Understanding the precipitation mechanism of copper-bearing phases in Al-Mg-Si system during thermo-mechanical treatment

doi:10.1016/j.jmst.2021.04.026

Abstract

Abstract:

We modified the morphology and distribution of copper-bearing precipitates in an Al-Mg-Si-Cu alloy and obtained ultra-high comprehensive properties through a modified thermo-mechanical treatment composed of aging and cold deformation. The precipitate evolution and atomic structure was observed by employing atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Our results proved the existence of β′′/Q′ composite precipitates in rod-like morphology after aging treatment due to the inhibition of β′′ phase formation by copper atoms. We also revealed that the peak-aged phases transformed into in-situ reverse-transformed Guinier Preston zones (GP zones) during the deformation process. By the means of modifying the precipitates, we finally simultaneously obtained ultra-high strength (424.40 MPa) as well as favorable conductive properties (52.78%IACS), both of which surpassed three mainstream standards for high strength aluminum conductor.

Key words: Al-Mg-Si-Cu alloy, Aging, Copper-bearing precipitates, GP zones, HAADF-STEM

Hongmei Jin, Renguo Guan, Xianxiang Huang, Ying Fu, Jin Zhang, Xiaolin Chen, Yu Wang, Fei Gao, Di Tie. Understanding the precipitation mechanism of copper-bearing phases in Al-Mg-Si system during thermo-mechanical treatment[J]. J. Mater. Sci. Technol., 2022, 96: 226-232.

Figures/Tables 8

Fig. 1. The yield strength changes of the alloy with increasing aging time.

Table 1 Chemical composition (wt%) of the test alloy.

Mg	Si	Cu	Al	Other
0.638	0. 712	0.049	Bal.	<0.003

Fig. 2. TEM images (bright-field) of the aged samples at 175 °C for 3 h (a), 6 h (c) and 9 h (e) and the thermo-mechanical treated (aging + deformation) sample (g) with their corresponding enlarged drawings (b, d, f, h).

Fig. 3. HAADF-STEM micrographs of β″ (a), β″/Q′ (c), Q′ (e) and L (g) and their corresponding FFT patterns (b, d, f, h) of the 9 h aged samples.

Fig. 4. The HAADF-STEM image with corresponding pixel intensity profile and FFT pattern of the in-situ reverse-transformed GP zones corresponding to previous β″ (a), β″/Q′ (b) and L (c) formed by dislocation motion during deformation process. The GP zone spots are circled in red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5. The schematic diagrams of the precipitate evolution (a) and the size distribution of the GP zones formed during different processing routes (b).

Table 2 Mechanical properties and electrical conductivity of the alloy under different conditions.

Conditions	Tensile strength (MPa)	Yield strength (MPa)	Elongation(%)	Electrical conductivity (%IACS)
Untreated	152.28	97.35	21.79	60.25
Aging	312.12	289.61	10.56	52.85
Aging + deformation	424.40	410.72	6.71	52.78
Standard: GB/T LHA1 [20]	325	NA	3	52.5
Standard: ASTM B398 [21]	330	NA	3	52.5
Standard: EN50183 AL2 [22]	325	NA	3	52.5

Fig. 6. Stress-strain curves of the alloy under different conditions.

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