Effect of Natural Aging on Artificial Aging of an Al-2.95Cu-1.55Li-0.57Mg-0.18Zr Alloy at 160oC
GONG Xiangpeng, WU Cuilan(), LUO Shifang, SHEN Ruohan, YAN Jun
Center for High-Resolution Electron Microscopy, College of Materials Science and Engineering, Hunan University, Changsha 410012, China
Cite this article:
GONG Xiangpeng, WU Cuilan, LUO Shifang, SHEN Ruohan, YAN Jun. Effect of Natural Aging on Artificial Aging of an Al-2.95Cu-1.55Li-0.57Mg-0.18Zr Alloy at 160oC. Acta Metall Sin, 2023, 59(11): 1428-1438.
Due to their excellent combination of low density, high strength, and stiffness, third-generation Al-Cu-Li-Mg alloys are important lightweight materials in the aerospace industry. Precipitation strengthening or hardening, which is controlled by precipitates, including the structure, size, morphology, distribution, and volume fraction of precipitates, is mainly responsible for the alloy's excellent mechanical properties. The precipitates in Al-Cu-Li-Mg alloys mainly include T1, S, δ', δ'/θ'/δ', θ', GPB, and various metastable phases. In practice, the Al alloys are inevitably stored for a period at ambient temperature before subsequent processing during which natural aging occurs. Natural aging has an important effect on the precipitation behavior of subsequent artificial aging in Al-Cu-Li-Mg alloys, but the related mechanism is still highly controversial. To solve this problem, the effects of natural aging treatment on the microstructure and mechanical properties of an Al-2.95Cu-1.55Li-0.57Mg-0.18Zr alloy treated by artificial aging at 160oC were investigated using TEM, three-dimensional atom probe (3DAP), three-dimensional electron tomography (3DET), and mechanical property testing. It was discovered that natural aging significantly changed the artificial aging hardening behavior and caused two strengthening peaks in the alloy's hardness curve. The Mg-rich and Cu-Mg clusters and δ' precipitates formed during natural aging were first dissolved at the initial stage of artificial aging, which resulted in a decrease in hardness. Then, large numbers of GPB zones formed uniformly and dispersedly, followed by the formation of T1 precipitates with the increase of aging time, which caused the increase in hardness. The hardness reached its first peak value at 96 h. Following that, GPB zones dissolved and the hardness decreased again. When the aging time was continuously exceeded, the volume fraction of T1 precipitates and the number of lath-like S precipitates increased, so the hardness increased again and reached the second peak value at 192 h. In other words, the atomic clusters that form during natural aging can significantly modify precipitation behaviors and the evolution of mechanical properties during artificial aging.
Fund: National Natural Science Foundation of China(51831004);National Natural Science Foundation of China(52171006);National Natural Science Foundation of China(11427806)
Corresponding Authors:
WU Cuilan, professor, Tel: (0731)88664010, E-mail: cuilanwu@hnu.edu.cn
Fig.1 Mechanical properties of the alloys with T6 and T66 treatment (a) age-hardening curves vs aging time (b) engineering stress-strain curves of peak-aged T6 alloys at 96 and 192 h, respectively (c) engineering stress-strain curves of peak-aged T66 alloys at 96 and 192 h, respectively (d) diagram of tensile properties
Fig.2 Microstructures and T1 diameter distribution of the peak-aged T6 sample for 96 h (a) HAADF image of precipitates viewed along the [001]Al direction (b) atomic-resolution HAADF images of rod-like S precipitates and GPB zones viewed along the [001]Al direction (c) HAADF image of T1 precipitates viewed along the [110]Al direction (Inset is the atomic-resolution image of a T1 precipitate) (d) T1 diameter distribution
Fig.3 Microstructures and T1 diameter distribution of the peak-aged T6 sample at 192 h (a) HAADF image of precipitates viewed along the [001]Al direction (Inset is the corresponding atomic-resolution HAADF image of a δ'/θ'/δ' composite precipitate) (b) HAADF image of T1 precipitates viewed along the [110]Al direction (c) T1 diameter distribution
Fig.4 TEM images of microstructures of the sample which is natural aged (NA) for 7 d (a) atomic-resolution HAADF image of δ'-phase precipitates viewed along the [001]Al orientation (Inset is the selected area electron diffraction (SAED) pattern) (b) atomic-resolution HAADF image of GPI zones viewed along the [001]Al orientation
Fig.5 3DAP analyses of atomic clusters in the sample which is natural aged for 7 d (dpair—distance between nearest-neighbor atoms) (a) nearest-neighbor analysis of Mg (b) nearest-neighbor analysis of Cu (c) distribution of Mg clusters and Mg-Cu co-clusters
Fig.6 HAADF-STEM images and SAED pattern of the T66 sample aged for 8 h (a) morphology of precipitates viewed along [001]Al direction and SAED pattern (inset) (b) morphology of T1 precipitates viewed along [110]Al direction
Fig.7 HAADF-STEM images of the first-peak-aged T66 sample at 96 h (a) morphology of precipitates viewed along the [001]Al direction (b) atomic-resolution HAADF images of the precipitates viewed along the [001]Al direction (c) morphology of T1 precipitates viewed along the [110]Al direction (Inset is the atomic-resolution HAADF image of a T1 precipitate)
Fig.8 HAADF-STEM images of the valley-aged T66 sample at 144 h (a) morphology of precipitates viewed along the [001]Al direction (Insets are the corresponding atomic-resolution HAADF images of the precipitates) (b) morphology of T1 precipitates viewed along the [110]Al direction (Inset is the atomic-resolution HAADF image of a T1 precipitate)
Fig.9 HAADF-STEM images of the second-peak-aged T66 sample at 192 h (a) morphology of precipitates viewed along the [001]Al direction (Inset is the atomic-resolution HAADF image of a lath-like S precipitate) (b) morphology of T1 precipitates viewed along [110]Al direction (Inset is the atomic-resolution HAADF image of a T1 precipitate)
Fig.10 Statistics of T1 precipitate diameter of samples with T66 treatment (a) first-peak-aged sample (b) valley-aged sample (c) second-peak-aged sample (d) aspect ratios of T1 phases
Fig.11 3DET images of precipitates in the two peak-aged T66 samples and the number density of T1 precipitates (a, b) plain view of peak 1 (a) and peak 2 (b), respectively (c, d) top-down view of peak 1 (c) and peak 2 (d), respectively (e) number density of T1 precipitates
Fig.12 Schematic of mechanical properties and microstructure evolution of the T66 alloy
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