Effect of Creep Aging on Mechanical Properties of Under-Aged 7075 Aluminum Alloy
GAO Chuan1, DENG Yunlai1,2, WANG Fengquan1, GUO Xiaobin2()
1.Light Alloy Research Institute, Central South University, Changsha 410083, China 2.School of Materials Science and Engineering, Central South University, Changsha 410083, China
Cite this article:
GAO Chuan, DENG Yunlai, WANG Fengquan, GUO Xiaobin. Effect of Creep Aging on Mechanical Properties of Under-Aged 7075 Aluminum Alloy. Acta Metall Sin, 2022, 58(6): 746-759.
The 7075 alloy is widely used in the manufacture of aerospace components, such as aircraft wings and fuselage plates, owing to its high strength and light weight. Moreover, it is well-suited for manufacturing these massive aviation components using creep aging forming (CAF) technology. In this present study, the effect of creep aging on the mechanical properties of under-aged 7075 alloy was systematically studied in detail by means of a uniaxial creep tensile test and a stress-free artificial aging test. EBSD, SEM, and TEM observations were used to characterize the evolution of dislocations and precipitates with creep aging time. A quantitative analysis was performed on the relationship between mechanical properties and microstructure evolution. The results show that creep aging greatly improves the plasticity of the under-aged 7075 aluminum alloy while maintaining its high strength. The mechanical properties of the alloy are sensitive to creep stress. The sample aged for 6 h under 260 MPa and 426 K has the maximum yield strength, reaching 537.9 MPa. In comparison to the artificial aging sample, the dimple distribution of the creep aging sample is denser and the grain is more inclined toward a high Schmid factor orientation, which is 15% higher than the artificial aging sample. TEM results show that the primary phase in the crystal is η′ phase. The size of the precipitated phase in the crystal grows with increasing creep aging time from 3.04 nm for 2 h to 4.27 nm for 6 h and the volume fraction increases from 0.22% to 0.46%. The size of the grain boundary precipitates increases and the transition from continuous to discontinuous occurs. The EBSD results show that no significant change in the recrystallization and subgrain ratio occurred in any of the samples, and the average grain size remains approximately 80 μm. The distribution of geometrically necessary dislocations (GND) decreases first and subsequently increases with the extension of creep aging time. The contribution of grain boundary strengthening to the yield strength contribution model is shown to be essentially constant at about 17 MPa, and the coupling effect of dislocation and precipitation strengthening is the primary reason for the increase in strength.
Fig.1 Evolution of material preparation and precipitation behavior (SSSS—supersaturated solid solution)
Fig.2 7075 aluminum alloy creep specimen (RD—rolling direction, ND—normal direction, TD—transverse direction) (a) sampling position of creep specimen (b) dimensions of creep specimen, tensile specimen, and sampling position of subsequent experiment (unit: mm)
Fig.3 Creep strain behavior (a) and corresponding creep rate (b) of under-aged 7075 aluminum alloy under different stresses at 426 K
Fig.4 Hardening curves of 7075 aluminum alloy after the aging treatments of UA + AA (a) and UA + CA (b) (UA—under aged, CA—creep aging, AA—artificial aging)
Fig.5 Mechanical properties of 7075 aluminum alloy samples with different aging treatments (a) yield strength (b) tensile strength (c) elongation
Fig.6 Typical fracture morphologies of samples under different aging treatments (a) UA + CA/260 MPa-2 h (b) UA + CA/260 MPa-6 h (c) UA + CA/260 MPa-10 h (d) UA + AA/8 h
Fig.7 OM images of microstructures under solution state (a) and aging state (b) of 7075 aluminum alloy
Fig.8 EBSD inverse pole figures (a-d) and corresponding Taylor factor diagrams (e-f) of samples under different aging treatments (a, e) UA + CA/260 MPa-2 h (b, f) UA + CA/260 MPa-6 h (c, g) UA + CA/260 MPa-10 h (d, h) UA + AA/8 h
Fig.9 Bright field TEM images and corresponding SAED pattern (inset) along [011] of 7075 aluminum alloy under different aging treatments (a) UA + CA/260 MPa-2 h (b) UA + CA/260 MPa-6 h (c) UA + CA/260 MPa-10 h (d) UA + AA/8 h
Sample
Number density of precipitated phase
Volume fraction
η′ radius
1022 mm-3
%
nm
UA + CA/260 MPa-2 h
1.50 ± 0.04
0.22 ± 0.07
3.04 ± 0.12
UA + CA/260 MPa-6 h
1.13 ± 0.03
0.46 ± 0.05
4.27 ± 0.21
UA + CA/260 MPa-10 h
0.93 ± 0.05
0.39 ± 0.08
4.32 ± 0.36
UA + AA/8 h
1.12 ± 0.03
0.46 ± 0.08
4.29 ± 0.28
Table 1 Statistics of precipitates in creep aging and artificial aging at different time
Fig.10 Kernel average misorientation (KAM) diagrams of samples with different aging treatments (a) UA + CA/260 MPa-2 h (b) UA + CA/260 MPa-6 h (c) UA + CA/260 MPa-10 h (d) UA + AA/8 h
Fig.11 Yield strength model calculation results (σp—precipitation strength, σd—dislocation strength, σss—solution strength, σgb—grain boundary strength) (a) total strength prediction (b) precipitation strengthening and dislocation strengthening error
Fig.12 Schmid factor maps under different aging treatments (a) UA + CA/260 MPa-2 h (b) UA + CA/260 MPa-6 h(c) UA + CA/260 MPa-10 h (d) UA + AA/8 h
Fig.13 Schmid factor distribution diagram (a) and the average diameter of dimples corresponding to the average Schmid factor of samples with different aging treatments (b)
Fig.14 TEM images of grain boundaries of 7075 aluminum alloy along <110> under different aging treatments (PFZ—precipitation free zone) (a) UA + CA/260 MPa-2 h (b) UA + CA/260 MPa-6 h(c) UA + CA/260 MPa-10 h (d) UA + AA/8 h
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