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Effects of creep-aging processing on the corrosion resistance and mechanical properties of an Al–Cu–Mg alloy
Materials Science & Engineering A 605 (2014) 192–202
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effects of creep-aging processing on the corrosion resistance and mechanical properties of an Al–Cu–Mg alloy Y.C. Lin a,b,n, Yu-Qiang Jiang a,b, Yu-Chi Xia a,b, Xian-Cheng Zhang c, Hua-Min Zhou d, Jiao Deng a,b a
School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China State Key Laboratory of High Performance Complex Manufacturing, Changsha 410083, China c Key Laboratory of Pressure Systems and Safety, Ministry of Education, East China University of Science and Technology, Shanghai 200237, China d State Key Laboratory of Material Processing and Die & Mould Technology, Wuhan 430074, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 10 November 2013 Received in revised form 23 January 2014 Accepted 16 March 2014 Available online 22 March 2014
Creep-aging forming, combining both the aging treatment and forming process, has recently drawn much attention of researchers. In this study, the effects of creep-aging processing on the corrosion resistance and mechanical properties of a typical Al–Cu–Mg alloy (2024 aluminium alloy) are investigated by electrochemical corrosion, exfoliation corrosion and uniaxial tensile experiments. The results show that the corrosion resistance and mechanical properties of the studied Al–Cu–Mg alloy are strongly sensitive to the creep-aging processing parameters. The creep-aging processing can change the dimension and density of nanoprecipitates, which greatly affects the corrosion resistance and mechanical properties of the studied Al–Cu–Mg alloy. With the increase of creep-aging temperature, the corrosion resistance and the elongation decrease, while the yield strength and ultimate tensile strength increase. For the materials creep-aged in “under aging” and “peak aging” conditions, the applied stress can increase the ultimate tensile strength, but deteriorate the corrosion resistance. & 2014 Elsevier B.V. All rights reserved.
Keywords: Creep-aging Aluminum alloy Corrosion resistance Mechanical property Nanoprecipitate
1. Introduction Due to their optimal combination of physical and mechanical properties, Al–Cu–Mg alloys are primarily developed for aerospace vehicles structures, such as wing skins, machined fuselage bulkheads and engine areas [1–3]. The preferential disposition towards the selection and usage of Al–Cu–Mg alloy is always attributed to the guaranteed fracture toughness, improved short transverse properties, relatively high strength and good corrosion resistance [4–6]. In past, some investigations are carried out to study the relationships between the external conditions, microstructural evolution and mechanical properties of Al–Cu–Mg alloys [7–18]. Marceau et al. [7] demonstrated a diffusion couple approach for the combinatorial study of the compositional dependence of the rapid hardening phenomena in Al–Cu–Mg alloys, and a series of Al–Cu/Al–Cu–Mg and Al–Mg/Al–Cu–Mg diffusion couples have been successfully fabricated to contain a gradient in Mg or Cu concentration, respectively. Feng et al. [8] studied the
n Corresponding author at: School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China. Tel.: þ 86 13469071208. E-mail addresses: [email protected], [email protected] (Y.C. Lin).
http://dx.doi.org/10.1016/j.msea.2014.03.055 0921-5093/& 2014 Elsevier B.V. All rights reserved.
heterogeneous nucleation and growth of precipitates at dislocations in 2024 aluminum alloy, and found that the precipitation sequence of S (Al2CuMg) phase along dislocations is SSS-GPB zones-S(Type I)-S(Type I) þS(Type II). Jiang et al. [9] studied the creep-fracture behavior of 2124 aluminum alloy, and a Monkman– Grant relationship between the minimum creep rate and time for reaching 1.5% creep strain was proposed. Alexopoulos [10] studied the corrosion-induced mechanical degradation of 2024 aluminum alloy experienced different artificial aging conditions. Banerjee et al. [11] studied the hot compressive deformation behaviors of some typical Al–Cu–Mg alloys microalloyed with Sn. Moy et al. [12] investigated how the precipitation hardening affects the mechanical properties and formability of two 2024 aluminum sheets, and concluded that the anisotropy resulting from precipitation hardening can be used to improve the formability of aluminum sheets while keep the minimal loss in strength through the suitable aging time. Also, Moy et al. [13] presented an inverse analysis approach to identify the mechanical properties of 2024 aluminum alloy prepared using different age-hardening treatments. Rosales and Iannuzzi [14] correlated the distributions of alloying elements with the MIC attack morphology of 2024-T351 aluminum alloy by the fungus Hormoconis resinae. Lin et al. [15,16] studied the creep behaviors of Al–Cu–Mg alloys, and established
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the constitutive models to predict the high-temperature creep strain based on the θ projection method. Also, considering the effects of the applied stress on the creep damage during tertiary stage, Li et al. [17] proposed the modified continuum damage mechanics models to estimate the high-temperature creep behavior of 2124 aluminum alloy. Maximov et al. [18] studied the strain hardening and creep behavior of 2024-T3 aluminum alloy at room and high temperature, and a FE simulation model was established to study the residual stress relaxation. Additionally, some investigations were carried out to accurately describe the macroscopical deformation and damage processes during the creep of some other metals or alloys [19–21], as well as the effects of aging processing parameters on the microstructures and mechanical properties of different metals or alloys [22–33]. Marlaud et al. [22] studied the relationship between the alloy composition, microstructure and exfoliation corrosion in Al–Zn– Mg–Cu alloy, and found that the exfoliation corrosion occurs via two different mechanisms, i.e., inter-granular dissolution and inter-granular fracture induced damage. Xiao et al. [23] studied the retrogression and re-aging treatment on the corrosion behavior of an Al–Zn–Mg–Cu alloy, and found that the discontinuous grain boundary precipitates can improve the corrosion resistance. Rokni et al. [24] studied the microstructural evolution and mechanical properties of back extruded 7075 aluminum alloy. Demira and Gündüzb [25] investigated the effects of artificial aging (as-received, solution heat treated and then artificial aging) on the machinability of 6061 aluminum alloy. Whittaker et al. [26] investigated the high-temperature creep fracture behavior of 316H stainless steel. Jiang et al. [27] studied the creep behavior of Mg–5Li–3Al–(0,1)Ca alloys, and found that the grain boundary sliding and twinning are the main creep mechanisms. Whittenberger et al. [28] studied the elevated temperature creep deformation in solid solution strengthened 〈001〉 NiAl–3.6Ti single crystals, and a forced temperature-compensated power law fit using the activation energy for diffusion was established to adequately (4 90%) predict the observed creep properties. Mahmudi et al. [29] investigated the impression creep behavior of Cu–0.3Cr–0.1Ag in the as-processed and aging-treated conditions. They found that the creep resistance of the aging-treated material was much higher than that of the as-processed condition, because the Cr-rich particles precipitate in the Cu matrix. Also, Mahmudi et al. [30] studied the creep behavior of Sn–9Zn, Sn–8Zn–3Bi, and Sn–37 Pb solders at room-temperature, and found that Sn–9Zn and Sn–8Zn–3Bi alloys exhibit the better creep resistances, attributing to both solid solution hardening and precipitation of Bi in the Sn matrix. Oliveira et al. [31] studied the correlations between the corrosion resistance and the semiconducting properties of the oxide film formed on AZ91D alloy after solution treatment. Chen et al. [32] studied the high-temperature creep behavior of low-temperature-sintered nano-silver paste films, and the creep rupture life was predicted by the Monkman–Grant relationship and the θ projection concept. Wang et al. [33] studied the tensile creep behavior and microstructure evolution of the extruded Mg–10Gd–3Y–0.5Zr (wt%, GW103) alloy, and found that the aging treatment exerted limited effect on its creep performance. In aerospace industry, there is an increasing demand for the excellent performance aluminum alloy integral panels, and creepaging forming is an ideal method to manufacture these complex aircraft panels [15–17,34]. Creep-aging forming, combining both the aging treatment and forming process, has recently drawn much attention of researchers [15–17,35–42]. Jeshvaghani et al. [35] investigated the springback and mechanical properties of 7075 aluminum alloy in creep forming process, found that the appropriate forming cycle is 150 1C/24 h among all forming conditions. Chen et al. [36] investigated the effects of age-forming on
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microstructures and mechanical properties of 7050 aluminum alloy, and found that the applied stress induces the coarsening of precipitates in 7050 Al alloy. Also, the texture in the alloy was influenced by age-forming. Jeshvaghani et al. [37] studied the effects of microstructural changes on the hardening and creep deformation behaviors of 7075 aluminum alloy, and their results indicated that the size of globular shaped phase (known as GP zones) increases and transforms into η0 precipitates with increasing the forming time and temperature. Further, increasing forming time leads to a decrease in the number of η0 precipitates and an increase in precipitate spacing, where the large equilibrium η phase forms and the precipitate free zones became wider. Lin et al. [38,39] investigated the effects of external stress and creep-aging temperature on the hardness and precipitation process in typical Al–Cu–Mg alloys (2124 and 2024 aluminum alloys), and found that the hardness and precipitation process are sensitive to the external stress and creep aging temperature, and S phase (CuMgAl2) is the main precipitate under the tested conditions. Meanwhile, the creep aging leads to the discontinuous distribution of precipitation phase in grain boundary, which can improve the corrosionresistance of Al–Cu–Mg alloys. Also, Lin et al. [40] investigated the effects of external stress and creep-aging temperature on the precipitation process in7075 aluminum alloy, and found that the precipitates are discontinuously distributed on the grain boundary and the width of precipitate free zone increases with the increase of creep-aging temperature and applied stress. Liu et al. [41] investigated the creep age formability of friction stir welded 2A12 aluminum alloy structures, and found that the springback of the integral part decreases with increasing the aging time and temperature. Guo et al. [42] studied the associated materials aspects-age hardening during the age-forming process, particularly the stress-aging effects on precipitation and Vickers microhardness of Al–Zn–Mg–Cu alloys. Therefore, the effects of creepaging processing parameters (creep-aging temperature and applied stress) on the final heterogeneous microstructure are significant, which greatly affects the corrosion resistance and properties of alloys to some extent. Despite some efforts invested on the effects of external stress and creep-aging temperature on the hardness and precipitation processes in some Al–Cu–Mg alloys, little studies have been reported on the effects of creep-aging processing on the corrosion resistance and mechanical properties, as well as the relationships between the creep-aging processing, corrosion resistance and mechanical properties of Al–Cu–Mg alloys. In this study, the effects of creep-aging processing on the corrosion resistance and mechanical properties of a typical Al–Cu–Mg alloy are investigated by electrochemical corrosion, exfoliation corrosion, and uniaxial tensile experiments. A comprehensive insight into the relationships between the creep-aging processing, corrosion resistance and mechanical properties of the studied Al–Cu–Mg alloy are obtained.
2. Material and experiments A typical commercial Al–Cu–Mg alloy (2024 aluminium alloy) was used in this investigation. The chemical compositions (wt%) of the studied aluminum alloy are 4.59Cu–1.45Mg–0.63Mn–0.11Fe– 0.05Zn–0.02Ti–0.12Si–(bal.)Al. The heat treatment processing is T3, which means the solution treatment, cold deformation and then natural aging. The specimens with a rectangular cross section of 2.5 mm 15 mm and a gauge length of 50 mm were machined along the rolling direction of the studied aluminum alloy sheet. Based on the previously optimized creep-aging parameters [16,38], three different creep temperatures (423, 448 and 473 K) and three different stresses (185, 205 and 225 MPa) were selected in this study.
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The specimens were creep-aged for 24 h on MTS-GWT2105 test machine according to ASTM E 139-06 standard [43]. The accuracies of loading and heating system of the MTS-GWT2105 test machine are 0.1 MPa and 1 K, respectively. The specimen was firstly heated to the experimental temperature at a heating rate of 10 K/min and held for 30 min. Then, the extra stress was applied. After the 24-h creep-aging, the applied loading was released, and the specimen was naturally cooled down to the room temperature in the furnace. In order to study the effects of the creep-aging processing on the corrosion resistance and mechanical properties of the studied aluminum alloy, the electrochemical corrosion, exfoliation corrosion and uniaxial tensile experiments were carried out. The specimens for uniaxial tensile tests and corrosion resistance tests were directly machined from the gauge of creep-aged materials. The rectangular cross section of the dog-bone flat tensile specimens is 3.2 mm 2.5 mm and the gauge length is 16 mm. The uniaxial tensile experiments were carried out at room temperature on the MTS-810 landmark test system, and the tensile velocity is 2 mm/min. The tensile test under each creep-aging condition repeated five times, and the standard error (SE) is computed to represent the overall distribution of measured data. The standard (SE) error is calculated by dividing the standard deviation (SD) by the square root of experimental number, i.e., sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ∑ni¼ 1 ðX i XÞ2 SD ¼ ð1Þ n SD SE ¼ pffiffiffi n
ð2Þ
where X i and X are the i and mean experimental results, respectively. n is the number of the repeated experiments (n ¼ 5). Graphically, the standard error is represented by the error bar. Based on the tensile experimental results, the standard errors (SE) of the ultimate tensile strength, yield strength and elongation under different deformation conditions are evaluated, as shown in Table 1. Obviously, the repeatability of tensile experiments is satisfactory. The potentiodynamic polarization experiments were performed under a scan rate of 0.01 V/s using CHI660C electrochemical testing instrument. The sample was degreased with ethanol after the mirror polishing. The non-tested surfaces were protectively coated using a stop-off lacquer. Standard calomel electrode (SCE) was used as the reference electrode. Platinum was regarded as the auxiliary electrode, and the sample served as the working electrode. The exposed area was 100 mm2 and the experiments were carried out in 3.5 wt% NaCl solution at 293 K. The accelerated exfoliation corrosion (EXCO) experiments were performed according to the standard EXCO test, as described in ASTM G34-01 [44]. The sample was firstly degreased with ethanol
after mirror polishing. The non-tested surfaces were protectively coated using a stop-off lacquer to avoid the crevice corrosion beneath the coating. The EXCO test solution was prepared as 4.0 M NaCl þ0.5 M KNO3 þ0.1 M HNO3. The solution temperature was 298 K, and the ratio of electrolyte volume to electrode surface area was 25 mL/cm2. After the 96-h immersion in the EXCO test solution, the corrosion morphologies were observed. The fracture surface morphologies after tensile experiments were examined using TESCAN MIRA 3 Scanning Electron Microscope (SEM). The transmission electron microscope (TEM) observations were performed on the JEM-2100F microscope operating at 200 kV. Samples for TEM observations were cut from the creepaged specimens, and then grinded into 0.7–0.8 mm thin foils. Several disks with a diameter of 3 mm were punched out from these thin foils, and subsequently electro-polished using a solution of hydrogen nitrate and methanol (1:4 in volume).
3. Results and discussion 3.1. Effects of creep-aging processing on corrosion resistance According to the authors' previous studies [38], it is found that the precipitation process is very sensitive to the applied stress and creep-aging temperature. 423, 448, and 473 K are validated as the “under aging”, “peak aging” and “over aging” temperatures, respectively. An applied stress can accelerate the precipitation hardening (also called age hardening) of Al–Cu–Mg alloy. The large applied stress and high creep-aging temperature easily make the preferential precipitation process as SSS-GPB-S″-S0 -S, and S phase (CuMgAl2) is the main precipitate under the experimental conditions. With the increase of applied stress and creep-aging temperature, S phase easily grows up and coarsens. The change of precipitates can affect the corrosion resistance of aluminum alloy. In this section, the effects of creep-aging processing parameters (the applied stress and aging temperature) on the corrosion resistance of the studied alloy will be discussed. 3.1.1. Effects of creep-aging temperature on corrosion resistance Fig. 1 shows the potentiodynamic polarization curves of the creep-aged Al–Cu–Mg alloy under the applied stress of 185 MPa and different creep-aging temperatures. The electrochemical characteristics derived from the polarization curves are listed in Table 2. Generally, the electrochemical corrosion resistance gradually increases with the increase of corrosion potential (towards anodic direction), and the pitting potentials have been commonly used to evaluate the pitting resistance of alloys. From Fig. 1 and Table 2, it can be easily found that the creep-aging processing has a substantial impact on the electrochemical corrosion resistance of the studied aluminum alloy. The pitting potential of the as-received
Table 1 Standard errors (SE) of tensile experiments under different deformation conditions.
As-received 423
448
473
Applied stress (MPa)
Standard errors of ultimate tensile strength
Standard errors Standard errors of of yield elongation strength
185 205 225
2.78 3.07 1.69 3.29
3.82 3.07 5.41 4.97
0.91 1.01 0.64 0.81
185 205 225
1.86 4.00 1.70
3.92 4.95 4.40
0.87 0.72 0.53
185 205 225
3.79 1.98 1.50
4.12 3.78 4.96
0.49 0.58 0.75
Potential (VSCE)
Creep-aging temperature (K)
-0.2
-0.4
423 K As-received
-0.6 448 K 473 K
-0.8 10
-10
10
-8
10
-6
10
Current Density
-4
10
-2
1
(A/cm2)
Fig. 1. Polarization curves of the studied aluminum alloy under the applied stress of 185 MPa and different creep-aging temperatures.
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Table 2 Corrosion characteristics of the studied aluminum alloy under the applied stress of 185 MPa. Creep-aging temperature (K)
Epit (VSCE)
As-received 423 448 473
0.518 0.506 0.552 0.601
EXCO rating
ED
As-received 423 K 448 K 473 K
EC EB EA P N 0
20
40
60
80
100
Immersion time (h) Fig. 2. Effects of creep-aging temperature on the exfoliation corrosion rating of the studied aluminum alloy under the applied stress of 205 MPa.
sample is 0.518 VSCE. When the sample is creep-aged under the temperature of 423 K, the pitting potential is 0.506 VSCE. However, when the creep-aging temperature is 448 K, the pitting potential is 0.552 VSCE, which indicates that the corrosion resistance decreases compared with that of the as-received sample. When the creep-aging temperature is increased to 473 K, the pitting potential is 0.601 VSCE. So, it can be concluded that the corrosion resistance of the creep-aged Al–Cu–Mg alloy decreases with the increase of creep-aging temperature. This is because the creepaging temperature can change the size and density of aging precipitates in the aluminum matrix, which results in the changed corrosion resistance of the studied alloy. Fig. 2 shows the effects of creep-aging temperature on the exfoliation corrosion rating of the creep-aged alloy under the applied stress of 205 MPa. Also, the visual exfoliation corrosion morphologies of the creep-aged samples are observed, as shown in Fig. 3. The EXCO rating P indicates the occurrence of pitting corrosion. EA shows that there is only slight pitting corrosion on the surface of specimens. EB indicates the moderate surface corrosion. EC stands for the notable peeled-off laying and penetration. ED is similar to EC except for much greater penetration and loss of metal. With the increase of EXCO rating from EA to ED, the exfoliation corrosion resistance gradually decreases. Obviously, the exfoliation corrosion susceptibility of the studied aluminum alloy is sensitive to the creep-aging temperature. From Fig. 3, it can be found that the severe exfoliation corrosion (with a corrosion rating of EB) occurs on the as-received sample after immersion in the EXCO solution for 96 h. A notable peeled-off layering can be found on the sample surface, as shown in Fig. 3(a). Meanwhile, when the sample is creep-aged under the temperature of 423 K, relative less material is peeled off from the sample surface (Fig. 4b), and the exfoliation corrosion rating decreases to EB-, which indicates that the exfoliation corrosion resistance increases, compared with that of the as-received sample. However, when the creep-aging temperatures are 448 and 473 K, the exfoliation corrosion ratings are EC and ED, respectively. Large peeled-off layering can be found on the surface of the creep-aged samples, and the peeled-off layering penetrates to a considerable depth into the material, as shown in Fig. 3(c) and (d). So, the increase of
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creep-aging temperature can deteriorate the corrosion resistance of the studied aluminum alloy. 3.1.2. Effects of applied stress on corrosion resistance Fig. 4 shows the potentiodynamic polarization curves of the creep-aged Al–Cu–Mg alloy under different applied stresses. The electrochemical characteristics derived from the polarization curves are listed in Table 3. Fig. 4 and Table 3 indicate that the applied stress has significant effects on the corrosion resistance of the studied alloy under all the tested creep-aging conditions. Under the creep-aging temperature of 423 K, when the applied stress is increased from 185 MPa to 225 MPa, the pitting potential decrease from 0.506 VSCE to 0.521 VSCE, and the exfoliation corrosion rating (Fig. 5) increases from EA þ to EB . Meanwhile, the corrosion depth increases with the increase of applied stress, as shown in Fig. 6(a) and (b). Under the creep-aging temperature of 448 K, when the applied stress is increased from 185 MPa to 225 MPa, the pitting potential decrease from 0.552 VSCE to 0.581 VSCE, and the exfoliation corrosion rating (Fig. 5) increases from EC to EC þ . So, the corrosion resistance decreases with the increase of applied stress under relative low creep-aging temperatures (423 and 448 K). However, under the creep-aging temperature of 473 K, when the applied stress is increased from 185 MPa to 225 MPa, the pitting potential increases from 0.601 VSCE to 0.516 VSCE. Furthermore, with the increase of applied stress, the exfoliation corrosion rating decreases from ED þ to ED, and the corrosion depth decreases, as shown in Fig. 6(c) and (d), which indicates the corrosion resistance increases with the increase of applied stress. 3.2. Effects of creep-aging processing on mechanical properties The effects of creep-aging processing on the yield strength, ultimate tensile strength and elongation are shown in Fig. 7(a)–(c). The mechanical properties of the as-received sample are also plotted in dash lines in Fig. 7 for comparison with the creepaged samples. Obviously, the mechanical properties of the studied aluminum alloy are significantly affected by the creep-aging processing parameters. This is because the creep-aging temperature and applied stress can promote the precipitation and coarsen of precipitates in the studied alloy. From Fig. 7(a) and (b), it can be found that the yield strength and ultimate tensile strength increase with the increase of creep-aging temperature. When the creep-aging temperature is 423 K, the yield strength and ultimate tensile strength are smaller than those of the as-received sample. The yield strength and ultimate tensile strength increase with the increase of applied stress under the creep-aging temperatures of 423 and 448 K. However, when creep-aging temperature is 473 K (“over aging” temperature [38]), the yield strength and ultimate tensile strength slightly decreases with the increase of the applied stress. Additionally, the elongation decreases with the increase of creep-aging temperature. When the creep-aging temperature is 423 K, the elongation decreases with the increase of applied stress, and is larger than that of the as-received sample. However, when the creep-aging temperatures are 448 and 473 K, the elongations are greatly smaller than those of the as-received sample. Meanwhile, they are not very sensitive to the applied stress. 3.2.1. Effects of creep-aging temperature on mechanical properties Fig. 8 shows the tensile fracture morphologies of the creepaged samples under the applied stress of 205 MPa and temperatures of 423 and 473 K. From Fig. 8(a), it can be found that small dimples distribute on the fracture morphology, and the tearing edges can also be found. However, Fig. 8(b) shows that the fracture surface is covered with large dimples and no obvious tearing edges
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Fig. 3. Exfoliation corrosion morphologies of the samples under the applied stress of 205 MPa and creep-aging temperatures of (a) as-received, (b) 423 K, (c) 448 K, and (d) 473 K.
-0.2
Potential (VSCE)
Potential (VSCE)
-0.2
-0.4
225 MPa 185 MPa -0.6
-0.8
-0.4 185 MPa
-0.6
225 MPa
-0.8 -8
10
-6
10
-4
-2
10
10
Current Density
-8
1
-6
10
-4
10
(A/cm2)
10
-2
10
1
Current Density (A/cm2)
Potential (VSCE)
-0.2
-0.4 225 MPa
-0.6
185 MPa
-0.8 -10
10
-8
10
-6
10
-4
10
Current Density
-2
10
1
(A/cm2)
Fig. 4. Polarization curves of the studied aluminum alloy under the creep-aging temperatures of (a) 423 K, (b) 448 K, and (c) 473 K.
can be found. Furthermore, the dimples in Fig. 8(b) are bigger than those in Fig. 8(a). Meanwhile, the fracture morphology in Fig. 8 (b) is relative flatter than that in Fig. 8(a). Under the creep-aging temperature of 473 K (“over aging” condition [38]), the high creepaging temperature promotes the quick precipitation and growth of aging precipitates. Due to the incompatibility between the precipitates and aluminum matrix during the tensile deformation, the precipitates impede the dislocations sliding, and thus cause the stress concentration in the vicinity of them. When the concentrated stress exceeds a critical value, the interfacial debonding between the precipitates and aluminum matrix or the breaking of
precipitates occur, which deteriorates the plastic deformation capability of the studied alloy. So, under the “over aging” condition, coarsen aging precipitates impede the dislocation sliding, and deteriorate the plastic deformation capability. As a result, with the increase of creep-aging temperature, the yield strength and ultimate tensile strength increase, while the elongation decreases, as shown in Fig. 7. Fig. 9 shows the fracture morphologies of samples under different creep-aging conditions. From Fig. 9(a), it can be found that the big dimples caused by the interfacial debonding between the precipitates and aluminum matrix are surrounded by shallow dimples.
Y.C. Lin et al. / Materials Science & Engineering A 605 (2014) 192–202
Meanwhile, such big dimples do not link together until the final fracture. This indicates that the creep-aged sample has a good plastic deformation capability. Whereas, due to the coalescence of big dimples caused by the interfacial debonding between the precipitates and aluminum matrix [45], large dimples and cracks appear, as shown in Fig. 9(b) and (c), which severely reduce the homogeneous deformation ability of the aluminum matrix. This is because the high Table 3 Corrosion characteristics of the studied aluminum alloy under different applied stresses. Creep-aging temperature (K)
Applied stress (MPa)
Epit (VSCE)
423
185 225
0.506 0.521
448
185 225
0.552 0.581
473
185 225
0.601 0.516
EXCO rating
ED
185 MPa 225 MPa
EC EB EA P N
423
448
473
Creep-aging temperature (K) Fig. 5. Effects of applied stress on the exfoliation corrosion rating of the studied aluminum alloy.
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creep-aging temperature promotes the precipitation and growth of the aging precipitates. With the sustained increase of the plastic strain, the micro-voids grow up and shrink the linkage regions among voids, which reduce the effective bearing area of tensile specimen. Once the effective bearing area is reduced to a critical value, the aluminum matrixes quickly fracture. Simultaneously, the nucleation and growth of secondary cracks accelerate the fracture of materials and reduce the plastic deformation capacity, as shown in Fig. 7(c).
3.2.2. Effects of applied stress on mechanical properties Fig. 10 shows the fracture morphologies of the creep-aged samples under the creep temperature of 448 K and different applied stresses. With the increase of applied stress, the obvious difference can be found from the fracture morphologies. Small dimples and fractured precipitates can be clearly found in Fig. 10(a). However, both dimples and cleavage fracture morphologies appear on the tensile fracture morphology, as shown in Fig. 10(b–d). For Al–Cu–Mg alloys, the preferential growth orientation of precipitates is sensitive to the applied stress [38,39]. With the increase of applied stress, the aging precipitates grow up in a preferential orientation, namely, stress-orientation effect, as shown in Fig. 11. Due to the semi-incoherent and incoherent relationships between the precipitates and aluminum matrix, the integrity of the crystal structure is destroyed and the binding force between the atoms decreases. The original nucleation of cleavage cracks occurs in different orientation of crystalline grain, and then the cracks extend at a certain crystal surface. The cleavage cracks extend along the low surface energy crystalline planes, as shown in Figs. 10(c) and 12. Generally, the applied stress can promote the precipitation and growth of the aging precipitates. With the increase of applied stress, the density and dimension of aging precipitates increase, and more big dimples can be found from the fracture morphologies (Fig. 10a and d). The work hardening effect is obvious under the high applied stress,
Fig. 6. Exfoliation corrosion morphologies of the studied aluminum alloy under the creep-aging conditions of (a) 423 K—185 MPa, (b) 423 K—225 MPa, (c) 473 K—185 MPa, and (d) 473 K—225 MPa.
Y.C. Lin et al. / Materials Science & Engineering A 605 (2014) 192–202
Yield strength (MPa)
430 400 370
As-received
340 423 K 448 K 473 K
310 280 180
190
200
210
220
230
Ultimate tensile strength (MPa)
198
520
500
As-received
480
423 K 448 K 473 K
460 180
190
200
210
220
230
Applied stress (MPa)
Applied stress (MPa)
Elongation (%)
24
As-received
20 16 12
423 K 448 K 473 K
8 180
190
200
210
220
230
Applied stress (MPa) Fig. 7. Effects of creep-aging processing on the mechanical properties of the studied aluminum alloy. (a) Yield strength, (b) ultimate tensile strength, and (c) elongation.
Fig. 8. Fracture morphologies (2000 ) of samples under the applied stress of 205 MPa and creep-aging temperatures of (a) 423 Kand (b) 473 K.
Fig. 9. Fracture morphologies (4000 ) of samples under different creep-aging conditions. (a) 423 K—205 MPa, (b) as-received, and (c) 473 K—205 MPa.
Y.C. Lin et al. / Materials Science & Engineering A 605 (2014) 192–202
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Fig. 10. Fracture morphologies of samples under the creep-aging temperature of 448 K and applied stresses of (a) 185 MPa and (b–d) 225 MPa.
Fig. 11. TEM micrographs of samples under the creep-aging temperature of 448 K and applied stresses of (a) 185 MPa and (b) 225 MPa.
and the yield strength and ultimate tensile strength increase. However, too many aging precipitates will damage the alloy and decrease the plastic deformation capability, and then the elongation decreases slightly.
creep-aging temperature, S phase easily grows up and coarsens. During the creep-aging heat treatment, the temperature and stress fields change the dimension and distribution of aging precipitates, which affects the corrosion resistance and mechanical properties of Al–Cu–Mg alloys.
3.3. Relationships between creep-aging processing, corrosion resistance and mechanical properties The studied Al–Cu–Mg alloy used in this investigation is a typical age hardenable aluminum alloy. Some investigators [38,39,46] found that the large applied stress and high aging temperature easily make the preferential precipitation process as SSS-GPB-S″-S0 -S. With the increase of applied stress and
3.3.1. Creep-aging processing and corrosion resistance The aging precipitates in Al–Cu–Mg alloy have different corrosion potential, compared to the aluminum matrix. The corrosion resistance of Al–Cu–Mg alloy is affected by the alloying elements, dimension and distribution of aging precipitates. The creep-aging processing can change the dimension and distribution of aging
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precipitates, which greatly affects the corrosion resistance of aluminum alloy [47]. Generally, the Mg and Zn elements in the aluminum matrix shift the corrosion potential to the anodic direction, and Cu-rich precipitates often act as local cathodes, which ultimately drive the anodic dissolution of the surrounding matrix material [48]. The precipitation and growth of S phase (Al2CuMg) will decrease the corrosion resistance of Al–Cu–Mg alloys. Fig. 13 shows the TEM micrographs of the aging precipitates of the studied aluminum alloy under the applied stress of 185 MPa and different creep-aging temperatures. From Fig. 13(a), it can be found that the GPB (Guinier–Preston–Bagaryatsky) zone dissolves back into aluminum matrix under the creep-aging temperature of 423 K [38] and the precipitates grow up and coarsen with the increase of creep-aging temperature, as shown in Fig. 13(b) and (c).
Under the creep-aging temperature of 423 K, the GPB zone dissolves back into the aluminum matrix and it is hard to form the local galvanic cells on the surface of the alloy. So, the corrosion resistance is enhanced when compared with that of the asreceived sample. Nevertheless, with the increase of creep-aging temperature, S phases grow up, which reduces the corrosion resistance of the studied alloy. As a result, the corrosion resistance decreases with the increase of creep-aging temperature. Fig. 14 shows the effects of applied stress on the precipitation of the studied aluminum alloy. From Figs. 13(b) and 14(a), it can be found that the applied stress will promote the precipitation and growth of S phase. The density and dimension of precipitates increase with increasing the applied stress. As mentioned above, S phase can deteriorate the corrosion resistance of aluminum alloys. So, the corrosion resistance decreases with the increase of applied stress, as shown in Figs. 4 and 6. However, when the dimension of precipitates exceeds a critical value, the density of precipitates decreases because the total volume fraction of alloying elements is a constant. According to the authors' previous studies [38], under “over aging” temperature (473 K), the density of precipitates decreases with the increase of applied stress. So, the number of local galvanic cells decreases, the corrosion resistance is enhanced.
3.3.2. Creep-aging processing and mechanical properties Precipitation strengthening provides the most important contribution to the yield strength of age hardenable aluminum alloys. The strengthening mechanisms focus on the interactions between the dislocations and precipitation particles. It is adequate to accept that the contribution of aging precipitates to the yield strength of alloy can be expressed as [49,50]
Δsy ¼ C 1 f m1 rm2
Fig. 12. Cleavage planes and cleavage crack under the creep-aging temperature of 448 K and applied stresses of 225 MPa.
ð3Þ
where Δsy is the yield strength increment, C 1 is a material constant, f is the volume fraction of the shear particles, r is the radius of the shear particles, m1 and m2 are material constants. For most interactions between the dislocations and particles, both m1 and m2 are in the vicinity of 0.5. Consequently, with the increase of particle's dimension and density, the yield strength of the alloy
Fig. 13. TEM micrographs of the studied aluminum alloy under the applied stress of 185 MPa and creep-aging temperatures of (a) 423 K, (b) 448 K, and (c) 473 K.
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Fig. 14. TEM micrographs of the studied aluminum alloy under creep-aging temperature of 448 K and the applied stresses of (a) 205 MPa and (b) 225 MPa.
will increase. As shown in Fig. 13(a), under the creep-aging temperature of 423 K, the precipitates dissolve back into the aluminum matrix. So, the volume fraction of particles decreases, the yield strength and ultimate tensile strength are smaller than those of the as-received specimen. With the increase of creep-aging temperature, the driving force for the growth and coarsen of precipitates is enhanced. The volume fraction and the radius of shear particles increase. So, the yield strength and ultimate tensile strength increase. Meanwhile, the applied stress can promote precipitation process of the studied aluminum alloy and easily make the preferential precipitation process as SSS-GPB-S″-S0 -S. The dimension and density of S phase (Al2CuMg) increase with the increase of applied stress, and the interactions between the dislocation and precipitates will be enhanced. Thus, the yield and ultimate tensile strengths increase.
4. Conclusion The creep-aging behavior of a typical Al–Cu–Mg alloy (2024 aluminium alloy) is studied in this work. The effects of creep-aging processing on the corrosion resistance and mechanical property of the creep-aged Al–Cu–Mg alloy are analyzed. Some important conclusions can be made as follows: (1) The corrosion resistance of the studied Al–Cu–Mg alloy is sensitive to the creep-aging heat treatment. With the increase of creep-aging temperature, the corrosion resistance decreases. For the “under aging” and “peak aging” conditions, the corrosion resistance decreases with the increase of applied stress. However, the applied stress improves the corrosion resistance of the studied alloy under “over aging” condition. (2) With the increase of creep-aging temperature and applied stress, the aging precipitates from the aluminum matrix rapidly grow up. The yield strength and ultimate tensile strength of the creep-aged Al–Cu–Mg alloy increase. However, the elongation decreases with the increase of creep-aging temperature. (3) With the increase of creep-aging temperature, the micro-voids easily grow up and link together, which accelerates the fracture of materials. Meanwhile, the high applied stress easily results in the cleavage failure of the creep-aged Al–Cu–Mg alloy.
Acknowledgments This work was supported by the National Natural Science Foundation Council of China (Grant no. 51125021), the National Key Basic Research Program (Grant no. 2010CB731702), Sheng-hua Yu-ying Program of Central South University, and State Key
Laboratory of Materials Processing and Die & Mold Technology, Huazhong University of Science and Technology (No. 2012P04), China.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effects of creep-aging processing on the corrosion resistance and mechanical properties of an Al–Cu–Mg alloy Y.C. Lin a,b,n, Yu-Qiang Jiang a,b, Yu-Chi Xia a,b, Xian-Cheng Zhang c, Hua-Min Zhou d, Jiao Deng a,b a
School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China State Key Laboratory of High Performance Complex Manufacturing, Changsha 410083, China c Key Laboratory of Pressure Systems and Safety, Ministry of Education, East China University of Science and Technology, Shanghai 200237, China d State Key Laboratory of Material Processing and Die & Mould Technology, Wuhan 430074, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 10 November 2013 Received in revised form 23 January 2014 Accepted 16 March 2014 Available online 22 March 2014
Creep-aging forming, combining both the aging treatment and forming process, has recently drawn much attention of researchers. In this study, the effects of creep-aging processing on the corrosion resistance and mechanical properties of a typical Al–Cu–Mg alloy (2024 aluminium alloy) are investigated by electrochemical corrosion, exfoliation corrosion and uniaxial tensile experiments. The results show that the corrosion resistance and mechanical properties of the studied Al–Cu–Mg alloy are strongly sensitive to the creep-aging processing parameters. The creep-aging processing can change the dimension and density of nanoprecipitates, which greatly affects the corrosion resistance and mechanical properties of the studied Al–Cu–Mg alloy. With the increase of creep-aging temperature, the corrosion resistance and the elongation decrease, while the yield strength and ultimate tensile strength increase. For the materials creep-aged in “under aging” and “peak aging” conditions, the applied stress can increase the ultimate tensile strength, but deteriorate the corrosion resistance. & 2014 Elsevier B.V. All rights reserved.
Keywords: Creep-aging Aluminum alloy Corrosion resistance Mechanical property Nanoprecipitate
1. Introduction Due to their optimal combination of physical and mechanical properties, Al–Cu–Mg alloys are primarily developed for aerospace vehicles structures, such as wing skins, machined fuselage bulkheads and engine areas [1–3]. The preferential disposition towards the selection and usage of Al–Cu–Mg alloy is always attributed to the guaranteed fracture toughness, improved short transverse properties, relatively high strength and good corrosion resistance [4–6]. In past, some investigations are carried out to study the relationships between the external conditions, microstructural evolution and mechanical properties of Al–Cu–Mg alloys [7–18]. Marceau et al. [7] demonstrated a diffusion couple approach for the combinatorial study of the compositional dependence of the rapid hardening phenomena in Al–Cu–Mg alloys, and a series of Al–Cu/Al–Cu–Mg and Al–Mg/Al–Cu–Mg diffusion couples have been successfully fabricated to contain a gradient in Mg or Cu concentration, respectively. Feng et al. [8] studied the
n Corresponding author at: School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China. Tel.: þ 86 13469071208. E-mail addresses: [email protected], [email protected] (Y.C. Lin).
http://dx.doi.org/10.1016/j.msea.2014.03.055 0921-5093/& 2014 Elsevier B.V. All rights reserved.
heterogeneous nucleation and growth of precipitates at dislocations in 2024 aluminum alloy, and found that the precipitation sequence of S (Al2CuMg) phase along dislocations is SSS-GPB zones-S(Type I)-S(Type I) þS(Type II). Jiang et al. [9] studied the creep-fracture behavior of 2124 aluminum alloy, and a Monkman– Grant relationship between the minimum creep rate and time for reaching 1.5% creep strain was proposed. Alexopoulos [10] studied the corrosion-induced mechanical degradation of 2024 aluminum alloy experienced different artificial aging conditions. Banerjee et al. [11] studied the hot compressive deformation behaviors of some typical Al–Cu–Mg alloys microalloyed with Sn. Moy et al. [12] investigated how the precipitation hardening affects the mechanical properties and formability of two 2024 aluminum sheets, and concluded that the anisotropy resulting from precipitation hardening can be used to improve the formability of aluminum sheets while keep the minimal loss in strength through the suitable aging time. Also, Moy et al. [13] presented an inverse analysis approach to identify the mechanical properties of 2024 aluminum alloy prepared using different age-hardening treatments. Rosales and Iannuzzi [14] correlated the distributions of alloying elements with the MIC attack morphology of 2024-T351 aluminum alloy by the fungus Hormoconis resinae. Lin et al. [15,16] studied the creep behaviors of Al–Cu–Mg alloys, and established
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the constitutive models to predict the high-temperature creep strain based on the θ projection method. Also, considering the effects of the applied stress on the creep damage during tertiary stage, Li et al. [17] proposed the modified continuum damage mechanics models to estimate the high-temperature creep behavior of 2124 aluminum alloy. Maximov et al. [18] studied the strain hardening and creep behavior of 2024-T3 aluminum alloy at room and high temperature, and a FE simulation model was established to study the residual stress relaxation. Additionally, some investigations were carried out to accurately describe the macroscopical deformation and damage processes during the creep of some other metals or alloys [19–21], as well as the effects of aging processing parameters on the microstructures and mechanical properties of different metals or alloys [22–33]. Marlaud et al. [22] studied the relationship between the alloy composition, microstructure and exfoliation corrosion in Al–Zn– Mg–Cu alloy, and found that the exfoliation corrosion occurs via two different mechanisms, i.e., inter-granular dissolution and inter-granular fracture induced damage. Xiao et al. [23] studied the retrogression and re-aging treatment on the corrosion behavior of an Al–Zn–Mg–Cu alloy, and found that the discontinuous grain boundary precipitates can improve the corrosion resistance. Rokni et al. [24] studied the microstructural evolution and mechanical properties of back extruded 7075 aluminum alloy. Demira and Gündüzb [25] investigated the effects of artificial aging (as-received, solution heat treated and then artificial aging) on the machinability of 6061 aluminum alloy. Whittaker et al. [26] investigated the high-temperature creep fracture behavior of 316H stainless steel. Jiang et al. [27] studied the creep behavior of Mg–5Li–3Al–(0,1)Ca alloys, and found that the grain boundary sliding and twinning are the main creep mechanisms. Whittenberger et al. [28] studied the elevated temperature creep deformation in solid solution strengthened 〈001〉 NiAl–3.6Ti single crystals, and a forced temperature-compensated power law fit using the activation energy for diffusion was established to adequately (4 90%) predict the observed creep properties. Mahmudi et al. [29] investigated the impression creep behavior of Cu–0.3Cr–0.1Ag in the as-processed and aging-treated conditions. They found that the creep resistance of the aging-treated material was much higher than that of the as-processed condition, because the Cr-rich particles precipitate in the Cu matrix. Also, Mahmudi et al. [30] studied the creep behavior of Sn–9Zn, Sn–8Zn–3Bi, and Sn–37 Pb solders at room-temperature, and found that Sn–9Zn and Sn–8Zn–3Bi alloys exhibit the better creep resistances, attributing to both solid solution hardening and precipitation of Bi in the Sn matrix. Oliveira et al. [31] studied the correlations between the corrosion resistance and the semiconducting properties of the oxide film formed on AZ91D alloy after solution treatment. Chen et al. [32] studied the high-temperature creep behavior of low-temperature-sintered nano-silver paste films, and the creep rupture life was predicted by the Monkman–Grant relationship and the θ projection concept. Wang et al. [33] studied the tensile creep behavior and microstructure evolution of the extruded Mg–10Gd–3Y–0.5Zr (wt%, GW103) alloy, and found that the aging treatment exerted limited effect on its creep performance. In aerospace industry, there is an increasing demand for the excellent performance aluminum alloy integral panels, and creepaging forming is an ideal method to manufacture these complex aircraft panels [15–17,34]. Creep-aging forming, combining both the aging treatment and forming process, has recently drawn much attention of researchers [15–17,35–42]. Jeshvaghani et al. [35] investigated the springback and mechanical properties of 7075 aluminum alloy in creep forming process, found that the appropriate forming cycle is 150 1C/24 h among all forming conditions. Chen et al. [36] investigated the effects of age-forming on
193
microstructures and mechanical properties of 7050 aluminum alloy, and found that the applied stress induces the coarsening of precipitates in 7050 Al alloy. Also, the texture in the alloy was influenced by age-forming. Jeshvaghani et al. [37] studied the effects of microstructural changes on the hardening and creep deformation behaviors of 7075 aluminum alloy, and their results indicated that the size of globular shaped phase (known as GP zones) increases and transforms into η0 precipitates with increasing the forming time and temperature. Further, increasing forming time leads to a decrease in the number of η0 precipitates and an increase in precipitate spacing, where the large equilibrium η phase forms and the precipitate free zones became wider. Lin et al. [38,39] investigated the effects of external stress and creep-aging temperature on the hardness and precipitation process in typical Al–Cu–Mg alloys (2124 and 2024 aluminum alloys), and found that the hardness and precipitation process are sensitive to the external stress and creep aging temperature, and S phase (CuMgAl2) is the main precipitate under the tested conditions. Meanwhile, the creep aging leads to the discontinuous distribution of precipitation phase in grain boundary, which can improve the corrosionresistance of Al–Cu–Mg alloys. Also, Lin et al. [40] investigated the effects of external stress and creep-aging temperature on the precipitation process in7075 aluminum alloy, and found that the precipitates are discontinuously distributed on the grain boundary and the width of precipitate free zone increases with the increase of creep-aging temperature and applied stress. Liu et al. [41] investigated the creep age formability of friction stir welded 2A12 aluminum alloy structures, and found that the springback of the integral part decreases with increasing the aging time and temperature. Guo et al. [42] studied the associated materials aspects-age hardening during the age-forming process, particularly the stress-aging effects on precipitation and Vickers microhardness of Al–Zn–Mg–Cu alloys. Therefore, the effects of creepaging processing parameters (creep-aging temperature and applied stress) on the final heterogeneous microstructure are significant, which greatly affects the corrosion resistance and properties of alloys to some extent. Despite some efforts invested on the effects of external stress and creep-aging temperature on the hardness and precipitation processes in some Al–Cu–Mg alloys, little studies have been reported on the effects of creep-aging processing on the corrosion resistance and mechanical properties, as well as the relationships between the creep-aging processing, corrosion resistance and mechanical properties of Al–Cu–Mg alloys. In this study, the effects of creep-aging processing on the corrosion resistance and mechanical properties of a typical Al–Cu–Mg alloy are investigated by electrochemical corrosion, exfoliation corrosion, and uniaxial tensile experiments. A comprehensive insight into the relationships between the creep-aging processing, corrosion resistance and mechanical properties of the studied Al–Cu–Mg alloy are obtained.
2. Material and experiments A typical commercial Al–Cu–Mg alloy (2024 aluminium alloy) was used in this investigation. The chemical compositions (wt%) of the studied aluminum alloy are 4.59Cu–1.45Mg–0.63Mn–0.11Fe– 0.05Zn–0.02Ti–0.12Si–(bal.)Al. The heat treatment processing is T3, which means the solution treatment, cold deformation and then natural aging. The specimens with a rectangular cross section of 2.5 mm 15 mm and a gauge length of 50 mm were machined along the rolling direction of the studied aluminum alloy sheet. Based on the previously optimized creep-aging parameters [16,38], three different creep temperatures (423, 448 and 473 K) and three different stresses (185, 205 and 225 MPa) were selected in this study.
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The specimens were creep-aged for 24 h on MTS-GWT2105 test machine according to ASTM E 139-06 standard [43]. The accuracies of loading and heating system of the MTS-GWT2105 test machine are 0.1 MPa and 1 K, respectively. The specimen was firstly heated to the experimental temperature at a heating rate of 10 K/min and held for 30 min. Then, the extra stress was applied. After the 24-h creep-aging, the applied loading was released, and the specimen was naturally cooled down to the room temperature in the furnace. In order to study the effects of the creep-aging processing on the corrosion resistance and mechanical properties of the studied aluminum alloy, the electrochemical corrosion, exfoliation corrosion and uniaxial tensile experiments were carried out. The specimens for uniaxial tensile tests and corrosion resistance tests were directly machined from the gauge of creep-aged materials. The rectangular cross section of the dog-bone flat tensile specimens is 3.2 mm 2.5 mm and the gauge length is 16 mm. The uniaxial tensile experiments were carried out at room temperature on the MTS-810 landmark test system, and the tensile velocity is 2 mm/min. The tensile test under each creep-aging condition repeated five times, and the standard error (SE) is computed to represent the overall distribution of measured data. The standard (SE) error is calculated by dividing the standard deviation (SD) by the square root of experimental number, i.e., sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ∑ni¼ 1 ðX i XÞ2 SD ¼ ð1Þ n SD SE ¼ pffiffiffi n
ð2Þ
where X i and X are the i and mean experimental results, respectively. n is the number of the repeated experiments (n ¼ 5). Graphically, the standard error is represented by the error bar. Based on the tensile experimental results, the standard errors (SE) of the ultimate tensile strength, yield strength and elongation under different deformation conditions are evaluated, as shown in Table 1. Obviously, the repeatability of tensile experiments is satisfactory. The potentiodynamic polarization experiments were performed under a scan rate of 0.01 V/s using CHI660C electrochemical testing instrument. The sample was degreased with ethanol after the mirror polishing. The non-tested surfaces were protectively coated using a stop-off lacquer. Standard calomel electrode (SCE) was used as the reference electrode. Platinum was regarded as the auxiliary electrode, and the sample served as the working electrode. The exposed area was 100 mm2 and the experiments were carried out in 3.5 wt% NaCl solution at 293 K. The accelerated exfoliation corrosion (EXCO) experiments were performed according to the standard EXCO test, as described in ASTM G34-01 [44]. The sample was firstly degreased with ethanol
after mirror polishing. The non-tested surfaces were protectively coated using a stop-off lacquer to avoid the crevice corrosion beneath the coating. The EXCO test solution was prepared as 4.0 M NaCl þ0.5 M KNO3 þ0.1 M HNO3. The solution temperature was 298 K, and the ratio of electrolyte volume to electrode surface area was 25 mL/cm2. After the 96-h immersion in the EXCO test solution, the corrosion morphologies were observed. The fracture surface morphologies after tensile experiments were examined using TESCAN MIRA 3 Scanning Electron Microscope (SEM). The transmission electron microscope (TEM) observations were performed on the JEM-2100F microscope operating at 200 kV. Samples for TEM observations were cut from the creepaged specimens, and then grinded into 0.7–0.8 mm thin foils. Several disks with a diameter of 3 mm were punched out from these thin foils, and subsequently electro-polished using a solution of hydrogen nitrate and methanol (1:4 in volume).
3. Results and discussion 3.1. Effects of creep-aging processing on corrosion resistance According to the authors' previous studies [38], it is found that the precipitation process is very sensitive to the applied stress and creep-aging temperature. 423, 448, and 473 K are validated as the “under aging”, “peak aging” and “over aging” temperatures, respectively. An applied stress can accelerate the precipitation hardening (also called age hardening) of Al–Cu–Mg alloy. The large applied stress and high creep-aging temperature easily make the preferential precipitation process as SSS-GPB-S″-S0 -S, and S phase (CuMgAl2) is the main precipitate under the experimental conditions. With the increase of applied stress and creep-aging temperature, S phase easily grows up and coarsens. The change of precipitates can affect the corrosion resistance of aluminum alloy. In this section, the effects of creep-aging processing parameters (the applied stress and aging temperature) on the corrosion resistance of the studied alloy will be discussed. 3.1.1. Effects of creep-aging temperature on corrosion resistance Fig. 1 shows the potentiodynamic polarization curves of the creep-aged Al–Cu–Mg alloy under the applied stress of 185 MPa and different creep-aging temperatures. The electrochemical characteristics derived from the polarization curves are listed in Table 2. Generally, the electrochemical corrosion resistance gradually increases with the increase of corrosion potential (towards anodic direction), and the pitting potentials have been commonly used to evaluate the pitting resistance of alloys. From Fig. 1 and Table 2, it can be easily found that the creep-aging processing has a substantial impact on the electrochemical corrosion resistance of the studied aluminum alloy. The pitting potential of the as-received
Table 1 Standard errors (SE) of tensile experiments under different deformation conditions.
As-received 423
448
473
Applied stress (MPa)
Standard errors of ultimate tensile strength
Standard errors Standard errors of of yield elongation strength
185 205 225
2.78 3.07 1.69 3.29
3.82 3.07 5.41 4.97
0.91 1.01 0.64 0.81
185 205 225
1.86 4.00 1.70
3.92 4.95 4.40
0.87 0.72 0.53
185 205 225
3.79 1.98 1.50
4.12 3.78 4.96
0.49 0.58 0.75
Potential (VSCE)
Creep-aging temperature (K)
-0.2
-0.4
423 K As-received
-0.6 448 K 473 K
-0.8 10
-10
10
-8
10
-6
10
Current Density
-4
10
-2
1
(A/cm2)
Fig. 1. Polarization curves of the studied aluminum alloy under the applied stress of 185 MPa and different creep-aging temperatures.
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Table 2 Corrosion characteristics of the studied aluminum alloy under the applied stress of 185 MPa. Creep-aging temperature (K)
Epit (VSCE)
As-received 423 448 473
0.518 0.506 0.552 0.601
EXCO rating
ED
As-received 423 K 448 K 473 K
EC EB EA P N 0
20
40
60
80
100
Immersion time (h) Fig. 2. Effects of creep-aging temperature on the exfoliation corrosion rating of the studied aluminum alloy under the applied stress of 205 MPa.
sample is 0.518 VSCE. When the sample is creep-aged under the temperature of 423 K, the pitting potential is 0.506 VSCE. However, when the creep-aging temperature is 448 K, the pitting potential is 0.552 VSCE, which indicates that the corrosion resistance decreases compared with that of the as-received sample. When the creep-aging temperature is increased to 473 K, the pitting potential is 0.601 VSCE. So, it can be concluded that the corrosion resistance of the creep-aged Al–Cu–Mg alloy decreases with the increase of creep-aging temperature. This is because the creepaging temperature can change the size and density of aging precipitates in the aluminum matrix, which results in the changed corrosion resistance of the studied alloy. Fig. 2 shows the effects of creep-aging temperature on the exfoliation corrosion rating of the creep-aged alloy under the applied stress of 205 MPa. Also, the visual exfoliation corrosion morphologies of the creep-aged samples are observed, as shown in Fig. 3. The EXCO rating P indicates the occurrence of pitting corrosion. EA shows that there is only slight pitting corrosion on the surface of specimens. EB indicates the moderate surface corrosion. EC stands for the notable peeled-off laying and penetration. ED is similar to EC except for much greater penetration and loss of metal. With the increase of EXCO rating from EA to ED, the exfoliation corrosion resistance gradually decreases. Obviously, the exfoliation corrosion susceptibility of the studied aluminum alloy is sensitive to the creep-aging temperature. From Fig. 3, it can be found that the severe exfoliation corrosion (with a corrosion rating of EB) occurs on the as-received sample after immersion in the EXCO solution for 96 h. A notable peeled-off layering can be found on the sample surface, as shown in Fig. 3(a). Meanwhile, when the sample is creep-aged under the temperature of 423 K, relative less material is peeled off from the sample surface (Fig. 4b), and the exfoliation corrosion rating decreases to EB-, which indicates that the exfoliation corrosion resistance increases, compared with that of the as-received sample. However, when the creep-aging temperatures are 448 and 473 K, the exfoliation corrosion ratings are EC and ED, respectively. Large peeled-off layering can be found on the surface of the creep-aged samples, and the peeled-off layering penetrates to a considerable depth into the material, as shown in Fig. 3(c) and (d). So, the increase of
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creep-aging temperature can deteriorate the corrosion resistance of the studied aluminum alloy. 3.1.2. Effects of applied stress on corrosion resistance Fig. 4 shows the potentiodynamic polarization curves of the creep-aged Al–Cu–Mg alloy under different applied stresses. The electrochemical characteristics derived from the polarization curves are listed in Table 3. Fig. 4 and Table 3 indicate that the applied stress has significant effects on the corrosion resistance of the studied alloy under all the tested creep-aging conditions. Under the creep-aging temperature of 423 K, when the applied stress is increased from 185 MPa to 225 MPa, the pitting potential decrease from 0.506 VSCE to 0.521 VSCE, and the exfoliation corrosion rating (Fig. 5) increases from EA þ to EB . Meanwhile, the corrosion depth increases with the increase of applied stress, as shown in Fig. 6(a) and (b). Under the creep-aging temperature of 448 K, when the applied stress is increased from 185 MPa to 225 MPa, the pitting potential decrease from 0.552 VSCE to 0.581 VSCE, and the exfoliation corrosion rating (Fig. 5) increases from EC to EC þ . So, the corrosion resistance decreases with the increase of applied stress under relative low creep-aging temperatures (423 and 448 K). However, under the creep-aging temperature of 473 K, when the applied stress is increased from 185 MPa to 225 MPa, the pitting potential increases from 0.601 VSCE to 0.516 VSCE. Furthermore, with the increase of applied stress, the exfoliation corrosion rating decreases from ED þ to ED, and the corrosion depth decreases, as shown in Fig. 6(c) and (d), which indicates the corrosion resistance increases with the increase of applied stress. 3.2. Effects of creep-aging processing on mechanical properties The effects of creep-aging processing on the yield strength, ultimate tensile strength and elongation are shown in Fig. 7(a)–(c). The mechanical properties of the as-received sample are also plotted in dash lines in Fig. 7 for comparison with the creepaged samples. Obviously, the mechanical properties of the studied aluminum alloy are significantly affected by the creep-aging processing parameters. This is because the creep-aging temperature and applied stress can promote the precipitation and coarsen of precipitates in the studied alloy. From Fig. 7(a) and (b), it can be found that the yield strength and ultimate tensile strength increase with the increase of creep-aging temperature. When the creep-aging temperature is 423 K, the yield strength and ultimate tensile strength are smaller than those of the as-received sample. The yield strength and ultimate tensile strength increase with the increase of applied stress under the creep-aging temperatures of 423 and 448 K. However, when creep-aging temperature is 473 K (“over aging” temperature [38]), the yield strength and ultimate tensile strength slightly decreases with the increase of the applied stress. Additionally, the elongation decreases with the increase of creep-aging temperature. When the creep-aging temperature is 423 K, the elongation decreases with the increase of applied stress, and is larger than that of the as-received sample. However, when the creep-aging temperatures are 448 and 473 K, the elongations are greatly smaller than those of the as-received sample. Meanwhile, they are not very sensitive to the applied stress. 3.2.1. Effects of creep-aging temperature on mechanical properties Fig. 8 shows the tensile fracture morphologies of the creepaged samples under the applied stress of 205 MPa and temperatures of 423 and 473 K. From Fig. 8(a), it can be found that small dimples distribute on the fracture morphology, and the tearing edges can also be found. However, Fig. 8(b) shows that the fracture surface is covered with large dimples and no obvious tearing edges
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Fig. 3. Exfoliation corrosion morphologies of the samples under the applied stress of 205 MPa and creep-aging temperatures of (a) as-received, (b) 423 K, (c) 448 K, and (d) 473 K.
-0.2
Potential (VSCE)
Potential (VSCE)
-0.2
-0.4
225 MPa 185 MPa -0.6
-0.8
-0.4 185 MPa
-0.6
225 MPa
-0.8 -8
10
-6
10
-4
-2
10
10
Current Density
-8
1
-6
10
-4
10
(A/cm2)
10
-2
10
1
Current Density (A/cm2)
Potential (VSCE)
-0.2
-0.4 225 MPa
-0.6
185 MPa
-0.8 -10
10
-8
10
-6
10
-4
10
Current Density
-2
10
1
(A/cm2)
Fig. 4. Polarization curves of the studied aluminum alloy under the creep-aging temperatures of (a) 423 K, (b) 448 K, and (c) 473 K.
can be found. Furthermore, the dimples in Fig. 8(b) are bigger than those in Fig. 8(a). Meanwhile, the fracture morphology in Fig. 8 (b) is relative flatter than that in Fig. 8(a). Under the creep-aging temperature of 473 K (“over aging” condition [38]), the high creepaging temperature promotes the quick precipitation and growth of aging precipitates. Due to the incompatibility between the precipitates and aluminum matrix during the tensile deformation, the precipitates impede the dislocations sliding, and thus cause the stress concentration in the vicinity of them. When the concentrated stress exceeds a critical value, the interfacial debonding between the precipitates and aluminum matrix or the breaking of
precipitates occur, which deteriorates the plastic deformation capability of the studied alloy. So, under the “over aging” condition, coarsen aging precipitates impede the dislocation sliding, and deteriorate the plastic deformation capability. As a result, with the increase of creep-aging temperature, the yield strength and ultimate tensile strength increase, while the elongation decreases, as shown in Fig. 7. Fig. 9 shows the fracture morphologies of samples under different creep-aging conditions. From Fig. 9(a), it can be found that the big dimples caused by the interfacial debonding between the precipitates and aluminum matrix are surrounded by shallow dimples.
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Meanwhile, such big dimples do not link together until the final fracture. This indicates that the creep-aged sample has a good plastic deformation capability. Whereas, due to the coalescence of big dimples caused by the interfacial debonding between the precipitates and aluminum matrix [45], large dimples and cracks appear, as shown in Fig. 9(b) and (c), which severely reduce the homogeneous deformation ability of the aluminum matrix. This is because the high Table 3 Corrosion characteristics of the studied aluminum alloy under different applied stresses. Creep-aging temperature (K)
Applied stress (MPa)
Epit (VSCE)
423
185 225
0.506 0.521
448
185 225
0.552 0.581
473
185 225
0.601 0.516
EXCO rating
ED
185 MPa 225 MPa
EC EB EA P N
423
448
473
Creep-aging temperature (K) Fig. 5. Effects of applied stress on the exfoliation corrosion rating of the studied aluminum alloy.
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creep-aging temperature promotes the precipitation and growth of the aging precipitates. With the sustained increase of the plastic strain, the micro-voids grow up and shrink the linkage regions among voids, which reduce the effective bearing area of tensile specimen. Once the effective bearing area is reduced to a critical value, the aluminum matrixes quickly fracture. Simultaneously, the nucleation and growth of secondary cracks accelerate the fracture of materials and reduce the plastic deformation capacity, as shown in Fig. 7(c).
3.2.2. Effects of applied stress on mechanical properties Fig. 10 shows the fracture morphologies of the creep-aged samples under the creep temperature of 448 K and different applied stresses. With the increase of applied stress, the obvious difference can be found from the fracture morphologies. Small dimples and fractured precipitates can be clearly found in Fig. 10(a). However, both dimples and cleavage fracture morphologies appear on the tensile fracture morphology, as shown in Fig. 10(b–d). For Al–Cu–Mg alloys, the preferential growth orientation of precipitates is sensitive to the applied stress [38,39]. With the increase of applied stress, the aging precipitates grow up in a preferential orientation, namely, stress-orientation effect, as shown in Fig. 11. Due to the semi-incoherent and incoherent relationships between the precipitates and aluminum matrix, the integrity of the crystal structure is destroyed and the binding force between the atoms decreases. The original nucleation of cleavage cracks occurs in different orientation of crystalline grain, and then the cracks extend at a certain crystal surface. The cleavage cracks extend along the low surface energy crystalline planes, as shown in Figs. 10(c) and 12. Generally, the applied stress can promote the precipitation and growth of the aging precipitates. With the increase of applied stress, the density and dimension of aging precipitates increase, and more big dimples can be found from the fracture morphologies (Fig. 10a and d). The work hardening effect is obvious under the high applied stress,
Fig. 6. Exfoliation corrosion morphologies of the studied aluminum alloy under the creep-aging conditions of (a) 423 K—185 MPa, (b) 423 K—225 MPa, (c) 473 K—185 MPa, and (d) 473 K—225 MPa.
Y.C. Lin et al. / Materials Science & Engineering A 605 (2014) 192–202
Yield strength (MPa)
430 400 370
As-received
340 423 K 448 K 473 K
310 280 180
190
200
210
220
230
Ultimate tensile strength (MPa)
198
520
500
As-received
480
423 K 448 K 473 K
460 180
190
200
210
220
230
Applied stress (MPa)
Applied stress (MPa)
Elongation (%)
24
As-received
20 16 12
423 K 448 K 473 K
8 180
190
200
210
220
230
Applied stress (MPa) Fig. 7. Effects of creep-aging processing on the mechanical properties of the studied aluminum alloy. (a) Yield strength, (b) ultimate tensile strength, and (c) elongation.
Fig. 8. Fracture morphologies (2000 ) of samples under the applied stress of 205 MPa and creep-aging temperatures of (a) 423 Kand (b) 473 K.
Fig. 9. Fracture morphologies (4000 ) of samples under different creep-aging conditions. (a) 423 K—205 MPa, (b) as-received, and (c) 473 K—205 MPa.
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Fig. 10. Fracture morphologies of samples under the creep-aging temperature of 448 K and applied stresses of (a) 185 MPa and (b–d) 225 MPa.
Fig. 11. TEM micrographs of samples under the creep-aging temperature of 448 K and applied stresses of (a) 185 MPa and (b) 225 MPa.
and the yield strength and ultimate tensile strength increase. However, too many aging precipitates will damage the alloy and decrease the plastic deformation capability, and then the elongation decreases slightly.
creep-aging temperature, S phase easily grows up and coarsens. During the creep-aging heat treatment, the temperature and stress fields change the dimension and distribution of aging precipitates, which affects the corrosion resistance and mechanical properties of Al–Cu–Mg alloys.
3.3. Relationships between creep-aging processing, corrosion resistance and mechanical properties The studied Al–Cu–Mg alloy used in this investigation is a typical age hardenable aluminum alloy. Some investigators [38,39,46] found that the large applied stress and high aging temperature easily make the preferential precipitation process as SSS-GPB-S″-S0 -S. With the increase of applied stress and
3.3.1. Creep-aging processing and corrosion resistance The aging precipitates in Al–Cu–Mg alloy have different corrosion potential, compared to the aluminum matrix. The corrosion resistance of Al–Cu–Mg alloy is affected by the alloying elements, dimension and distribution of aging precipitates. The creep-aging processing can change the dimension and distribution of aging
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precipitates, which greatly affects the corrosion resistance of aluminum alloy [47]. Generally, the Mg and Zn elements in the aluminum matrix shift the corrosion potential to the anodic direction, and Cu-rich precipitates often act as local cathodes, which ultimately drive the anodic dissolution of the surrounding matrix material [48]. The precipitation and growth of S phase (Al2CuMg) will decrease the corrosion resistance of Al–Cu–Mg alloys. Fig. 13 shows the TEM micrographs of the aging precipitates of the studied aluminum alloy under the applied stress of 185 MPa and different creep-aging temperatures. From Fig. 13(a), it can be found that the GPB (Guinier–Preston–Bagaryatsky) zone dissolves back into aluminum matrix under the creep-aging temperature of 423 K [38] and the precipitates grow up and coarsen with the increase of creep-aging temperature, as shown in Fig. 13(b) and (c).
Under the creep-aging temperature of 423 K, the GPB zone dissolves back into the aluminum matrix and it is hard to form the local galvanic cells on the surface of the alloy. So, the corrosion resistance is enhanced when compared with that of the asreceived sample. Nevertheless, with the increase of creep-aging temperature, S phases grow up, which reduces the corrosion resistance of the studied alloy. As a result, the corrosion resistance decreases with the increase of creep-aging temperature. Fig. 14 shows the effects of applied stress on the precipitation of the studied aluminum alloy. From Figs. 13(b) and 14(a), it can be found that the applied stress will promote the precipitation and growth of S phase. The density and dimension of precipitates increase with increasing the applied stress. As mentioned above, S phase can deteriorate the corrosion resistance of aluminum alloys. So, the corrosion resistance decreases with the increase of applied stress, as shown in Figs. 4 and 6. However, when the dimension of precipitates exceeds a critical value, the density of precipitates decreases because the total volume fraction of alloying elements is a constant. According to the authors' previous studies [38], under “over aging” temperature (473 K), the density of precipitates decreases with the increase of applied stress. So, the number of local galvanic cells decreases, the corrosion resistance is enhanced.
3.3.2. Creep-aging processing and mechanical properties Precipitation strengthening provides the most important contribution to the yield strength of age hardenable aluminum alloys. The strengthening mechanisms focus on the interactions between the dislocations and precipitation particles. It is adequate to accept that the contribution of aging precipitates to the yield strength of alloy can be expressed as [49,50]
Δsy ¼ C 1 f m1 rm2
Fig. 12. Cleavage planes and cleavage crack under the creep-aging temperature of 448 K and applied stresses of 225 MPa.
ð3Þ
where Δsy is the yield strength increment, C 1 is a material constant, f is the volume fraction of the shear particles, r is the radius of the shear particles, m1 and m2 are material constants. For most interactions between the dislocations and particles, both m1 and m2 are in the vicinity of 0.5. Consequently, with the increase of particle's dimension and density, the yield strength of the alloy
Fig. 13. TEM micrographs of the studied aluminum alloy under the applied stress of 185 MPa and creep-aging temperatures of (a) 423 K, (b) 448 K, and (c) 473 K.
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Fig. 14. TEM micrographs of the studied aluminum alloy under creep-aging temperature of 448 K and the applied stresses of (a) 205 MPa and (b) 225 MPa.
will increase. As shown in Fig. 13(a), under the creep-aging temperature of 423 K, the precipitates dissolve back into the aluminum matrix. So, the volume fraction of particles decreases, the yield strength and ultimate tensile strength are smaller than those of the as-received specimen. With the increase of creep-aging temperature, the driving force for the growth and coarsen of precipitates is enhanced. The volume fraction and the radius of shear particles increase. So, the yield strength and ultimate tensile strength increase. Meanwhile, the applied stress can promote precipitation process of the studied aluminum alloy and easily make the preferential precipitation process as SSS-GPB-S″-S0 -S. The dimension and density of S phase (Al2CuMg) increase with the increase of applied stress, and the interactions between the dislocation and precipitates will be enhanced. Thus, the yield and ultimate tensile strengths increase.
4. Conclusion The creep-aging behavior of a typical Al–Cu–Mg alloy (2024 aluminium alloy) is studied in this work. The effects of creep-aging processing on the corrosion resistance and mechanical property of the creep-aged Al–Cu–Mg alloy are analyzed. Some important conclusions can be made as follows: (1) The corrosion resistance of the studied Al–Cu–Mg alloy is sensitive to the creep-aging heat treatment. With the increase of creep-aging temperature, the corrosion resistance decreases. For the “under aging” and “peak aging” conditions, the corrosion resistance decreases with the increase of applied stress. However, the applied stress improves the corrosion resistance of the studied alloy under “over aging” condition. (2) With the increase of creep-aging temperature and applied stress, the aging precipitates from the aluminum matrix rapidly grow up. The yield strength and ultimate tensile strength of the creep-aged Al–Cu–Mg alloy increase. However, the elongation decreases with the increase of creep-aging temperature. (3) With the increase of creep-aging temperature, the micro-voids easily grow up and link together, which accelerates the fracture of materials. Meanwhile, the high applied stress easily results in the cleavage failure of the creep-aged Al–Cu–Mg alloy.
Acknowledgments This work was supported by the National Natural Science Foundation Council of China (Grant no. 51125021), the National Key Basic Research Program (Grant no. 2010CB731702), Sheng-hua Yu-ying Program of Central South University, and State Key
Laboratory of Materials Processing and Die & Mold Technology, Huazhong University of Science and Technology (No. 2012P04), China.
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