The effect of vacancy‐rich clusters on the decomposition processes in Al‐Zn‐Mg alloys

Abstract

1ntroduct)ionSince the early works of HARDY and POLMEAR the existence of vacancy-solute atom clusters (VRC) often turned up in the literature, in inost cases to explain the differences of the precipitate structures after different quenching treatments. The aim of this paper is to review the results concernirtg the forrnation of the VHC and their effect on the decortipsition processes. VRC as nucleation sites for metastable and stable precipitatesThe existence of VRC has been assumed to explain the differences in the precipitate structure of Al-Zn-Mg alloys after direct quench, UQ, and indirect quench, 1&, to the ageing temperature, T,. I n case of 1Q the alloy is generally water quenched to 0 or 20 "C, kept at this temperature for a short time (order of one minute) and thereafter aged at T,. 1982) we can conclude that VRC play an important role in the precipitation of the metastable (7') and even of the stable ( q ) phase when G P zones are not formed prior to or a t the beginning of the ageing at T,. I A o~~~,~;~ 1982.) investigations. The observations were interpreted in most cases by the formation of V l t C during the quench and/or during short ageing times (order of 10 sec) a t room temperature. The VltC then act as nucleation sites for the precipitation of the metastable or stable precipitates when the alloy is up-quenched to T, > TI,. I t is generally agreed (L~FELER et al. 1983) that the number of VRC depends sensitively on the quench procedure, i.e. on the temperature of solution treatment and of the quenching bath, on the quenching rate (e.g. air cooling, oil quench, water quench etc.) or on conditions of step quenching. Above a certain critical temperature, which depends on the alloy composition and on the quenching procedure, VRC are not formed and consequently the precipitation of the metastable or stable precipitates l)

Microstructural, mechanical and electrochemical behaviour of a 7017 Al–Zn–Mg alloy of different tempers

“…% NaCl solution at neutral pH 7 and in 3.5 wt. % NaCl at pH 12 at a A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 9 scan rate of 0.5 mV/sec. The solution pH 12 was made by adding a small amount of NaOH in 3.5…”

“…The formation of the η' precipitate and its variants depends on the alloy composition, artificial ageing temperature, ageing time and nucleation conditions etc. The ageing behavior, sequence of precipitation and dissolution reactions and microstructural features of the Al-Zn-Mg alloys have been assessed by various techniques [9][10][11][12][13][14]. However, the precipitation sequence and the microstructural features comprising such a large number of metastable and stable phases are very complex and are dependent on alloy composition, solute content, Zn / Mg ratio, presence impurities (such as Fe,…”

Differential scanning calorimetry and electron diffraction investigation on low-temperature aging in Al-Zn-Mg alloys

“…During decomposition of the supersaturated solid solution of Al-Zn-Mg alloys, two types of cluster, namely solute ACKNOWLEDGMENTS clusters GP(I) zones and vacancy-rich clusters GP(II), are formed simultaneously, [1,2,3] The present study has provided This work is supported by the Norwegian Research Council. It is a pleasure to thank Dr. Vidar Hansen and Professor further evidence for this mechanism of decomposition of the solid solution.…”

“…of metastable and stable precipitate particles, [1,2,3,7,8,9] and Ryum [7] has proposed the following precipitation paths for THE 7000 Al-Zn-Mg system has been widely studied Al-Zn-Mg alloys: due to the excellent mechanical properties developed in the alloys after age hardening. [1,2,3] A large number of investiga-SSS → ␣reaction tions have been concerned with the mechanisms of zone and precipitate formation in the alloys, and models for the SSS → VRC → Ј → ␤reaction decomposition process have been proposed.…”

“…[1,2,3] A large number of investiga-SSS → ␣reaction tions have been concerned with the mechanisms of zone and precipitate formation in the alloys, and models for the SSS → VRC → Ј → ␤reaction decomposition process have been proposed. [4][5][6][7][8][9] However, SSS → GP(I) → Ј → ␥reaction results reported in the literature are contradictory, especially those concerning the early stages of the decomposition Recently, Stiller et al [8,9] have carried out an atom probe process.…”