Revisiting the Hierarchical Microstructures of an Al-Zn-Mg Alloy Fabricated by Pre-deformation and Aging

doi:10.1007/s40195-020-01082-4

Abstract

Abstract:

Pre-deformation before aging has been demonstrated to have a positive effect on the mechanical strength of the 7N01 alloy in our previous study, which is rather different from the general negative effects of pre-deformation on high-strength 7XXX aluminum alloys. In order to explain the strengthening mechanism relating to the positive effect, in the present study, the microstructure of the aged 7N01 alloy with different degrees of pre-deformation was investigated in detail using advanced electron microscopy techniques. Our results show that, without pre-deformation, the aged alloy is strengthened mainly by the η′ type of hardening precipitates. In contrast, with pre-deformation, the aged alloy is strengthened by the hierarchical microstructure consisting of the GP-η′ type of precipitates formed inside sub-grains, the η_p type of precipitates formed at small-angle boundaries, and the dislocation introduced by pre-deformation (residual work-hardening effect). By visualizing the distribution of the η_p precipitates through three-dimensional electron tomography, the 3D microstructures of dislocation cells are clearly revealed. Proper combinations of η_p precipitates, GP-η′ precipitates and residual dislocations in the alloy are responsible for the positive effect of pre-deformation on its mechanical properties.

Key words: Aluminum alloys, Precipitates, Pre-deformation, Three-dimensional electron tomography (3DET), Dislocation cell

Xiong-Wei Yu, Jiang-Hua Chen, Wen-Quan Ming, Xiu-Bo Yang, Tian-Tian Zhao, Ruo-Han Shen, Yu-Tao He, Cui-Lan Wu. Revisiting the Hierarchical Microstructures of an Al-Zn-Mg Alloy Fabricated by Pre-deformation and Aging[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(11): 1518-1526.

Figures/Tables 7

Table 1 Chemical composition of the Al-Zn-Mg alloy (wt%)

Zn	Mg	Fe	Si	Cr	Zr	Ti	Mn	Al
4.50	1.32	0.08	0.06	0.12	0.10	0.08	0.37	Bal.

Fig. 1 Microstructures of the T6 samples: a low-magnification HAADF STEM image of the T6 sample (120 °C/100 h); b HAADF STEM image of the precipitates in a grain. The inset is the atomic resolution HAADF STEM image of a precipitate. c Statistical chart of precipitates’ diameter distribution. The electron beam is parallel to the <011> Al zone axis

Fig. 2 Microstructures of the R20%-100 h sample: a, b low- and high-magnification HAADF images of the R20%-100 h sample showing the dislocation cells; c, d TEM and HAADF image of the same area showing the heterogeneous nucleation of precipitates on dislocations; e statistical chart of diameter of small precipitates and large precipitates

Fig. 3 Atomic-resolution HAADF image of the R20%-100 h samples: a uniformly distributed precipitate in the matrix; b heterogeneously nucleated precipitate at dislocations. All images are recorded with the electron beam along the <112> Al zone axis

Fig. 4. 3DET images of the R20%-100 h sample: a overall morphology of the reconstructed volume; b projection of the tilted volume observed in another angles showing the grain boundary; c, d 3D sectioning of the arrowed area in a along z-axis; c, d displaying the red arrow marked area and the yellow arrow marked area, respectively

Fig. 5 HAADF images of the Rx%-100 h (x = 40, 60 and 80) samples with different pre-deformation degrees: a-c R40%-100 h, d-f R60%-100 h, g-i R80%-100 h. b, e, h are enlarged images of corresponding boxed area marked in a, d, g. The statistical charts of precipitate diameter of the large precipitates are shown in c, f, i, respectively. The images were recorded with the electron beam along the <112> Al zone axis

Fig. 6 Diagrammatic sketch of the forming of dislocation cells and precipitation morphology in the R20%-T6 and R80%-T6 sample upon aging treatment

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