Stress corrosion cracking susceptibility of 7A52 aluminum alloy
ZHAO Jun-jun(赵军军), WANG Wei-xin(王卫欣), CAI Zhi-hai(蔡志海), ZHANG Ping(张 平)
Institute of Remanufacture Engineering, Academy of Armored Force Engineering, Beijing 100072, China
Received 28 July 2006; accepted 15 September 2006
Abstract: The stress corrosion sensitivity of 7A52 aluminum alloy was investigated in the artificial sea water through slow stain rate test(SSRT). The stress corrosion cracking(SCC) susceptibility was estimated with the loss of elongation and stress corrosion sensitivity index. The results show that the susceptibility of 7A52 aluminum alloy is always high when the strain rate is in the range of 10-5-10-7 s-1. It reaches the maximum at the strain rate of 8.7×10-7 s-1, and the sensitivity index reaches 0.346. The characteristics of stress corrosion can be observed clearly on the fracture of tensile specimen. The process of SCC is depicted according to the fracture morphology. The SCC initiates at the edge of the specimen. Then the SCC grows rapidly because of the anode dissolving and stress concentration. When the area of specimen cannot support the tensile stress, it ruptures suddenly. The secondary cracks and quasi-cleavage surface can be found on the fracture morphology.
Key words: 7A52 aluminum alloy; stress corrosion cracking(SCC); stress corrosion susceptibility
1 Introduction
7A52 aluminum alloy is a high strength aluminum alloy of Al-Zn-Mg series. It was originally designed for the applications to the structures in land. But there are now some needs to use it as the ship shell material. It is well known to all that the high strength aluminum alloys have higher stress corrosion susceptibility to Cl- in sea water [1-4]. It has the hidden trouble in the structure security to use it as the material of ship body. Therefore, the stress corrosion susceptibility of 7A52 must be investigated as soon as possible. However, the studies on 7A52 were mainly centralized on the heat treatment, structure and mechanical properties[4-7]. Only a few investigations were about the stress corrosion cracking [8-9]. According to these researches, the stress corrosion properties of Al-Zn-Mg series high strength aluminum alloys are related to the total content of Zn+Mg and the ratio of Zn to Mg[10]. In order to improve the strength, the total content of Zn+Mg is about 6%-7% in 7A52 aluminum alloy. It made the stress corrosion susceptibility increase obviously. In addition, the stain rate is an important parameter which has great influence on the behavior of stress corrosion cracking[11-12]. For almost all materials, the stress corrosion cracking usually occurs in the range of 10-4-10-8 s-1. When the stain rate is about 10-6 s-1, it has the highest value of the stress corrosion susceptibility. However, the stress corrosion sensitive rate varies with the material and environment. In this paper, four different strain rates in the range of 10-5-10-7 s-1 were selected to study the stress corrosion susceptibility of 7A52 aluminum alloy in the artificial sea water with slow stain rate test(SSRT).
2 Experimental
A GYF-30 type slow stain rate tester was used as test equipment. Its maximal tensile load was 30 kN, and the moving rate of the crossbeam ranged from 4.275×10-2 to 1.99×10-8 mm/s. The tensile load on the specimen was measured and displayed with a stress sensor and a millivoltmeter. The displacement was measured with a linear inductosyn, and showed on a digital display. The automatic data treatment system could collect the data of load and displacement, draw the load/displacement curve, and calculate the energy to failure and printed all kinds of the test results.
7A52 aluminum alloy plates were used as test material with the nominal composition (mass fraction, %) of 4.35Zn, 2.40Mg, 0.35Mn, 0.20Cr, 0.10Zr, 0.12Ti, 0.12Cu, and balanced Al. The heat treatment was aging after forged. It was heated for 1 h at 460 ℃, water quenched to room temperature, and then artificially aged for 24 h at 120 ℃.
Fig.1 shows the schematic of SSRT sample. The gauge length is 16.0 mm, and the thickness is 4.0 mm. The tension direction of the specimen is along the longitudinal direction of the plate.
Fig.1 Schematic diagram of SSRT sample (mm)
The gauge length of the samples was burnished with waterproof abrasive paper and then polished before test. The width and thickness of the gauge length were measured after the surface being polished. The tensile tests were carried out at four strain rates of 4.2×10-5, 8.7×10-6, 4.2×10-6 and 8.7×10-7 s-1. The corresponding moving rates of crossbeam were respectively 0.05, 0.01, 0.005 and 0.001 mm/min. The test media were artificial seawater (3.5% NaCl solution) and inert gas (1 010 kPa N2), and the test temperature was room temperature.
The loads and displacements with the time were recorded in the test. The curve between load and displacement was drawn and the energy to failure was calculated according to it. The maximal load, the maximal elongation and the loss of elongation then were obtained.
3 Results and discussion
3.1 Evaluation of stress corrosion susceptibility
The test results at four strain rates are listed in Table 1. The stress-strain curves in the inert gas and the artificial seawater are shown in Fig.2.
Although there is no commonly accepted method to estimate the stress corrosion susceptibility of materials at present, the loss of elongation Eloss is usually used as a parameter to compare with each other. It is defined as follows[10]:
(1)
When the SCC susceptibility of a material is very low, the difference of elongation in inert gas and in sea- water is very little, and the value of Eloss is less than 10%. When the value of Eloss is more than 10%, whereas, it means the material has high SCC susceptibility[13,14]. A high value of Eloss means a high stress corrosion cracking susceptibility of a material.
As shown in Table 1, 7A52 aluminum alloy shows the high stress corrosion susceptibility at four strain rates because Eloss values are quite high. But the susceptibility varies at different strain rates. When the strain rate is 8.7?10-7 s-1, the value of Eloss reaches 31.41%, and the stress corrosion susceptibility of 7A52 aluminum alloy is the highest. But it falls down when the strain rate is respectively 4.2?10-5, 8.7?10-6 and 4.2?10-6 s-1. The same conclusion can be drawn from Fig.2. Compared with the stress-strain curves at the same strain rate in different environment media, it is obvious that the distinction at strain rate of 8.7?10-7 s-1 is the largest. It shows that the susceptibility under this condition is the highest.
Another method to evaluate the stress corrosion susceptibility of materials is to compare the capabilities on the curve in a corrodent with those on the curve in the inert gas. Their ratios can be used as a parameter to show the corrosion resistance of materials in this corrodent [15]. The smaller the ratio, generally, the higher the stress corrosion susceptibility is. The capabilities usually include the ultimate tensile stress, the maximal energy to failure, section shrinkage rate, elongation, test time and so on. By calculating the ratios of parameters in Table 1, the results are obtained as listed in Table 2.
Table 1 Test results of SSRT at different strain rates
Fig.2 Stress—strain curves at different strain rates: (a) ε=8.7×10-7 s-1; (b) ε=4.2×10-6 s-1; (c) ε=8.7×10-6 s-1; (d) ε=4.2×10-5 s-1
Table 2 Ratio of test results in artificial seawater and those in inert gas
All the four sets of the results in Table 2 have the similar tendency with the strain rate. The ratios of test time and elongation, especially, even have the same values. That is to say, when the strain rate is in the range of 10-5-10-7 s-1, 7A52 aluminum alloy possesses high stress corrosion cracking susceptibility. The ratios of all parameters at the strain rate of 8.7?10-7 s-1 are the lowest, showing the highest stress corrosion susceptibility under this condition.
All the evaluations for the stress corrosion susceptibility made above are established on single property parameters. In order to evaluate the stress corrosion susceptibility more concretely, more properties of material in SSRT were proposed in one expression. So the stress corrosion sensitivity index ISSRT was put forward to be used as an important criterion. Its expression is as follows[16]:
(2)
The value of ISSRT increases from 0 to 1, representing the growth of stress corrosion susceptibility. Through calculation, the values of ISSRT at different strain rates are respectively 0.226 (ε=4.2?10-5 s-1), 0.180 (ε=8.7?10-6 s-1), 0.179 (ε=4.2?10-6 s-1) and 0.346 (ε=8.7?10-7 s-1). Obviously, they share the same tendency with the data in Table 2.
3.2 Fracture analysis
The fractures of the tensile specimen were analyzed by SEM. The morphology photos of the specimen in inert gas are shown in Fig.3.
Fig.3(a) displays the section at the edge of specimen, and Fig.3(b) displays the centre part. Many dimples can be found on the fracture. This means that the fracture is ductile. There exist more dimples at the centre part in Fig.3(b) than those at the edge in Fig.3(a). And some transgranular cracks can be seen which cannot be found at the center. The phenomenon is caused by the filamentary structure of 7A52 aluminum alloy. Transgranular cracks are firstly brought up at the initial stage. Then the cracks grew to the centre of specimen slowly. When the stress at the centre of sample reached the ultimate tensile strength of the material, the sample ruptured suddenly. So the fracture at the center displayed dimples.
The fracture morphologies of the specimen in artificial seawater are shown in Fig.4. The characteristics of brittle rupture are fully shown in the photos, revealing a typical stress corrosion fracture. A whole stress corrosion crack is visible in Fig.4(a). The origin of the crack locates at the edge of specimen. It extends to the centre slowly under tensile stress. The stress concentration is present at the tip of the crack. When the crack grows to some extent length, the stress concentration is high enough to break the specimen, it fractures suddenly. It can be found that the corrosion area at the edge of sample is plain, and the centre area is full of dimples in Fig.4(a). There is a step between the corrosion area and the centre area. Fig.4(b) shows a local magnification of Fig.4(a). Dimples can be viewed clearly on the left. It shows that the fracture mechanism at the centre is ductile rupture. In the crack growth area, secondary cracks can be seen in Fig.4(c) and the quasi-cleavage surface is shown in Fig.4(d). All these are the typical characteristics of stress corrosion cracking. Therefore, the formation process of SCC in SSRT can be summarized as follows. The stress concentration is firstly produced at the weak spots of the surface such as grinding traces in the action of tensile stress. The oxide film in these areas is broken and the matrix metal is exposed to the corrosive medium. So the stress corrosion cracking is generated. Because of the different electric potential between the matrix metal and oxide film, a self-corrosion battery was formed in the corrodent which has small anodes and large cathodes. It dissolves the anode and the cracks spread rapidly. In addition, the corrodent can penetrate to the weak parts due to the defects in the material. Thus the secondary cracking and quasi-cleavage surface are formed. Then the cracks grow continually, and the real area of the specimen is reduced. When the sample can not bear the tensile load, it ruptures. The fracture section usually displays, therefore, one or a few crack initiation zones, growing zone and shear lip zone can be found on the fracture surface, but dimples owing to ductile fracture appear at the specimen centre, as shown in Fig.4.
Fig.3 Sectional morphologies of specimen in inert gas: (a) At edge of specimen; (b) In centre part
Fig.4 Sectional morphologies of specimen in artificial seawater: (a) Stress corrosion cracking; (b) Local magnification of Fig.4(a); (c) Secondary cracks; (d) Quasi-cleavage surface
4 Conclusions
1) The SCC susceptibility of 7A52 aluminum alloy is always high when the strain rate varies in the range of 10-5-10-7 s-1, and has the maximal value at 8.7×10-7 s-1, its sensitive index reaches 0.346.
2) The characteristic of stress corrosion can be observed clearly on the fracture of 7A52 aluminum alloy.
3) The SCC is firstly brought up at the edge of the specimen. Then the SCC spreads rapidly because of the anode dissolving effect and stress concentration.
4) When the area of specimen reduces and can not support the tensile stress, it ruptures suddenly. The
secondary cracks and quasi-cleavage surface can be found in the fracture morphology.
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(Edited by LI Xiang-qun)
Foundation item: Project(404010202.3C) supported by the Tenth Five-year Plan Program of China
Corresponding author: ZHAO Jun-jun; Tel: +86-10-66719249; E-mail: zhaojj0182@sina.com
