Synchronous improvement of mechanical properties and stress corrosion resistance by stress-aging coupled with natural aging pre-treatment in an Al-Zn-Mg alloy with high recrystallization fraction

doi:10.1016/j.jmst.2021.11.068

Abstract

Abstract:

Enhancing the strength of Al-Zn-Mg alloys is critical to the weight-lightening of structural components in the application of high-speed trains and aerospace industries, while high stress corrosion cracking (SCC) susceptibility of Al-Zn-Mg alloys (especially the alloy with high recrystallization fraction) with high strength makes it difficult. In this study, the influence of tensile stress-aging coupled with natural aging pre-treatment on the mechanical properties and SCC resistance of Al-Zn-Mg alloy with high recrystallization fraction has been investigated. The results show that tensile stress-aging at 160 ℃ can inhibit the dissolution of clusters/Guinier-Preston (GP) zones formed during long-term natural aging pre-treatment, which increases the number density of matrix precipitates (MPts), narrow the width of precipitate free zone (PFZ), and dramatically improve the mechanical properties of the experimental Al-Zn-Mg alloy. Meanwhile, the precipitation of the high density of MPts within the matrix will assume a large number of solute atoms during artificial aging, which will reduce the supplement of solute atoms to grain boundaries. As a result, the volume of anodic active grain boundary precipitates (GBPs) and the content of free solute atoms at grain boundaries are reduced, which reduces the possibility of initial nucleation and propagation of SCC crack. The coupled treatment method proposed in this study proves efficient in resolving the contradiction between the strength and SCC resistance in Al-Zn-Mg alloy.

Key words: Al-Zn-Mg alloy, Natural aging, Stress-aging, Precipitates, Stress corrosion cracking

D. Zhang, H.C. Jiang, Z.J. Cui, D.S. Yan, Y.Y. Song, L.J. Rong. Synchronous improvement of mechanical properties and stress corrosion resistance by stress-aging coupled with natural aging pre-treatment in an Al-Zn-Mg alloy with high recrystallization fraction[J]. J. Mater. Sci. Technol., 2022, 121: 40-51.

Figures/Tables 16

Fig. 1. Schematic diagram for the stress-aging process of Al-Zn-Mg alloys.

Fig. 2. Mechanical performance of Al-Zn-Mg alloys with stress-aged for 12 h at 160 ℃ after different natural aging pre-treatment.

Fig. 3. Age hardening curves of Al-Zn-Mg alloy aged at 160 ℃ after natural aging for 75 days.

Fig. 4. Precipitate size and number density as a function of the natural aging time for Al-Zn-Mg alloy stress-aged for 12 h at 160 ℃.

Fig. 5. Precipitate information obtained from the SAXS measurements after artificial aging at 160 ℃: (a) radius of the assumed sphere as a function of aging time; (b) number density of precipitates as a function of aging time.

Fig. 6. Atom probe tomography analysis of samples artificial aging for 12 h at 160 ℃: (a), (c), (e) and (g) isocomposition surface of 10 at.% Mg & Zn embedded in the Zn matrix. (b), (d), (f) and (h) corresponding proximity histograms of a representative phase marked in (a), (c), (e) and (g), respectively. (a, b) 10-TSA; (c, d) 75-TSA; (e, f) 75-SFA; (g, h) 120-TSA.

Fig. 7. TEM micrographs and statistical results of the subgrain boundaries for Al-Zn-Mg alloy aged for 12 h at 160 ℃ under different heat treatment conditions: (a) 3-TSA; (b) 10-TSA: (c) 30-TSA; (d) 75-TSA; (e) 120-TSA; (f) 75-SFA days; (g) average PFZ width; (h) average GBPs size.

Fig. 8. TEM micrographs of the typical recrystallized grain boundaries for studied Al-Zn-Mg alloy: (a) 10-TSA, and (b) corresponding partial enlarged detail marked in (a); (c) 120-TSA, and (d) corresponding partial enlarged detail marked in (c); (e) 75-SFA, and (f) corresponding partial enlarged detail marked in (e).

Fig. 9. Atom probe tomography analysis of the tensile stress-aged samples with different natural aging treatment: (a) and (d) 3D reconstructed Mg & Zn atom maps of 10-TSA and 120-TSA specimens, respectively, with precipitates defined by isoconcentration surfaces of 10 at.% Mg &Zn (in dark cyan); (b) and (e) 1D composition profile showing the compositions of the selected grain boundary precipitate (GBP) in a 5 nm-diameter cylinder (marked in (a) and (d), respectively) along the z-direction. (c) and (f) 1D Composition profiles across the grain boundary along the direction of the pink arrows (marked in (a) and (b), respectively) using a 5 nm-diameter cylinder.

Fig. 10. Typical SSRT curves of the stress-aged Al-Zn-Mg alloys subjected to different natural aging pre-treatment: (a) in air; (b) in 3.5 wt.% NaCl aqueous solution at pH 3.0.

Table 1. SSRT results of the tensile stress-aged samples after different natural aging pre-treatment.

Alloy temper	Environment	Yield strength(MPa)	Tensile strength(MPa)	Elongation(%)	I_SCC(%)
3-TSA	Air	202.1 ± 6.9	262.8 ± 5.1	19.9 ± 0.4	14.4
3-TSA	Solution	203.8 ± 4.5	264.4 ± 5.3	17.0 ± 0.4	14.4
10-TSA	Air	253.8 ± 10.9	297.0 ± 13.3	16.6 ± 0.2	6.3
10-TSA	Solution	244.4 ± 15.2	286.8 ± 12.3	15.6 ± 0.3	6.3
30-TSA	Air	298.9 ± 6.5	333.5 ± 5.4	16.5 ± 0.2	5.5
30-TSA	Solution	296.7 ± 4.1	329.8 ± 3.1	15.6 ± 0.3	5.5
75-TSA	Air	303.4 ± 5.7	338.5 ± 5.4	17.1 ± 0.5	3.3
75-TSA	Solution	299.6 ± 9.7	333.8 ± 10.5	16.5 ± 0.2	3.3
120-TSA	Air	316.7 ± 1.9	352.0 ± 2.6	15.2 ± 0.0	—
120-TSA	Solution	325.4 ± 6.2	360.3 ± 7.1	15.4 ± 0.1	—
75-SFA	Air	285.8 ± 1.1	322.5 ± 1.7	17.9 ± 0.2	6.0
75-SFA	Solution	288.3 ± 2.0	323.2 ± 2.6	16.8 ± 0.0	6.0

Fig. 11. SEM image of fracture surfaces of the SSRT specimens tested in 3.5 wt.% NaCl aqueous solution at pH 3: (a) 3-TSA; (b) 10-TSA; (c) 30-TSA; (d) 75-TSA; (e) 120-TSA; (f) 75-SFA.

Fig. 12. 3D maps showing the distribution of Zn (in gray) and Mg (in purple) atoms in clusters from the reconstruction after natural aging for (a) 3 days; (b) 30 days; (c) 75 days; (d) 120 days. All boxes are of the same size of 25 mm × 25 mm × 50 nm to facilitate visual comparisons.

Fig. 13. Cluster size distribution from the reconstruction after natural aging for: (a) 3 days; (b) 30 days; (c) 75 days; (d) 120 days.

Fig. 14. DSC heat flux profiles for studied Al-Zn-Mg alloy carried out after various aging times at room temperature with a heat rate of 10 ℃ / min.

Fig. 15. Schematic illustration showing the remarkable influence of stress-aging coupled with natural aging on grain boundary characteristics of Al-Zn-Mg alloy.

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