Materials Chemistry and Physics 81 (2003) 286–288 TEM investigation of age-hardenable Al 2519 alloy subjected to stress corrosion cracking tests Stanisław Dymek a,∗ , Marek Dollar b b a University of Mining and Metallurgy, al. Mickiewicza 30, 30-059 Cracow, Poland School of Engineering and Applied Science, Miami University, Kreger Hall, Oxford, OH 45056, USA Abstract The influence of changes in chemical composition and pre-aging deformation on the resistance to stress corrosion cracking in the age-hardenable aluminum alloy 2519 was investigated by transmission electron microscopy. The improvement of this resistance may be accomplished by keeping the Cu concentration on the lower side of the allowed limit for the 2519 alloy. Also, plastic deformation prior to aging, comprising both cold rolling and stretching, seems to be beneficial since it promotes a more homogeneous distribution of the precipitates and reduces the number of precipitates on the grain boundaries and thus shrinks the total volume of precipitation-free zones at grain and subgrain boundaries. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Stress corrosion cracking; Precipitation hardening; Aluminum alloys 1. Introduction Aluminum alloys have been the primary material for structural components of military aircrafts, helicopters, amphibians, etc. for several decades. This was because of the low specific gravity of aluminum which favors the selection of aluminum alloys in weight-critical applications [1]. Wrought heat-treatable (age-hardenable) alloys are of particular interest because they develop the highest specific strength. The alloy 2519, which is used for armor plates applications, belongs to this group. This application calls for a balance of properties which comprise weldability, acceptable ballistic resistance and mechanical properties as well as resistance to stress corrosion cracking. To meet all these requirements an improved understanding of the relationships between composition, processing, microstructure and properties is required. In particular importance, microstructural features for control are: size, coherence, volume fraction and distribution of strengthening precipitates, degree of recrystallization, grain size and shape, crystallographic texture as well as the presence of intermetallic constituent particles. Investigation of numerous versions of the 2519 alloy let to find ranges for chemical compositions and processing parameters which suit the basic requirements for this alloy. However, it ∗ Corresponding author. Tel.: +48-12-617-2696; fax: +48-12-617-3190. E-mail address: gmdymek@kinga.cyf-kr.edu.pl (S. Dymek). turned out that some variants of this alloy are susceptible to stress corrosion cracking. This work was attempted to find microstructural reasons (if any) of susceptibility to stress corrosion cracking. The investigation was carried out by means of analytical transmission electron microscopy (TEM) supplemented with energy-dispersive spectrometry (EDS). 2. Material and experimental procedure Two alloys designated as 2519 with slightly different chemical compositions, given in Table 1, were examined. The 2519 alloy is usually used in T8 temper. So, deformation is carried out to aid the nucleation of precipitates and thus to increase the strength at a reduced time of aging. By reducing the aging time to peak strength, the amount and size of grain boundary precipitates are also reduced. Thus the strength/fracture toughness relationship is improved [1]. Both alloys were subjected to solution treatment at 532 ◦ C followed by cold working and subsequent aging at 160 ◦ C for 14 h. The cold working was of two kinds: rolling with 10% reduction in thickness followed by stretching to about 1.5–3.0% and stretching alone to about 10% strain. The samples were subjected to tensile, ballistic and stress corrosion cracking tests. The stress corrosion cracking tests were performed on tensile as well as C-ring samples. The alloy was regarded as resistant to stress corrosion cracking, when 0254-0584/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(02)00603-X S. Dymek, M. Dollar / Materials Chemistry and Physics 81 (2003) 286–288 287 Table 1 Chemical composition of the examined alloys (in wt.%) Alloy Cu Mg Zr V Mn Zn Fe Si 1 2 5.56 5.80 0.36 0.23 0.11 0.10 0.07 0.07 0.44 0.43 0.02 0.02 0.14 0.12 0.04 0.04 three out of three tested specimens did not show evidence of cracking. The specimen for TEM investigation from tensile samples were cut out from sections perpendicular to the tensile axis while disks from the C-ring samples were cut out from the section parallel to the lateral surface of the ring. The disks were subsequently ground down to about 0.05–0.07 mm and electropolished in a solution of 33% nitric acid in methanol at a temperature of about −30 ◦ C and at a voltage of 30 V. The TEM investigation was carried out using a Phillips CM 200 analytical scanning transmission electron microscope operating at 200 kV. The structures were observed by conventional transmission or scanning modes while scanning and/or nanoprobe modes were utilized for chemical analysis (EDS). The probe size for the scanning and nanoprobe modes was approaching a few nanometers in diameter (practically 10–20 nm because at smaller electron probes the number of X-ray counts was usually too low for analysis). A Cliff–Lorimer standard-less method for thin section was used for a quantitative analysis. Standard-less methods always total selected elements to 100% which makes all results approximate since minor constituents are not taken into consideration. 3. Results and discussion Stress corrosion cracking may be defined as a phenomenon which results in brittle failure of alloys, normally considered ductile, when they are exposed to a simultaneous action of surface tensile stress and corrosive environment, neither of which when operating separately could cause damage [2]. Stress corrosion cracking in Al alloys is characteristically intergranular [3]. However, the exact mechanism responsible for stress corrosion cracking of a susceptible aluminum alloy remains controversial in a particular environment. There are three main theories [4]: (1) Anodic dissolution: this theory assumes that the cracking is due to preferential corrosion along grain boundaries. (2) Hydrogen-induced cracking: this theory postulates that atomic hydrogen is absorbed and weakens the grain boundaries. (3) Rupture of the passive film. Since the role of grain boundaries is essential in either case, the examination was focused on the structure of grain and subgrain boundaries, precipitates on grain boundaries, and changes in chemical composition in regions near grain boundaries. The Al 2519 alloy is a typical age-hardenable alloy which is strengthened by the coherent θ phase which precipitates Fig. 1. The hardening precipitates in the examined alloys. as thin plates on {1 0 0} planes. The evidence of a uniform distribution of this phase was found in the alloy (Fig. 1). Regardless of differences in composition and applied deformation prior to aging, the alloys exhibited similar mechanical properties: the tensile strength was about 490 MPa and the yield strength about 450 MPa, but alloy 1 exhibited a higher elongation than alloy 2 (13 vs. 11%). All samples passed ballistic tests, however, not all samples passed the stress corrosion cracking tests. Alloy 2 failed all tests regardless of the mode of deformation prior the aging. On the other hand, alloy 1 which was cold rolled and stretched prior the aging passed all test while samples which were only stretched failed. The microstructures in all examined samples were similar, especially with regard to grain boundary precipitates, but some differences between samples which passed and failed the stress corrosion cracking tests could be detected. Samples of all variants exhibited precipitates on grain boundaries and subgrain boundaries. These precipitates were usually Cu-rich (probably semicoherent θ or incoherent θ (Al2 Cu) phase). A feature of such a microstructure was the presence of precipitation-free zones along the grain boundaries (Fig. 2). Such zones were also found in samples which passed the tests. The Cu content within the precipitation-free zones was lower than in the surrounding matrix and did not Fig. 2. Precipitation-free zones at a grain boundary. 288 S. Dymek, M. Dollar / Materials Chemistry and Physics 81 (2003) 286–288 Fig. 3. Precipitates on a subboundary in alloy 2. exceed 2 wt.%. According to the literature the difference in electrochemical potentials of the Cu-depleted zone and the Cu-rich matrix form a strong galvanic cell with a potential difference of about 0.12 V [3]. The anodic Cu-depleted zone is small in area compared with the area of the cathodic matrix, resulting in a high driving force for rapid intergranular corrosion. Moreover, strain is likely to be concentrated in the zones because they are relatively soft. It is believed that the resistance to the stress corrosion cracking is inversely proportional to the total volume of precipitation-free zones. Some differences between the microstructures of samples which failed and passed the stress corrosion cracking tests were observed. However, these changes have not been correlated quantitatively with susceptibility. In samples which failed the test, almost continuous chains of precipitates were found on grain boundaries as well as subgrain boundaries (Fig. 3). On the other hand, precipitates on grain boundaries in samples which passed the test were distributed in a more discrete manner. Cu-depleted zones were rarely found along subgrain boundaries (or precipitation-free zones were very narrow) in samples which passed the test. It is believed that the more homogeneous distribution of Cu-rich precipitates in such samples is due to the more complex deformation mode (rolling and stretching) prior the aging. This produces a more homogeneous dislocation substructure which provides more uniformly distributed potential nucleation sites for the hardening precipitates. A larger number of huge particles (well above 1 m) was found in samples which failed the tests; the particles were often arranged in aligned stringers. Such particles were always Fe- and Mn-rich (probably isomorphous with Al7 Cu2 Fe). Compounds containing Fe are cathodic with respect to the Al matrix and promote electrochemical attack. The protective oxide film over them is very thin and may be easily broken by stress. However, the presence of Mn reduces the detrimental effect of Fe and it is not very likely that such particles enhance the susceptibility to stress corrosion cracking in an appreciable manner. All samples were fully recrystallized. Also a variety of precipitates was found in grain boundaries and grain interiors of all samples. The distribution of these precipitates was relatively uniform. Their size ranged between 0.2 and 0.5 m. The Cu-depleted zones were never found around such particles. Other precipitates contained Zr and sporadically V. These precipitates were found on grain boundaries. Their presence is beneficial since they control the recrystallization and grain size. It is believed that such particles do not influence the stress corrosion cracking behavior in a considerable manner. Neither segregation nor Mg-rich precipitates were found at grain boundaries. It means that the anticipated role of Mg to modify the precipitation process of the hardenable phase, and thus resulting in a greater age-hardening effect, came up to expectations. 4. Conclusions 1. Complex plastic deformation prior to aging, comprising both cold rolling and stretching, rather than stretching alone, is beneficial for the stress corrosion cracking resistance since it promotes a more homogeneous distribution of the precipitates and reduces the number of precipitates on grain boundaries. 2. Lowering the Cu concentration in the 2519 alloy improves the stress corrosion cracking resistance. 3. All examined samples, in both alloying variants, may be susceptible to stress corrosion cracking due to Cu-rich precipitates on grain boundaries and formation of Cudepleted zones adjacent to the boundaries. Acknowledgements We would like to thank McCook Metals for conducting all tests and for providing the material for the present study. References [1] E.A. Starke Jr., J.T. Staley, Prog. Aerospace Sci. 32 (1996) 131. [2] I.J. Polmear, Light Metals, Halsted Press, An Imprint of Wiley, New York, 1996, p. 62. [3] J.R. Davies, Corrosion of Aluminum and Aluminum Alloys, ASM International, Materials Park, OH, 1999, p. 99. [4] T.D. Burleigh, Corrosion 47 (1991) 89.