- Email: [email protected]
Effect of pre-deformation on the stress corrosion cracking susceptibility of aluminum alloy 2519
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RARE METALS Vol. 26, No. 4, Aug 2007, p . 385 E-mail: rm@ustb,edu.cn
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Effect of pre-deformation on the stress corrosion cracking susceptibility of aluminum alloy 2519 LI Huizhong, W A N G Xinming, CHEN Mingan, LI Yanfang, and HANG Xiaopeng School of Materials Science and Engineering, Central South Univmity, Changsha 410083, China (Received 2006-04-05)
Abstract:The effects of pre-deformation and strain rate on the stress corrosion cracking (SCC) behavior of aluminum alloy 2519 in air and in 3.5% NaCI water solution were investigated by means of slow strain rate tension (SSRT), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results indicate that the alloy is susceptible to SCC in 3.5% NaCl water solution and not in air. At the same predefonnation, the alloy is more susceptible to SCC at 1.33 x s-' than at 6.66 x s-'. Moreover, it is more susceptible to SCC at free predeformation than at 10% pre-deformation at the same strain rate. The number of 8 precipitated along the grain boundaries is reduced and distributed discontinuously, at the same time, the precipitate-free zones (PFZ)become narrow and the susceptibility to stress corrosion cracking is reduced after 10% pre-deformation.
Key words: 2519 aluminum alloy; pre-deformation; slow strain rate tension; susceptibilityto stress corrosion cracking
[Thisstudy wasjnuncially supported by the State Key Fundamental Research Program of China (No.2005CB623706).]
1. Introduction Aluminum alloy 2519, mainly used as the struc-
tural material for parts of aircraft skin, rocket, naval ships and boats, and the amphibious annored assault vehicle (AAAV), is a kind of high strength and high toughness alloy which was fust developed by the Americans in 1980s [l]. Certain scientists reported its welding properties, as well as, thermal strain properties [2-4]. Moreover, the effect of heat treatment on the intergranular corrosion of this alloy has been investigated and it was shown that this alloy has better intergranular corrosion resistance in T8 condition than in T6 condition [5]. Overaged alloys have better intergranular corrosion resistance than underaged and peak-aged alloys [6]. The resistance to stress corrosion became lower with the increase in copper content [7]. Kramer researched the influence of different hot working methods on the resistance to stress corrosion of 2519 alloy [8]. To date, there is Corresponding author: LI Huizhong
little research on the effect of pre-deformation on the stress corrosion of aluminum alloy 2519. The pre-crack specimen is not capable of offering nucleation information of stress corrosion, which is always used to study the stress corrosion cracking susceptibility of crack propagation behavior. But the nucleation process is very important for stress corrosion cracking susceptibility of materials with smooth surface in a corrosive environment [9]. Consequently, using slow strain rate tension test to evaluate the stress corrosion cracking susceptibility may reflect the objective behaviors, and short detecting period is cost. The effect of predeformation and strain rate on the SCC behavior of aluminum alloy 25 19 was investigated in this study.
2. Experimental 2.1. Experimental materials
E-mail: [email protected]
The material used in this study was a commercial
RARE METALS, Vol. 26, No. 4, Aug 2007
386
alloy 2519 rolled plate of 40 mm in thickness. The materials were tested in both free pre-deformation (alloy 1) and 10% pre-deformation (alloy 2) between solid solution and ageing, and solution was heat treated at 525°C for 60 min, followed by cold rolling and then artificial ageing at 180°C. The chemical composition of the experimental alloy is shown in Table 1. Table 1. Chemical composition of 2519 aluminum alloy wt.%
Cu
Mn
ME
Zr
Ti
V
Fe
Si
Al
5.80 0.29 0.22 0.22 0.06 0.06 0.15 0.05 Bal
2.2. Experimental process
According to the HB 7235-95 standard test method for measurement of SSRT [ 101, tensile-type specimens with longitudinal transverse (LT) orientation were selected for SSRT study. All the peak-aged specimens were machined from a 40 mm thick plate with the loading direction parallel to the rolling direction of the plate. The diameter of the specimens was 5 mm and the gauge length was 30 mm. The SSRT experiment was carried out on an Instron 8032 fatigue-testing machine. The tensile s-' and 6.66 x lo-' strain rate was set at 1.33 x s-' when the experiments were operated in air or in 3.5% NaCl water solution, and the temperature was controlled to 35 f 1OC. The fracture was observed using a Sirion 200 scanning electron microscopy (SEM). The microstructure was characterized by a Hitachi 800 transmission electron microscopy (TEM) analysis. TEM samples were electro-polished in 70% methanol and 30% nitric acid solution at -35°C using twin-jet equipment operated at 30 V.
strength and elongation of alloy 1 at 6.66 x lo-' s-' in 3.5% NaCl water solution decreased. Their ratios were 9 1% and 74%, respectively. This indicates that alloy 1 is susceptible to SCC in 3.5% NaCl water solution. As for alloy 2, its fracture strength and elongation in 3.5%NaC1 water solution are also lower than those in dry air. But the ratios are 96% and 81%, respectively, which has smaller decrement than alloy 1. Alloy 2 is less susceptible to SCC than alloy 1. According to Table 3, the fracture strength and the elongation of alloys 1 and 2 at 1.33 x lo-' s-' are lower than those at 6.66 x lo-' s-' in 3.5% NaCl water solution. On this account, alloy 25 19 at 1.33 x lo-' s-' is more susceptible to SCC than at 6.66 x s-' in 3.5% NaCl water solution. Stress corrosion index, which is an important criterion of SCC, can reflect the SCC susceptibility better than a single mechanical performance index. It is defined in expression [l I]: ISSRT
(1)
where, ofw is the fracture strength in environmental medium, Mpa, sfw is the elongation in environmental medium, %, o f A i sthe fracture strength in inert medium, MPa, JfA is the elongation in inert medium, %. Table 2. Mechanical properties of aluminum alloy 2519 Alloy
No.
Medium Dry air 3.5%
1
NaCl 3.5%
NaCl Dry air 3.5% 2
3. Results
= 1-[of, (1 + s f w )I / [ o f A (1 + 8 f A )I
NaCl 3.5% NaCl
Strain rate / (10-~.~-l)
Tensile Elongation/ strength/MPa %
6.66
480
20
6.66
435
15
1.33
425
11
6.66
500
12
6.66
480
10
1.33
475
8
._
3.1. Results of SSRT testing The results of SSRT tests in dry air and in 3.5% NaCl water solution are listed in Table 2. It can be seen that the strength and the elongation decrease with tensile in 3.5% NaCl water solution, and the decrement increases with the decrease in strain rate. Compared to an atmosphere of dry air, the fracture
The SCC susceptibility gradually increases, whereas ZssRT changes from 0 to 1 . The stress corrosion indexes of 2519 aluminum alloy with different pre-deformations are shown in Table 3. The results indicate that the stress corrosion indexes of alloy 1 are higher than those of alloy 2 and the stress corro-
Li H.Z. et al., Effect of pre-deformationon the stress corrosion cracking susceptibility of ...
sion indexes are higher at low stress rate than at high strain rate. Table 3. Results of SCC for aluminum alloy 2519 Alloy No. ~~
El
(lO".s-')
ISSRT
~
1 2
6.66
0.13
1.33
0.18
6.66
0.06
I .33
0.09
3.2. SEM analysis for the fracture surface of SSRT testing The fracture surfaces of 25 19 alloys with different pre-deformations present different features for samples tested under different conditions. Fig. 1 shows the fracture surface of two experimental alloys in dry air. Several dimples are visible with ductile features in the two alloys. But the plastic flow of alloy 1 is more drastic than that of alloy 2. This is coincident with the fact that the plasticity of alloy 1 is better. These two kinds of fracture in Fig. 1 are ductile fracture, which belongs to the pure mechanical fracture.
Fig. 1. Fracture surface SSRT testing in dry air (strain rate ::6.66 x s-I): (a) alloy 1; (b) alloy 2.
387
Fig. 2 shows the fracture surface of alloys in 3.5% NaCl water solution when the strain rate is 6.66 x lo-' s-'. The fracture surface of alloy 2 in this condition shows certain small and shallow dimples and the properties of a mechanical fracture (Fig. 2(b)). Whereas the fracture surface of alloy 1 shows certain intergranular cracks and their secondary cracks (Fig. 2(a)). The fracture surface of alloy 1 in this condition is constituted with crude intergranular cracks and certain small dimples. So alloy 1 is more susceptible to SCC than alloy 2 in this condition. Fig. 3 shows the fracture surfaces of alloys in 3.5% NaCl water solution at 1.33x s-'. The fracture surface of alloy 2 has certain small dimples, but the dimples are smaller than that at 1.33 x lo-' s-' and the fracture surface of alloy 1 is characterized by intergranular separation and is brittle fracture a.. shown in Fig. 3(a). The results indicate that the SCC susceptibility of alloys increases with the decrease in tensile strain rate.
Fig. 2. Fracture surface SSRT testing in 3.5 wt.% NaCl water solution (strain rate = 6-66 xlo4 s-'): (a) alloy 1; (b) alloy 2.
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Fig. 3. Fracture surface SSRT testing in 3.5 w t % NaCl water solution (strain rate = 133 xlOd s-'): (a) alloy 1; (b)alloy 2.
4.
Discussion
4.1. Effect of strain rate on the SCC behavior of aluminum alloy 2519 The operating range of tensile strain rate, which usually ranges between lo4 and lo-* s-', is primarily a parameter of SCC behavior [ 121. For most materials, it is most susceptibleto SCC when the tensile strain rate is between and lo4 s-', There is a suitable strain rate (or range) in certain environ-mental medium to distinguish the SCC susceptibility [13]. According to Table 3, the alloy with the same pre-deformation is more susceptible to SCC at low tensile strain rate than at high tensile strain rate. The SCC behavior occurs in a certain range of tensile strain rate. If the tensile strain rate is too high, the elongation will approach the value measured in air and the decrease in toughness will be small. It is not susceptible to SCC in this condition and results in the occurrence of ductile fracture before the reaction between environmental medium and the speci-
men. On the contrary, if the strain rate is too low, the materials will appear with a fresh surface and a passive film will form on the fresh surface and the 3.5% NaCl water solution cannot take effect in the stress corrosion crack pmess. In this condition, strength and toughness are almost the same as in air. According to Table 3, the SCC susceptibility of aluminum alloy 2519 increases with the decrease in tensile strain rate within the tension speed capability. There are two reasons for its more susceptibility to SCC at low tensile strain rate than at high tensile strain rate in 3.5% NaCl water solution. First, aluminum alloy 2519 in natural condition has a good corrosion resistance for it is easy to form a layer of oxide which has protective effect. Because of the difference in elongation between the oxide film and the base metal, the oxide film will break, although tensile stress is allowed. Cells will appear between the oxide film and the base metal in an environmental medium. As a result, anodic dissolution occurs and propagation of the cracks is accelerated. On the other hand, the fresh aluminum alloy 25 19 surface exposed in the corrosion medium may produce the following important electrochemical reactions with water in an environmental medium [ 141: 2Al+ 3H20 + A1203 + 6H' + 6e(2)
AI + A I ~++3e-
(3)
H++ e" + [HI
(4) The active hydrogen generated by the upper reaction is delivered into the inner part of the materials through adsorption, diffusion, and dislocation and leads to hydrogen embrittlement.The specimen has more time to react with the environmental medium at low strain rate and absorbs more hydrogen. So it is more susceptible to SCC at a low strain rate.
4.2. Effect of pre-deformation on the SCC behavior According to Table 3, the SCC susceptibility of alloy 2 with 10% pre-deformation before ageing decreases dramatically. Fig. 4 shows the TEM images of alloys with different extents of pre-deformation. Figs. 4(a) and 4(b) show the size and distribution of the precipitate phase 8' (8) within the grain boundary and inner crystal of alloy 1 in peak-aged
Li H.Z.et al., Effect of pre-deformation on the stress corrosion cracking susceptibility of ...
condition. The dimension of the 8 phase precipitated along the grain boundary is coarse and distributes continuously, which has wild precipitate free zones. The 8’ phase precipitated in intracrystalline is coarse and distributes inhomogeneously. The TEM images of alloy 2 are shown in Figs. 4(c) and 4(d). Compar-
389
ing Figs. 4(a) and 4(b) shows the following facts: (1) the size and the quantity of 8 phases precipitated along the grain boundary decreases; ( 2 ) the gap between 8 phases precipitated along the grain boundary increases obviously, whereas the distribution of it is no more continuous along the grain boundary
Fig. 4. TEM images showing the microstructures of the peak-aged 2519 aluminum alloy: (a) and (b) alloy 1; (c) and (d) alloy 2.
and the precipitate free zone becomes narrow; (3) as for the 8’ phase precipitated in intracrystalline, its size decreases, whereas the quantity increases, and it distributes uniformly and dispersively. Plenty of crystal defects came into being, such as vacancy and dislocation in solid solution alloy after cold working. As a result, the diffusion of the solute atoms is accelerated, and the rate of precipitation is 100 times faster than that in structure with free pre-deformation and magnitude dispersive 8’ phase is formed in intracrystalline. On the contrary, the nucleation of 8’ phase in intracrystalline alloy 1 with free pre-deformation has a long incubation period and the number of 8’ phase is small, and the precipitated phase on the grain boundary is easy to nucleate and grow, which leads to the equilibrium phases’ high-density distribution on the large-angle grain
boundary and shortened distance between phases. The common face slip becomes drastic that results in the increasing of slip shoulder height of the alloy surface and crack tip attributed to the coarseness of 8’ phase [15]. So, alloy 1 with free-deformation has low resistance to corrosion. As for alloy 2 with 10% pre-deformation, the potential drop between the grain boundary and inner crystal decreases and the optional dissolving of the precipitate phase on the grain boundary decelerates. At the same time, the abundant dispersive 8’ phase hinders common face slip and diminishes the slip shoulder height of the alloy surface and crack tip. So pre-deformation improves the structural properties and disperses common face slip. As a result, alloy 2 with pre-deformation is less susceptible to SCC.
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5. Conclusions (1) The elongation and the strength of the alloy in 3.5% NaCl water solution are lower at 1.33 x sd than at 6.66 x s-' with the same pre-deformation. The former is more susceptible to
SCC. (2) The 10% pre-deformation alloy in 3.5% NaCl water solution has a bigger resistance to corrosion than that with free-deformation at the same tensile strain rate. (3) 8' phase precipitated in crystal will be uniform and dispersive and its quantities increase whereas the size diminishes when the alloy is pre-deformed. At the same time, the value of 8' (8) phase precipitated along the grain boundary distributes discontinuously and the precipitate free zone becomes narrow. As a result, the drop of electrode potential between the grain boundary and crystal decreases. The alloy with 10% pre-deformation is less susceptible to SCC.
References James J.F, Lawrence S.K., and Joseph R.P., Aluminum alloy 2519 in military vehicles, Adv. Mater. Processes, 2002,160 (9): 43. Devincent S.M., Devletian J.H., and Gedeon S.A., Weld properties of the newly Developed 2519-T87 aluminum armor alloy, Weld. J., 1988,67 (7): 33. Li H.Z., Zhang X.M., Chen M.A., et al., Microsttuctures and properties of welded joint of 2519 aluminum alloy, The Chin. J. Nonferrous Met. (in Chinese), 2004,14 (6): 956. Li H.Z., Zhang X.M., Chen M.A., et al., Hot deformation behavior of 25 19 aluminum alloy, The Chin. J . Nonferrous Met. (in Chinese), 2005.15 (4):621. Li H.Z., Zhang X.M., Chen M.A., et al., Effect of heat treatment on intergranular corrosion of 2519
aluminum alloy, Trans. Mater. Heat Treat. (in Chinese), 2005.26 (1): 20. [6] Zhang X.M., Gong M.Y., and Li H.Z., Effect of ageing tempers of aluminum alloy 2519 sheet on intergranular cornsion, J. Cent. South Uni. (in Chinese), 2004,35 (3): 349. [7] Dymek S. and Dollar M., 'IEM investigation of age-hardenable Al 25 19 alloy subjected to stress corrosion cracking tests, Mater. Chern. Phys., 2003, 81 (Z3): 286. [8] Kramer L.S., Blair T.P., Blough S.D., et al., Stress-corrosion cracking susceptibility of various product forms of aluminum alloy 2519, J. Mater. Eng. Perform., 2002,11(6):645. [9] Preet M. and John J., Effects of treatment on stress corrosion cracking of a discontinuously reinforced aluminum (DRA) 7xxx alloy during slow strain rate testing, Scripta Metall. Mater.,1995,33 (9): 1393. [ 101 HB7235-95: The Method for Determining the Stress Corrosion Cracking by Using Slow Strain Rate Testing, Aviation Industry Department of China, Beijing, 1995, I . [ 111 Liu J.H., Li D., and Liu P.Y., Effect of ageing and retrogression treatments on mechanical and corrosion properties of 7075 aluminum alloy, Trans. Mater. Heat Treat. (in Chinese), 2005,26 (I): 20. [ 121 Liu J., Li D., and Guo B.L., Slow strain rate tension test of high-strength aluminum alloy 7075, Mater. Sci. Technol., 2001.9 (1): 37. [I31 Tsai W., Duh J., and Lee J., Effect of pH on stress corrosion cracking of 7050-"45 1 aluminum alloy in 3.5 wt.% NaCl solution, Corrosion, 1990, 46 (6): 444. [I41 Song R.G., Zhang B.J., and Zeng M.G., The stress corrosion and role of Mg segregated to grain boundary in 7175 aluminum alloy, Acta Metall. Sin. (in Chinese), 1997,33 (6):95. [ 151 Li R.S. and Wang Z.Y., The effect of warm-rolling prior to aging on stress corrosion cracking susceptibility of aluminum alloy 2091, J. Chin. Soc. Corros. Prof. (in Chinese), 1995,15 ( I ) : 15.
RARE METALS Vol. 26, No. 4, Aug 2007, p . 385 E-mail: rm@ustb,edu.cn
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ScienceDirect
Effect of pre-deformation on the stress corrosion cracking susceptibility of aluminum alloy 2519 LI Huizhong, W A N G Xinming, CHEN Mingan, LI Yanfang, and HANG Xiaopeng School of Materials Science and Engineering, Central South Univmity, Changsha 410083, China (Received 2006-04-05)
Abstract:The effects of pre-deformation and strain rate on the stress corrosion cracking (SCC) behavior of aluminum alloy 2519 in air and in 3.5% NaCI water solution were investigated by means of slow strain rate tension (SSRT), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results indicate that the alloy is susceptible to SCC in 3.5% NaCl water solution and not in air. At the same predefonnation, the alloy is more susceptible to SCC at 1.33 x s-' than at 6.66 x s-'. Moreover, it is more susceptible to SCC at free predeformation than at 10% pre-deformation at the same strain rate. The number of 8 precipitated along the grain boundaries is reduced and distributed discontinuously, at the same time, the precipitate-free zones (PFZ)become narrow and the susceptibility to stress corrosion cracking is reduced after 10% pre-deformation.
Key words: 2519 aluminum alloy; pre-deformation; slow strain rate tension; susceptibilityto stress corrosion cracking
[Thisstudy wasjnuncially supported by the State Key Fundamental Research Program of China (No.2005CB623706).]
1. Introduction Aluminum alloy 2519, mainly used as the struc-
tural material for parts of aircraft skin, rocket, naval ships and boats, and the amphibious annored assault vehicle (AAAV), is a kind of high strength and high toughness alloy which was fust developed by the Americans in 1980s [l]. Certain scientists reported its welding properties, as well as, thermal strain properties [2-4]. Moreover, the effect of heat treatment on the intergranular corrosion of this alloy has been investigated and it was shown that this alloy has better intergranular corrosion resistance in T8 condition than in T6 condition [5]. Overaged alloys have better intergranular corrosion resistance than underaged and peak-aged alloys [6]. The resistance to stress corrosion became lower with the increase in copper content [7]. Kramer researched the influence of different hot working methods on the resistance to stress corrosion of 2519 alloy [8]. To date, there is Corresponding author: LI Huizhong
little research on the effect of pre-deformation on the stress corrosion of aluminum alloy 2519. The pre-crack specimen is not capable of offering nucleation information of stress corrosion, which is always used to study the stress corrosion cracking susceptibility of crack propagation behavior. But the nucleation process is very important for stress corrosion cracking susceptibility of materials with smooth surface in a corrosive environment [9]. Consequently, using slow strain rate tension test to evaluate the stress corrosion cracking susceptibility may reflect the objective behaviors, and short detecting period is cost. The effect of predeformation and strain rate on the SCC behavior of aluminum alloy 25 19 was investigated in this study.
2. Experimental 2.1. Experimental materials
E-mail: [email protected]
The material used in this study was a commercial
RARE METALS, Vol. 26, No. 4, Aug 2007
386
alloy 2519 rolled plate of 40 mm in thickness. The materials were tested in both free pre-deformation (alloy 1) and 10% pre-deformation (alloy 2) between solid solution and ageing, and solution was heat treated at 525°C for 60 min, followed by cold rolling and then artificial ageing at 180°C. The chemical composition of the experimental alloy is shown in Table 1. Table 1. Chemical composition of 2519 aluminum alloy wt.%
Cu
Mn
ME
Zr
Ti
V
Fe
Si
Al
5.80 0.29 0.22 0.22 0.06 0.06 0.15 0.05 Bal
2.2. Experimental process
According to the HB 7235-95 standard test method for measurement of SSRT [ 101, tensile-type specimens with longitudinal transverse (LT) orientation were selected for SSRT study. All the peak-aged specimens were machined from a 40 mm thick plate with the loading direction parallel to the rolling direction of the plate. The diameter of the specimens was 5 mm and the gauge length was 30 mm. The SSRT experiment was carried out on an Instron 8032 fatigue-testing machine. The tensile s-' and 6.66 x lo-' strain rate was set at 1.33 x s-' when the experiments were operated in air or in 3.5% NaCl water solution, and the temperature was controlled to 35 f 1OC. The fracture was observed using a Sirion 200 scanning electron microscopy (SEM). The microstructure was characterized by a Hitachi 800 transmission electron microscopy (TEM) analysis. TEM samples were electro-polished in 70% methanol and 30% nitric acid solution at -35°C using twin-jet equipment operated at 30 V.
strength and elongation of alloy 1 at 6.66 x lo-' s-' in 3.5% NaCl water solution decreased. Their ratios were 9 1% and 74%, respectively. This indicates that alloy 1 is susceptible to SCC in 3.5% NaCl water solution. As for alloy 2, its fracture strength and elongation in 3.5%NaC1 water solution are also lower than those in dry air. But the ratios are 96% and 81%, respectively, which has smaller decrement than alloy 1. Alloy 2 is less susceptible to SCC than alloy 1. According to Table 3, the fracture strength and the elongation of alloys 1 and 2 at 1.33 x lo-' s-' are lower than those at 6.66 x lo-' s-' in 3.5% NaCl water solution. On this account, alloy 25 19 at 1.33 x lo-' s-' is more susceptible to SCC than at 6.66 x s-' in 3.5% NaCl water solution. Stress corrosion index, which is an important criterion of SCC, can reflect the SCC susceptibility better than a single mechanical performance index. It is defined in expression [l I]: ISSRT
(1)
where, ofw is the fracture strength in environmental medium, Mpa, sfw is the elongation in environmental medium, %, o f A i sthe fracture strength in inert medium, MPa, JfA is the elongation in inert medium, %. Table 2. Mechanical properties of aluminum alloy 2519 Alloy
No.
Medium Dry air 3.5%
1
NaCl 3.5%
NaCl Dry air 3.5% 2
3. Results
= 1-[of, (1 + s f w )I / [ o f A (1 + 8 f A )I
NaCl 3.5% NaCl
Strain rate / (10-~.~-l)
Tensile Elongation/ strength/MPa %
6.66
480
20
6.66
435
15
1.33
425
11
6.66
500
12
6.66
480
10
1.33
475
8
._
3.1. Results of SSRT testing The results of SSRT tests in dry air and in 3.5% NaCl water solution are listed in Table 2. It can be seen that the strength and the elongation decrease with tensile in 3.5% NaCl water solution, and the decrement increases with the decrease in strain rate. Compared to an atmosphere of dry air, the fracture
The SCC susceptibility gradually increases, whereas ZssRT changes from 0 to 1 . The stress corrosion indexes of 2519 aluminum alloy with different pre-deformations are shown in Table 3. The results indicate that the stress corrosion indexes of alloy 1 are higher than those of alloy 2 and the stress corro-
Li H.Z. et al., Effect of pre-deformationon the stress corrosion cracking susceptibility of ...
sion indexes are higher at low stress rate than at high strain rate. Table 3. Results of SCC for aluminum alloy 2519 Alloy No. ~~
El
(lO".s-')
ISSRT
~
1 2
6.66
0.13
1.33
0.18
6.66
0.06
I .33
0.09
3.2. SEM analysis for the fracture surface of SSRT testing The fracture surfaces of 25 19 alloys with different pre-deformations present different features for samples tested under different conditions. Fig. 1 shows the fracture surface of two experimental alloys in dry air. Several dimples are visible with ductile features in the two alloys. But the plastic flow of alloy 1 is more drastic than that of alloy 2. This is coincident with the fact that the plasticity of alloy 1 is better. These two kinds of fracture in Fig. 1 are ductile fracture, which belongs to the pure mechanical fracture.
Fig. 1. Fracture surface SSRT testing in dry air (strain rate ::6.66 x s-I): (a) alloy 1; (b) alloy 2.
387
Fig. 2 shows the fracture surface of alloys in 3.5% NaCl water solution when the strain rate is 6.66 x lo-' s-'. The fracture surface of alloy 2 in this condition shows certain small and shallow dimples and the properties of a mechanical fracture (Fig. 2(b)). Whereas the fracture surface of alloy 1 shows certain intergranular cracks and their secondary cracks (Fig. 2(a)). The fracture surface of alloy 1 in this condition is constituted with crude intergranular cracks and certain small dimples. So alloy 1 is more susceptible to SCC than alloy 2 in this condition. Fig. 3 shows the fracture surfaces of alloys in 3.5% NaCl water solution at 1.33x s-'. The fracture surface of alloy 2 has certain small dimples, but the dimples are smaller than that at 1.33 x lo-' s-' and the fracture surface of alloy 1 is characterized by intergranular separation and is brittle fracture a.. shown in Fig. 3(a). The results indicate that the SCC susceptibility of alloys increases with the decrease in tensile strain rate.
Fig. 2. Fracture surface SSRT testing in 3.5 wt.% NaCl water solution (strain rate = 6-66 xlo4 s-'): (a) alloy 1; (b) alloy 2.
388
RARE METALS, Vol. 26, No. 4, Aug 2007
Fig. 3. Fracture surface SSRT testing in 3.5 w t % NaCl water solution (strain rate = 133 xlOd s-'): (a) alloy 1; (b)alloy 2.
4.
Discussion
4.1. Effect of strain rate on the SCC behavior of aluminum alloy 2519 The operating range of tensile strain rate, which usually ranges between lo4 and lo-* s-', is primarily a parameter of SCC behavior [ 121. For most materials, it is most susceptibleto SCC when the tensile strain rate is between and lo4 s-', There is a suitable strain rate (or range) in certain environ-mental medium to distinguish the SCC susceptibility [13]. According to Table 3, the alloy with the same pre-deformation is more susceptible to SCC at low tensile strain rate than at high tensile strain rate. The SCC behavior occurs in a certain range of tensile strain rate. If the tensile strain rate is too high, the elongation will approach the value measured in air and the decrease in toughness will be small. It is not susceptible to SCC in this condition and results in the occurrence of ductile fracture before the reaction between environmental medium and the speci-
men. On the contrary, if the strain rate is too low, the materials will appear with a fresh surface and a passive film will form on the fresh surface and the 3.5% NaCl water solution cannot take effect in the stress corrosion crack pmess. In this condition, strength and toughness are almost the same as in air. According to Table 3, the SCC susceptibility of aluminum alloy 2519 increases with the decrease in tensile strain rate within the tension speed capability. There are two reasons for its more susceptibility to SCC at low tensile strain rate than at high tensile strain rate in 3.5% NaCl water solution. First, aluminum alloy 2519 in natural condition has a good corrosion resistance for it is easy to form a layer of oxide which has protective effect. Because of the difference in elongation between the oxide film and the base metal, the oxide film will break, although tensile stress is allowed. Cells will appear between the oxide film and the base metal in an environmental medium. As a result, anodic dissolution occurs and propagation of the cracks is accelerated. On the other hand, the fresh aluminum alloy 25 19 surface exposed in the corrosion medium may produce the following important electrochemical reactions with water in an environmental medium [ 141: 2Al+ 3H20 + A1203 + 6H' + 6e(2)
AI + A I ~++3e-
(3)
H++ e" + [HI
(4) The active hydrogen generated by the upper reaction is delivered into the inner part of the materials through adsorption, diffusion, and dislocation and leads to hydrogen embrittlement.The specimen has more time to react with the environmental medium at low strain rate and absorbs more hydrogen. So it is more susceptible to SCC at a low strain rate.
4.2. Effect of pre-deformation on the SCC behavior According to Table 3, the SCC susceptibility of alloy 2 with 10% pre-deformation before ageing decreases dramatically. Fig. 4 shows the TEM images of alloys with different extents of pre-deformation. Figs. 4(a) and 4(b) show the size and distribution of the precipitate phase 8' (8) within the grain boundary and inner crystal of alloy 1 in peak-aged
Li H.Z.et al., Effect of pre-deformation on the stress corrosion cracking susceptibility of ...
condition. The dimension of the 8 phase precipitated along the grain boundary is coarse and distributes continuously, which has wild precipitate free zones. The 8’ phase precipitated in intracrystalline is coarse and distributes inhomogeneously. The TEM images of alloy 2 are shown in Figs. 4(c) and 4(d). Compar-
389
ing Figs. 4(a) and 4(b) shows the following facts: (1) the size and the quantity of 8 phases precipitated along the grain boundary decreases; ( 2 ) the gap between 8 phases precipitated along the grain boundary increases obviously, whereas the distribution of it is no more continuous along the grain boundary
Fig. 4. TEM images showing the microstructures of the peak-aged 2519 aluminum alloy: (a) and (b) alloy 1; (c) and (d) alloy 2.
and the precipitate free zone becomes narrow; (3) as for the 8’ phase precipitated in intracrystalline, its size decreases, whereas the quantity increases, and it distributes uniformly and dispersively. Plenty of crystal defects came into being, such as vacancy and dislocation in solid solution alloy after cold working. As a result, the diffusion of the solute atoms is accelerated, and the rate of precipitation is 100 times faster than that in structure with free pre-deformation and magnitude dispersive 8’ phase is formed in intracrystalline. On the contrary, the nucleation of 8’ phase in intracrystalline alloy 1 with free pre-deformation has a long incubation period and the number of 8’ phase is small, and the precipitated phase on the grain boundary is easy to nucleate and grow, which leads to the equilibrium phases’ high-density distribution on the large-angle grain
boundary and shortened distance between phases. The common face slip becomes drastic that results in the increasing of slip shoulder height of the alloy surface and crack tip attributed to the coarseness of 8’ phase [15]. So, alloy 1 with free-deformation has low resistance to corrosion. As for alloy 2 with 10% pre-deformation, the potential drop between the grain boundary and inner crystal decreases and the optional dissolving of the precipitate phase on the grain boundary decelerates. At the same time, the abundant dispersive 8’ phase hinders common face slip and diminishes the slip shoulder height of the alloy surface and crack tip. So pre-deformation improves the structural properties and disperses common face slip. As a result, alloy 2 with pre-deformation is less susceptible to SCC.
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5. Conclusions (1) The elongation and the strength of the alloy in 3.5% NaCl water solution are lower at 1.33 x sd than at 6.66 x s-' with the same pre-deformation. The former is more susceptible to
SCC. (2) The 10% pre-deformation alloy in 3.5% NaCl water solution has a bigger resistance to corrosion than that with free-deformation at the same tensile strain rate. (3) 8' phase precipitated in crystal will be uniform and dispersive and its quantities increase whereas the size diminishes when the alloy is pre-deformed. At the same time, the value of 8' (8) phase precipitated along the grain boundary distributes discontinuously and the precipitate free zone becomes narrow. As a result, the drop of electrode potential between the grain boundary and crystal decreases. The alloy with 10% pre-deformation is less susceptible to SCC.
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