Investigation of mechanical and corrosion properties of an Al–Zn–Mg–Cu alloy under various ageing conditions and interface analysis of η′ precipitate

Abstract

The mechanical and corrosion properties under various ageing treatment conditions were investigated in an Al-6.0Zn-2.3Mg-1.8Cu-0.1Zr (wt.%) alloy. The results showed that the retrogression and re-ageing (RRA) were capable of providing higher strength and improved corrosion resistance in comparison with the conventional T6 and T74 ageing. The optimised ageing process had been found to be 120°C/24 h + 180°C/60 min + 120°C/24 h for the experimental alloy. The results obtained from the high resolution transmission electron microscopy (HRTEM) interface analysis revealed that a semi-coherent stress field between the η′ precipitate and the Al matrix was critical in controlling the strength of the Al-Zn-Mg-Cu alloy heat-treated under different conditions. Furthermore, Transition Matrix calculation showed that the η′ phases had only two zone axes: [1 ̅ 21

Effects of Ultrasonic Surface Rolling on the Localized Corrosion Behavior of 7B50-T7751 Aluminum Alloy

“…Owing to the discontinuous η-MgZn 2 precipitates at the grain boundary [41,42], 7B50-T7751 Al alloy exhibited low susceptibility to IGC in the NaCl + H 2 O 2 solution. However, apparent pitting corrosion occurred on the surface of the alloy, owing mainly to the reaction between the cathodic Fe-rich intermetallic particles and the nearby aluminum matrix [43].…”

A novel constitutive model for multi-step stress relaxation ageing of a pre-strained 7xxx series alloy

“…These precipitates are relatively small in size (i.e. around 2 -3.5 nm (Dumont et al, 2003;Sha and Cerezo, 2004;Werenskiold et al, 2000;Yang et al, 2015)), while extremely high in density (i.e. around 7 − 17 × 10 24 −3 (Sha and Cerezo, 2004)).…”

“…Platelet or cigar-like semi-coherent ʹ, which is believed to be the main strengthening phase, can be rapidly obtained within 1-3 hours of ageing at 177 °C. The rapid increase in the precipitate size from ~2-3.5 nm to ~5 nm (Werenskiold et al, 2000;Yang et al, 2015) is the main reason for the increase in the creep resistance from the under-aged to peak-aged states. The ordered ʹ continues to grow until the majority of the precipitate transforms into the incoherent phase and the samples reaching their over-aged T74 states (Polmear, 2005).…”

Insight into the partial solutionisation of a high pressure die-cast Al-Mg-Zn-Si alloy for mechanical property enhancement

“…Further, the corresponding select area diffraction patterns (SADP) in the area under the [112] Al zone axis in Figure 3b showed that several weak diffraction spots were located at the 1/3 and 2/3 220 Al positions in addition to the diffraction spots from the α-Al matrix. These typical diffraction information indicated that the fine precipitates were most likely the η' phase with a HPC structure, which was a metastable phase of MgZn 2 phase and was always found in Al-Zn-Mg-(Cu) alloys as the most important strengthening phase when the alloys reached a peak hardness [10] [11]. Based on the SADP, the orientation relationships between the η' phase and the Al matrix could be described as [ (Figure 3b).…”

“…The corresponding fast Fourier transform (FFT) of HRTEM image was displayed at the lower right corner in Figure 3c, where several clear diffraction streaks were also found at the 1/3 and 2/3 220 Al positions. Based on the previous research in the Al-Zn-Mg-Cu alloy [10] [11], it was known that η' phases actually had four equivalent variants, and the η' phase in Figure 3c only belonged to the one of four η' variants [11].…”