Precipitation behavior and properties of extruded 7136 aluminum alloy under different aging treatments

Huiyan LI, Lina JIA, Jiangnan HUANG, Yue MA

School of Materials Science and Engineering, Beihang University, Beijing 100083, China

KEYWORDS 7136 aluminum alloy;Electrical conductivity;Mechanical properties;Microstructure;Retrogression and re-aging

Abstract In this work, the mechanical properties and electrical conductivity of the extruded 7136 aluminum alloy treated by single-stage aging treatment (T6), retrogression and re-aging treatment(RRA),and multiple retrogression and re-aging treatment have been investigated by means of hardness measurements, electrical conductivity tests and tensile tests. The results have shown that the properties of the 7136 alloy such as hardness, tensile strength and electrical conductivity were sensitive to retrogression time (within 90 min). With prolonging the retrogression time, the tensile strength was enhanced first and then decreased, yet the electrical conductivity was continuously increased. The 60 min-treated alloy performed the highest tensile strength (716 MPa), whereas the 90 min-treated alloy possessed the highest electrical conductivity (33.95%IACS). Compared with the T6-treated alloy, the tensile strength and electrical conductivity were improved by 3.3%and 18.9%, respectively. The electrical conductivity showed an obvious increase with repetitious times of the RRA treatment. After 3RRA60 treatment, a good combination of tensile strength(705 MPa)and electrical conductivity(33.20%IACS)can be obtained.Compared with the T6 condition, the tensile strength and electrical conductivity were improved by 1.7% and 16.3%, respectively. The mechanism of microstructure evolution under different aging treatments has been discussed in detail.

1. Introduction

Al-Zn-Mg-Cu alloys are extensively applied in the aerospace industry, automotive industry as well as various critical military vehicles due to the high strength-to-weight ratio, fracture toughness,electrical conductivity and excellent stress corrosion cracking (SCC) resistance.1–5In recent years, several highalloying Al-Zn-Mg-Cu alloys have been developed and applied in the diverse industries with excellent comprehensive performances such as high strength, superior corrosion resistance and good fracture toughness.6,7Among them, the 7136 aluminum alloy with high Zn content(>8.4wt%)aims to replace conventional alloys of aerospace application.The precipitation hardening is the main strengthening mechanism of Al-Zn-Mg-Cu alloys.8–10In order to obtain a favorable combination of mechanical strength and corrosion resistance, a lot of heat treatment processes have been developed to tailor the evolved microstructures of Al-Zn-Mg-Cu alloys.11

The retrogression and re-aging treatment (RRA) has been developed to achieve a better combination of strength and corrosion resistance. After RRA treatment, the strength of alloy was equal to that of single-stage aging treatment (T6) and the stress corrosion resistance was close to that of two-stage over aging treatment (T7x).12–18The process parameters during the RRA treatment have a great influence on the precipitates.19–21Wen et al.22demonstrated that the different retrogression time and temperature could lead to the change of conductivity value and tensile strength. Peng et al.23reported that repetitious-RRA treatment could be used to improve the conductivity value without sacrificing strength.However, the alloy often fails to meet the actual service requirements of the material even if the electrical conductivity can be improved after RRA treatment.24Thus,how to further improve the electrical conductivity while maintaining the high strength is an urgent problem waiting to be solved.

The current literature about RRA treatment and multiple RRA treatment of 7136 alloy is still very limited, whereas AA 7075 and AA 7055 have been studied extensively.25–27In this paper,the effect of different RRA treatments on hardness,tensile strength and electrical conductivity of 7136 alloy has been investigated by comparison with the T6-treated alloy.Besides, the microstructure and precipitation behavior of 7136 alloy under multiple RRA treatment were discussed in detail.

2. Experimental details

The present study was carried out on the extruded 7136 aluminum alloy plate with nominal composition of Al-8.90Zn-2.50Mg-2.20Cu-0.15Zr (in wt%). The thickness of the plate is 15 mm. Specimens were solution treated at 450°C for 1 h and subsequently heated to 470°C with the heating rate of 40°C/h and solution treated for 1 h, then water quenched.The T6 treatment was performed at 120°C for 12 h. The precise regimes of RRA treatment were listed in Table 1. After aging treatments, all samples were treated by cold water quenching. The results showed that the RRA60 treated specimen possessed the highest strength. Therefore, the RRA60 treatment was chosen as the parameter of multiple RRA treat-ment.Fig.1 displayed the schematic diagram of multiple RRA treatment.

Table 1 Aging treatment specifications.

Fig.1 Scheme diagram of different aging treatments denoted by RRA60, 2RRA60, 3RRA60 and 4RRA60.

Vickers hardness testing was performed with a FM-800 Micro Vickers hardness tester and the electrical conductivity was measured by SIGMASCOPE®SMP10 metal conductivity meter. The tensile testing was carried out on an Instron-8801 universal testing machine at room temperature with a strain rate of 1.0 mm·min-1. To ensure the reliability of the data,at least three measurements were conducted for each sample to obtain the average values of ultimate tensile strength(UTS), yield strength (YS) and elongation. The fracture surfaces were observed by JSM-6010 scanning electron microscope (SEM) after tensile testing. A JEM-2100 transmission electron microscope (TEM) was applied to observe the structural characterization of matrix precipitates (Mpt), grain boundary precipitates (GBP) and precipitates free zone(PFZ). Specimens for TEM analyses were prepared with typical methods: slices were ground to 60–80 μm thickness,punched into 3 mm diameter foils and then prepared by twin-jet electropolishing in a solution of 25vol% nitric acid and 75vol% methanol solution cooled to -20°C.

3. Results and discussion

3.1. Effect of different aging treatments on microstructure

Fig. 2 displays the bright field TEM micrographs and the corresponding selected area diffraction patterns (SADPs) in<100>Aland<011>Alzone axes of 7136 alloy under different aging treatments.As shown in Fig.2(a),the microstructure of T6-treated alloy was characterized by fine η′precipitates within the grain and continuous η precipitates at grain boundary.The semi-coherent η′precipitates with the size of 3 nm to 8 nm are considered as the major precipitates of the T6-treated alloy28.As shown in Fig.2(b),when the retrogression time was 30 min, the GBP was discontinuous with the size of 17 nm to 25 nm, whereas the η′precipitates with the size of 4 nm to 12 nm homogeneously distributed in the matrix,similar to that of T6 condition. Meanwhile, some smaller precipitates with a nanoscale diameter distributed in the interspaces of η′phase.However, the PFZ near the grain boundary was unobvious.The change can be related to the re-precipitation of GP zones and η′precipitates along grain boundaries during the re-ageing stage.29The precipitates of RRA90 treated alloy are a little coarser as compared with the RRA30 sample and the coarsening of precipitates at grain boundary leads to the wide PFZ.30The size of precipitates inside the grain was ranged from 5 nm to 20 nm and the size of GPB was ranged from 26 nm to 53 nm.

Fig. 2 TEM micrographs and corresponding selected area diffraction patterns of 7136 alloy under different aging treatments. Selected area diffraction patterns were obtained in <100>Al and <011>Al zone axes of 7136 alloy.

The differences between the RRA30 and RRA90 treated samples in size, morphology and distribution of the precipitates indicate that the retrogression time significantly affect the precipitation behavior. The precipitates of the RRA30 treated sample are smaller than that of the RRA90 treated sample, implying that the increase of retrogression time could result in the coarsening tendency. Therefore, the retrogression time is an important factor during aging treatment.

The TEM micrographs of the multiple RRA treated samples are shown in Fig. 3. It can be observed from Fig. 3(a)–(d)that the mixture of fine precipitates and coarse precipitates was formed during the multiple RRA treatment and the number density of precipitates decreased with the increase of repetitious times. In addition, the PFZ near the grain boundary was unobvious when the repetitious times were within 2 times.When the repetitious times increased to 4 times, the GPB coarsened sharply and separated from each other obviously,and the size of GPB can be up to 20 nm. The results above showed that the multiple RRA treatment broadened the size distribution of precipitates (Table 2).

Fig. 3 TEM images of multiple RRA treated specimens.

Fig.4 shows the selected area diffraction patterns(SADPs)in <110>Aland <112>Alzone axes of the multiple RRA treated samples. As shown in Fig. 4(a)–(d), the main strong diffraction spots from the Al matrix have been processed.The weak patterns of spherical Al3Zr dispersoids were observed in <110>Aland <112>Alprojections. The spots at 1/3 and 2/3{220} positions in <110>Alprojections came from semi-coherent η′precipitates.12Meanwhile, in<112>Alprojections, the weak diffraction spots corresponding to η′precipitates can be observed at 1/3{220} and 2/3{220} positions along {111} directions.31Furthermore, some weak and diffuse diffraction spots can be observed near 1/2{311} positions in <112>Alprojections, which meant the presence of GP II zones.Previous research results showed that the precipitate sequence of the Al-Zn-Mg-Cu alloys during aging treatment was SSS,GP zone,η′phase,η phase.5,22Generally, GP zones can form as precursors to the metastable η′phase and are completely coherent with the matrix.32According to the different geometry and atomic arrangement,the GP zones can be classified into GP I and GP II zones.33During the decomposition of the supersaturated solid solution, the spherical GP I zones and plate-like GP II zones formed independently. The formation of GP II zones is associated with the interaction between solute atoms and quenched-in vacancies from the solution treatment.It has been reported that the high Zn:Mg ratio promoted the formation of GP II zones(Zn-rich clusters).34The GP II zones are more stable over a wider range of temperatures compared with GP I zones.33In Fig.3(a), the ellipse/rod-shaped precipitates homogeneously distributed in the Al matrix. According to the SADPs, the high-density pre-cipitates were GP II zones and η′precipitates (Fig. 4(a)). The streaks of GP II zone and η′phases became very weak as repetitious times increased to 4 times (Fig. 4(d)), indicating that most of the GP zone and η′phase has transformed to η′phase and η phase. The SADPs showed that GP II zone, η′phase and η phase are the main strengthening precipitates of the multiple RRA treated samples (within 3 times).

Table 2 The size of the precipitates after multiple RRA treatment.

3.2. Effect of different aging treatments on hardness and conductivity

The electrical conductivity is usually used as an important index for evaluating stress-corrosion cracking (SCC) resistance. Generally, higher conductivity represents better SCC resistance while high hardness implies high tensile strength.35Fig. 5 shows the influence of different aging treatments on the hardness and electrical conductivity of 7136 alloy. The hardness value and electrical conductivity of T6-treated specimen were 216.2 HV and 28.55%IACS,respectively. As shown in Fig. 5, retrogressed at 180°C, the hardness was increased within the initial 60 min and then decreased.The hardness values of samples retrogressed at 180°C for 30 min, 60 min and 90 min were 214.4 HV, 221.5 HV and 210.6 HV, respectively.Furthermore, the electrical conductivity was increased continuously with prolonging the retrogression time. The electrical conductivity values of the alloy retrogressed at 180°C for 30 min, 60 min and 90 min were 29.40%IACS, 31.65%IACS and 33.95%IACS, respectively. When the retrogression time was 90 min, the conductivity of the alloy was 18.9% higher than that of T6-treated alloy.However,after the long-time retrogression treatment, the coarse strengthening precipitates reduced the hardness of the alloy. Combining with the hardness and electrical conductivity, it can be found that the strengthening effect is significant when the retrogression time is 60 min.Compared with the T6-treated sample,the electrical conductivity and hardness of the RRA60 treated sample were improved by 10.9% and 2.5%, respectively. Therefore, multiple RRA treatment was carried out under this system(RRA60) to further improve the electrical conductivity and maintain the high strength.

Fig. 5 Conductivity and hardness of different aging treated specimens.

As shown in Fig. 6, the electrical conductivity of different aging treated samples followed the order: RRA60<2RRA60<3RRA60<4RRA60. Increasing the repetitious times of RRA60 treatment(within 3 times),the tensile strength of 7136 alloy decreased slightly, whereas the conductivity increased obviously.The hardness value and electrical conductivity of the 3RRA60 treated specimen were 217.4 HV and 33.20 %IACS, respectively. Compared with the T6-treated sample, the electrical conductivity and hardness of the 3RRA60 sample were improved by 16.3% and 0.6%, respectively. However, the hardness exhibited an obvious decrease as repetitious times reached to 4 times. It can be seen that the 4RRA60 treated alloy possessed the highest electrical conductivity (34.85%IACS) and the lowest hardness (205.8 HV).The results suggested that comparable strength as well as higher electrical conductivity (SCC resistance), as compared with that of T6 condition, can be obtained through 3RRA60 treatment.

Fig. 4 Selected area diffraction patterns (SADPs) in <112>Al and <110>Al zone axes of the 7136 alloy under multiple RRA treatment.

Fig. 6 Conductivity and hardness of 7136 alloy under multiple RRA treatment.

3.3. Effect of different aging treatments on tensile properties

As shown in Fig.7,the UTS,YS and elongation of T6-treated specimen were 693 MPa, 624 MPa and 12.6%, respectively.With prolonging the retrogression time (within 90 min), the tensile strength was increased gradually and then decreased obviously. The trend of the YS curve was in accordance with that of UTS curve. The 60 min-treated alloy (RRA60) had the highest tensile strength (716 MPa) and yield strength(683 MPa). Compared with the T6-treated sample, the tensile strength and yield strength were improved by 3.3% and 9.5%, respectively. In addition, the elongation of alloy has an unobvious variation under different aging treatments.

Fig. 7 Tensile properties of 7136 alloy under different aging treatments.

Fig. 8 Tensile properties of 7136 alloy under multiple RRA treatment.

The tensile properties of the samples under multiple RRA treatment are shown in Fig.8.The results showed that the tensile strength of multiple RRA treated samples followed the order: 4RRA60<3RRA60<2RRA60≈RRA60. The UTS,YS and elongation of 3RRA60 treated specimen were 705 MPa, 675 MPa and 14.0%, respectively. Compared with the T6-treated sample, the tensile strength and yield strength were increased by 1.7% and 8.2%, respectively. However,when the repetitious time of RRA treatment increased to 4 times, the UTS and YS decreased to 671 MPa and 645 MPa,respectively. What is more, it can be found from Fig. 8 that multiple RRA treatment has little effect on the elongation of 7136 alloy.

Based on the tensile tests,fracture morphology was used to figure out the fracture mechanism.Fig.9 showed that the fracture morphology of 7136 alloy was significantly different after different aging treatments.As shown in Fig.9(a),a lot of large flats presented on the fracture surface and the strip-shaped dimples distributed among the large flats, indicating the main fracture mechanism was intergranular. Increasing repetitious times of RRA60 treatment, the volume fraction of large flats was reduced and a large number of dimples were presented on the fracture surface. These results above indicated that the fracture mode is transformed from intergranular to transgranular.After 3RRA60 treatment,the fracture mode was the typical transgranular-intergranular fracture.

3.4. Discussion

It was reported that the mechanical properties of Al-Zn-Mg-Cu alloys were associated with the type, size and volume fraction of the precipitates.11After the appropriate T6 treatment,most of the GP zones have been transformed to η′phase inside the grain and the high density fine η′precipitates led to the high hardness and good tensile strength. But the continuous network of η precipitates in grain boundary regions may significantly increase the SCC susceptibility.20In order to obtain a favorable combination of mechanical strength and corrosion resistance, different RRA treatments have been explored to promote the comprehensive performances of 7136 alloy.

It is known that Al-Zn-Mg-Cu alloys with RRA treatment have ultrahigh strength and good corrosion resistance. The most critical process of the RRA treatment is the retrogression process. During the retrogression process, GP zones and fine semi-coherent η′precipitates dissolve in the Al matrix, and grain boundary precipitates (η precipitates) become coarser and more spaced. The short-time retrogression treatment led to the insufficient dissolution of GP zones and fine η′precipitates and reduced the re-precipitation kinetics of η′precipitates during the re-aging stage. However, the long-time retrogression treatment may lead to the coarsening and evolution of precipitates and reduced the number density of η′precipitates.The research results showed that the long-time retrogression treatment (90 min) obviously improved the electrical conductivity, but the excessively coarsened precipitates within the grain reduced the tensile strength. The multiple RRA treatment was explored to further improve the electrical conductivity (corrosion resistance) while maintaining high strength of the T6 condition.

Fig. 9 Fracture surfaces of tensile specimens.

When the retrogression time was 60 min,the GP zones and fine η′precipitates were dissolved sufficiently in the Al matrix.During the re-aging stage, the ever-present η′precipitates became coarser with the re-nucleation of GP zones and η′precipitates.The high density of η′precipitates and GP zones led to the high tensile strength (716 MPa). During the 3RRA60 treatment, the long-time regression process was interrupted by several low-temperature aging processes and the small size GP zones and fine η′precipitates within the grain can be protected by resolving and re-precipitating. In the final re-aging process, the fine GP zones and η′precipitates can be gained to maintain the high strength of the material. On the other hand, the relatively large GP zones and η′precipitates which cannot be dissolved during the retrogression process will continue to grow up. The large GP II zones served as preferential sites for the η′precipitates,promoting the formation of η′precipitates.36Therefore,the loss of strength caused by the coarsening of undissolved η′precipitates has been partly compensated by the transformation of GP II zone-to-η′phase in the re-aging stage. The η′precipitates with wide size distribution acted as effective obstacles to gliding the mobile dislocations.

However, when the repetitious times of RRA60 treatment increased to 4 times, the excessively coarsened Mpt and GBP led to the consumption of solute atoms in the Al matrix and new GP zones and η′phase cannot be re-precipitated at the final re-aging process. The fracture morphology showed that the coarsened Mpt contributed to the high density of dimples on the fracture surface(Fig.9(d)).The transformation of Mpt(from coherent phase to semi-coherent phase and from semicoherent phase to non-coherent phase) and the widening of PFZ led to the significant decrease of tensile strength.The multiple RRA treatment provided a new way to obtain an appropriate combination of tensile strength and electrical conductivity (corrosion resistance) of 7136 alloy.

4. Conclusions

The effect of different aging treatments on the precipitation behavior and mechanical properties of 7136 alloy has been investigated.The main conclusions were summarized as below:

(1) With prolonging the retrogression time (within 90 min),the tensile strength of 7136 alloy was increased first and then decreased, yet the electrical conductivity was increased continuously. The 60 min-treated alloy(RRA60) performed the highest tensile strength(716 MPa), whereas the 90 min-treated alloy (RRA90)possessed the highest electrical conductivity (33.95%IACS). Compared with the T6-treated alloy, the tensile strength and electrical conductivity were improved by 3.3% and 18.9%, respectively.

(2) Increasing the repetitious times of RRA60 treatment,the tensile strength of 7136 alloy decreased slightly and the electrical conductivity increased obviously. A good combination of tensile strength(705 MPa)and electrical conductivity (33.20%IACS) can been obtained after the 3RRA60 treatment. Compared with the T6-treated alloy, the tensile strength and electrical conductivity of 3RRA60 treated alloy were improved by 1.7% and 16.3%, respectively.

(3) The microstructure of 7136 alloy with favorable comprehensive performances consisted of η′precipitates with wide size distribution inside the grains as well as the coarser and sparser η precipitates at the grain boundary.

Acknowledgement

This work was financially supported by National Key Research and Development Program of China (No.2016YFB0300901).