Comparison of strength,stress corrosion cracking and microstructure of new generation 7000 series aluminium alloys

Abstract

The strength, stress corrosion cracking (SCC), and microstructure of four typical new generation 7000 series aluminium alloys (AA7037, AA7097, AA7056, and AA7085) under three-stage aging heat treatment have been investigated. The results showed that AA7085 exhibited the highest SCC resistance while AA7037 displayed the lowest SCC resistance. In addition, AA7056 presented the higher strength and SCC resistance compared with AA7037. The more susceptible to SCC of new generation 7000 series aluminium alloys was mainly ascribed to the high Zn and low Cu content in grain boundary precipitates. The results provided a new insight for the composition design of new generation 7000 series aluminium alloys.

Keywords

new generation 7000 series aluminium alloys stress corrosion cracking strength microstructure grain boundary precipitates Cu content composition design

Introduction

7000 series aluminium alloys are widely used in aerospace based on their attractive mechanical properties [1 -3]. Up to now, a series of new generation Al–Zn–Mg–Cu aluminium alloys, such as AA7085, AA7056, AA7037, and AA7097 have been developed for thickness sections products with high hardenability and strength. After carefully comparing the composition range of the new generation 7000 series alloys, there are similar composition characteristics, namely low Cu content and high Zn/Mg ratio, compared with the traditional AA7050 and AA7055 alloy [4]. Recently, it was reported that the new generation alloys were designed to surpass the aviation industry specified stress corrosion cracking (SCC) and exfoliation acceptance tests. However, the new generation alloys have the high SCC susceptibility compared with traditional alloys with low Zn and high Cu content [5 -7]. Particularly, European Aviation Safety Agency (EASA) [8] reported that there is potential SCC risk of the new generation 7000 series aluminium alloys in aggressive environment. However, the causes and mechanisms of SCC have not been described in detail. Additionally, Schwarzenböck et al. [9] investigated the hydrogen environmentally-assisted cracking (HEAC) behaviour of new generation thick-plate 7000-T7x alloys (AA7449, AA7085 and AA7037) and traditional industry benchmark 7050 alloy subjected to humid warm-air (relative humidity of 85%, at 70°C). They observed that the SCC cracking growth rates of new generation alloys were 6–20 times higher than that of AA7050-T7x alloy in humid air, while the initiation time under constant load can be reduced by at least an order of magnitude in new generation alloys. Herein, a strong association between crack growth rate and high Zn was also demonstrated. Notably, the aging heat treatment process parameters of new generation thick-plate7000-T7x alloys were not described in detail. Moreover, the microchemistry of grain boundary precipitates (GBPs) were only obtained by simulations carried out using the JMatPro software.

Reviewing the work on SCC of 7000 series aluminium alloys, the SCC resistance mainly depended on the alloy compositions (like Cu content, Zn/Mg ratio) and heat treatment conditions [10 -12]. Addition of Cu to Al–Zn–Mg alloys can improve the SCC resistance owing to Cu replacing Zn in the GBPs. Over-aging heat treatment increases the SCC resistance of 7000 series alloys primarily owing to Cu enrichment in GBPs [13 -15]. Besides, numerous studies have shown that the high Cu content and discontinuous distribution of GBPs are responsible for the excellent SCC resistance of 7000 series alloys [16,17]. However, the microstructure and corrosion properties, especially SCC behaviour, of the new generation 7000 series aluminium alloys were not studied in detail. Thus, research on the SCC resistance difference among these new generation alloys is critical for understanding the reasons why a high SCC susceptibility exist in alloys with high Zn/Mg ratio and low Cu. The purpose of the paper is to compare the microstructure, mechanical properties and SCC resistance of typical new generation 7000 series aluminium alloys (AA7037, AA7056, AA7097, and AA7085). Double cantilever beam (DCB) tests were conducted to evaluate the SCC resistance. The morphology, size, and microchemistry of GBPs were analysed by transmission electron microscope equipped with super-X energy dispersive spectroscopy. The correlations between the GBPs microchemistry and SCC resistance have been discussed.

Experimental

Materials and methods

The raw materials used in this study were prepared by 99.9% Al, 99.9% Zn, 99.9% Mg, 99.9% Cu. The Zr and Ti were added to Al melt in terms of Al-Zr and Al-Ti master alloys, respectively. The ingots were prepared by conventional casting procedures in relatively argon atmosphere and solidified in water cooled iron-mould with Φ100 mm. The actual compositions of investigated alloys are shown in Table 1. The cast ingots were homogenised at 410°C/8 h + 465°C/6 h + 470°C/36 h, and then hot extruded at 430°C/1 h with the extrusion ratio of 10:1. All specimens were given a solution heat treatment of 470 °C/1 h + 475°C/1 h in air furnace, then quenched into room temperature water, followed by three stage artificial aging heat treatment at 120°C/24 h (air cooling) + 160°C/8 h (room temperature water-cooled) + 120°C/24 h (air cooling)[18,19].

Table 1

Composition of investigated alloys (all in wt-%).

Alloy	Zn	Mg	Cu	Zr	Ti	Al	Zn/Mg
AA7085	7.36	1.46	1.54	0.15	0.034	Bal	5.04
AA7097	7.81	2.08	1.41	0.14	0.032	Bal	3.75
AA7037	8.15	1.57	1.07	0.15	0.033	Bal	5.19
AA7056	8.81	1.80	1.88	0.14	0.037	Bal	4.89

The Vickers hardness of the specimens was tested by a Micromet 5104 microhardness tester at room temperature, and the hardness of each specimen was acquired by the averaging five measurements. The tensile test specimens were cut along the longitudinal (L) direction and conducted at a Instron 3369 testing machine with a tensile speed of 2 mm/min at room temperature. The gauge length, the width and thickness of tensile samples were 25, 6, 2 mm, respectively. The tensile data here represented the average of three tests.

Corrosion tests

The specimens were subjected to double cantilever beam (DCB) tests according to GB12445.1-90 [20] to evaluate the SCC resistance. All the specimens were ground and polished perpendicular to the longitudinal (L) direction. The pre-crack with 2∼3 mm was performed before they were immersed in 3.5 wt-% NaCl solution at 35 ± 1°C, and the crack length was recorded by an OLYMPUS DSX500 optical microscope (OM). The corresponding stage II crack velocity values (V_II) and crack tip stress intensity factor K_I were calculated by cracking length, cracking propagation time and the following formula, respectively. (1) where δ is the loading displacement, E is the elasticity modulus of studied alloy, h is the half-height of DCB sample and a is the crack length.

Microstructural investigations

The fractographs of DCB SCC-tip regions and tensile fracture surfaces of samples were observed by scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (Nano SEM 230). The size and distribution of GBPs were analysed by transmission electron microscope (TEM-2100F). The chemical compositions of GBPs were detected by Titan G2 60–300 transmission electron microscopy (TEM) equipped with energy disperse X-ray spectroscopy (EDXS) system. The microchemistry of GBPs was the arithmetic mean of at least ten measured, and the size of the GBPs and precipitation free zone (PFZ) width were averaged using ImageJ software based on ten TEM images from different positions for each sample. Thin foils for TEM were prepared by mechanically ground to 0.07 mm, punched into 3 mm in diameter,and then thinned by a twin-jet electropolishing in 25% HNO₃ + 75% CH₃OH solution at −25°C.

Results

Mechanical properties

Figure 1 shows the results of tensile strength and ductility of new generation alloys after aging heat treatment. The AA7056 alloy has the highest ultimate tensile strength (UTS) and yield strength (YS) of 649 and 640 MPa, respectively, but exhibited the lowest elongation (7.6%) compared with the other alloys. In addition, the best combination tensile properties was obtained in AA7085 alloy. The AA7037 displayed the lowest strength compared with the other alloys. However, no significant differences were found between the tensile properties of AA7037 and AA7097.

SCC study

The dependence of the crack propagation rate (V_II) on the stress intensity factor (K_I) are shown in Figure 2. Generally, the critical stress intensity factor (K_ISCC) and V_II value are used to evaluate the SCC susceptibility. It can be seen that the crack propagation rate of the DCB samples at a stable state in the V_II stage, which can be regarded as a crack propagation platform. Notably, the V_II value of AA7085 was lower than that of AA7037, AA7056 and AA7097. In addition, the K_ISCC values of AA7037, AA7056, AA7097 and AA7085 were 4.33, 9.00, 7.9, and 14.33 MPa·m^1/2, respectively. It was indicated K_ISCC for AA7037 were greatly lower than that of AA7056 and AA7085. The AA7085 presented the highest SCC resistance compared with the other alloys.

Moreover, the SCC crack propagation rates (V_II) of AA7037, AA7097, AA7056 and AA7085 were 1.42 × 10⁻⁷ m/s, 2.99 × 10⁻⁸ m/s, 1.71 × 10⁻⁸ m/s, and 1.25 × 10⁻⁸ m/s, respectively (as shown in Figure 3). It can be seen that V_II value of AA7037 was about one order of magnitude higher than that of AA7085. The results indicated that AA7037 has the highest SCC susceptibility among the investigated alloys.

Figure 4 shows SEM morphologies of the crack tip regions of investigated alloys. The mixed fracture modes of investigated alloys were consist of intergranular and trangranular fracture. Additionally, numerous cleavage facets and parts of microvoids were observed on the fracture surface. In the crack-tip regions of the alloys, there were obvious inter-granular brittle fracture characteristics (region A in Figure 4(a)), and corrosion pits from anodic dissolution of second-phase particles (region B in Figure 4(b)).

TEM microstructures study

Figure 5 shows the TEM bright field images of precipitates within grains and the corresponding selected area diffraction patterns (SADPs) (<110>_Al). It can be seen that a high number density of precipitates were observed in the four alloys (as shown in Figure 5(b,d,f,h)). There are two types of η and precipitates were formed in all the alloys. Additionally, it was also observed that the denser rod-shaped phase were homogenously distributed in the matrix of AA7097 alloy with a average size of 15 nm. For AA7056 (Figure 5(d)), the dispersed rodlike were homogenously distributed in the matrix concomitant with a higher number density and the average size of 14 nm. However, AA7085 alloy has the relatively lower density in precipitates compared with AA7097 and AA7056. A little more η precipitates were formed in AA7097 and AA7056 compared with AA7037 and AA7085 in terms of SADPs (Figure 5(d,f)).

The morphologies of GBPs of investigated alloys are shown in Figure 6. The coarse and discontinuously distributed GBPs along the GBs are observed in AA7085, AA7097 and AA7056. In the AA7037 and AA7097 (Figure 6(a,b)), the average size of GBPs are 13 and 20 nm, and the PFZ width are 47 and 45 nm, respectively. AA7056 and AA7085 exhibited more coarse and discontinuous GBPs, and narrow PFZ, the inter-particle spacing and the size of GBPs were 29 and 142, 24 and 150 nm, respectively, the PFZ width were 40 and 43 nm respectively.

The GBPs microchemistry of the alloys after three-stages aging heat treatment were examined by high-angle angular dark field scanning transmission electron microscopy (HADDF-STEM) and EDXS, as shown in Figures 7 and 8. It can be observed that the content of Cu atoms in GBPs of AA7037 after three-stages aging heat treatment were lower than that of the other alloys, while a similar Zn content are exhibited in both AA7037 and AA7097. Additionally, the Cu contents in GBPs of AA7097 and AA7085 were also similar, the Cu content in GBPs of AA7056 is much higher than that of AA7097 and AA7085, while Zn content in GBPs are increased by 16 at.-% compared with AA7085 (Figure 8(b)). The GBPs of AA7085, AA7056, and AA7097 with a Mg content was about 10 at.-%, which was about two times higher than that of AA7037. The specific contents of Al, Zn, Mg and Cu in GBPs by EDXS linear scanning are shown in Table 2.

Table 2

Stoichiometry of the GBPs of AA7037, AA7097, AA7056 and AA7085 (all in at.-%).

Alloy	Al	Zn	Mg	Cu
AA7037	73.2 ± 3.4	18.1 ± 2.9	6.9 ± 0.5	1.8 ± 0.2
AA7097	66.2 ± 3.9	19.8 ± 2.3	10.8 ± 1.8	4.2 ± 0.8
AA7056	54.7 ± 4.7	23.3 ± 5.1	13.7 ± 1.5	8.3 ± 0.6
AA7085	59.9 ± 2.3	25.1 ± 0.71	11.2 ± 0.5	3.8 ± 0.2

The stoichiometry of GBPs can be approximately expressed by the relative ratio of Zn, Mg and Cu as Mg(Cu_xZn_1-x)_y (assuming that there is no Al in GBPs), as shown in Table 3. The average fraction of Cu in GBPs, 6.7 ± 0.7 at.-% in Mg(Cu_0.05-0.13Zn_0.87-0.95)_2.4-4.3 of AA7037 was much lower than of AA7097 (12.0 ± 2.2 at.-% in Mg(Cu_0.10-0.26Zn_0.74-0.90)_2.0-2.9), AA7056 (18.3 ± 1.3 at.-% in Mg(Cu_0.18-0.37Zn_0.63-0.82)_1.9-2.8), and AA7085 (9.5 ± 0.4 at.-% in Mg(Cu_0.11-0.15Zn_0.85-0.89)_2.5-2.8), respectively. It can be seen from Table 3 that the Cu content in GBPs of the alloy was proportional to the actual bulk alloy composition. The results were similar to previous studies [21].

Table 3

Average fraction of Zn, Mg and Cu in GBPs, and their calculated stoichiometry, assuming no Al exists in GBPs (all in at.-%).

Alloy	Zn	Mg	Cu	Precipitate stoichiometry
AA7037	67.6 ± 10.8	25.7 ± 1.8	6.7 ± 0.7	Mg(Cu_0.05-0.13 Zn_0.87-0.95)_2.4-4.3
AA7097	56.8 ± 6.6	31.0 ± 5.1	12.0 ± 2.2	Mg(Cu_0.10-0.26 Zn_0.74-0.90)_2.0-2.9
AA7056	51.5 ± 11.2	30.2 ± 3.3	18.3 ± 1.3	Mg(Cu_0.18-0.37 Zn_0.63-0.82)_1.9-2.8
AA7085	62.6 ± 1.7	27.9 ± 1.2	9.5 ± 0.4	Mg(Cu_0.11-0.15 Zn_0.85-0.89)_2.5-2.8

Discussion

The tensile strength of 7000 series aluminium alloys is closely correlated with the size, type, and number density of matrix precipitates. There are no significant difference in the size and type of matrix precipitates among the four alloys. Generally, a high number density of matrix precipitates provided high strength, AA7056 alloy with the high Zn, Mg, and Cu content, resulting in a higher precipitation kinetics for hardening η’ phase precipitation. Thus, AA7056 alloy has the highest UTS and YS. Additionally, Mg element is the one of key solute atoms that forming the MgZn₂ hardening precipitates and Cu element can greatly promote precipitation of η’, the AA7037 has the lower tensile strength owing to low Mg and Cu content compared with AA7097.

Extensive research indicated that the SCC corrosion mechanisms of 7000 series aluminium alloys are anodic dissolution and hydrogen embrittlement cracking [17,22 -24]. The anodic dissolution mechanism involved the galvanic cell between matrix and the GBPs during the corrosion process. However, hydrogen generation was accompanied with anodic dissolution owing to galvanic reaction among GBPs, PFZ and adjacent alloy matrix [6,25]. Although there still exist controversy over the exact SCC mechanism in Al–Zn–Mg–Cu alloys, researchers believe that the SCC mechanism of 7000 series aluminium alloys is the combination of anodic dissolution and hydrogen embrittlement [19,26]. The SCC behaviour of 7000 series aluminium alloys are related to the alloy composition, heat treatment condition, and corrosion environment. Most SCC cracking paths are occurred along the grain boundaries. Thus, the size, distribution and microchemistry of GBPs are mainly responsible for the SCC resistance of 7000 series aluminium alloys [27,28].

Herein, we selected four typical new generation 7000 series aluminium alloys as investigated materials on behalf of the trend of alloy composition design in recent years. Researchers focused mainly on achieved high strength and attractive hardenability by increased Zn and Mg content and decreased Cu content, however, few of them pay attention to the high SCC susceptibility exist in these alloys owing to low Cu content in GBPs. Thus, the AA7097 (registered in 2016) and AA7037 (registered in 2006) alloy with high Zn content combined with low Cu content were selected to compare SCC behaviour with AA7085 (relatively low Zn and high Cu, registered in 2002) and AA7056 (relatively high Zn, Mg, and Cu, registered in 2004). Based on the microstructure analysis and SCC performance results, low SCC resistance of AA7037 could be explained by considering that low Cu content in GBPs with relatively continous distribution. Low Cu content in GBPs resulting in high potential difference between GBPs and surrounding matrix, promoting the anodic dissolution rate and cracking propagation [29]. AA7097 alloy has higher Cu content in GBPs, larger size and wide inter-space of GBPs compared with AA7037 alloy, which are beneficial for delaying the crack growth rate. Previous research has reported that high Cu enrichment was beneficial for re-passivation [30] whereas high Zn enrichment in GBPs promoted the absorption of H in Al alloys and give rise to hydrogen embrittlement [31]. For AA7056 alloy, Cu enrichment in GBPs can increase the potential of GBPs and decrease its electrochemical activity, while higher Zn enrichment in GBPs. The beneficial effects of Cu on SCC resistance need to partly compensate the disadvantage effects form Zn enrichment in GBPs. Thus, the much higher V_II and lower K_ISCC value for AA7056 compared with AA7085 alloy. In addition, large GBPs (>25 nm) can act as irreversible hydrogen traps and release the atomic hydrogen concentration in crack-tip regions [32]. Holroyd et al. [25] reported that hydrogen embrittlement plays an important role on SCC resistance of alloys. Analysis of the fracture morphology of the DCB sample has confirmed that the failure path was occurred along the grain boundary. The large size η precipitates on the GBs can serve as trapping sites for hydrogen atoms and inhibit hydrogen embrittlement. Herein, the size of GBPs in the AA7085are much larger than that of AA7056, which are beneficial for capturing the hydrogen atom and suppressing hydrogen embrittlement [32]. Thus, AA7085 alloy has a better SCC resistance than AA7056 alloy.

Conclusion

Alloy with low Zn and Mg content combined with high Cu content was beneficial for reducing SCC susceptibility. The SCC plateau velocities V_II for the AA7085 alloy was about an order of magnitude lower than that of AA7037 alloy mainly because of large size, wide spacing, and high Cu content of GBPs.

AA7056 has the highest tensile strength compared with the other alloys owing to high density of nano-scale matrix precipitates.

The Cu content of GBPs increased with the increase in alloy Cu content.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the author(s).

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