”

High Strength Al-Alloys: Microstructure, Corrosion and Principles of Protection

“…The reason why compositional variation from batch to batch represents a difficulty is that the transition elements can be substitutional in many intermetallics. For example Ayer et al (Ayer, Koo et al 1985) found Zn and Ni in Al 7 Cu 2 Fe, and Gao et al (Gao, Feng et al 1998) found considerable compositional variation within a phase nominated as (Fe,Mn) x Si(Al,Cu) y with some particles containing mainly Fe and Cu with small amounts of Mn and others had significant Mn and Si; similar results were reported by Boag et al (Boag, Taylor et al 2010). Gao et al (Gao, Feng et al 1998) even found small amounts of Cr and Zn in Al 2 CuMg.…”

“…So there is an assumption that a particular composition has a specific crystallographic structure which may not be valid. For example, Wei and coworkers (Gao, Feng et al 1998;Wei, Liao et al 1998) used TEM to study IM particles in AA2024-T3 and AA7075-T6 and found that compositions containing Al, Cu, Fe, Mn and Si, had a rhomohedral structure which did not match the hexagonal structure previously reported for particles of this type for example Al 8 Fe 2 Si or Al 10 Mn 3 Si. Classifying IM particles by shape is used both in metallurgy and corrosion.…”

“…At the top of the Table 4. Composition of IM particles in AA2024-T3 determined by Gao et al (Gao, Feng et al 1998) Wei and co-workers (Gao, Feng et al 1998) identified S-phase, -phase and the remainder were categorised as (Fe,Mn) x Si(Al,Cu) y indicating no Al 7 Cu 2 Fe. Electron diffraction of these phases found an unidentified rhombohedral structure and particles with the general composition of (Fe,Mn) x Si(Al,Cu) y were reported as variants of Al 8 Fe 2 Si or Al 10 Mn 3 Si.…”