J. Cent. South Univ. Technol. (2011) 18: 279-284
DOI: 10.1007/s11771-011-0691-9
Microstructure and mechanical properties of Al-Zn-Cu-Mg-Sc-Zr alloy after retrogression and re-aging treatments
LI Wen-bin(李文斌)1,2, PAN Qing-lin(潘清林)1, XIAO Yan-ping(肖艳苹)1,
HE Yun-bin(何运斌)1, LIU Xiao-yan(刘晓艳)1
1. School of Materials Science and Engineering, Central South University, Changsha 410083, China;
2. School of Civil Engineering, Hunan City University, Yiyang 413000, China
? Central South University Press and Springer-Verlag Berlin Heidelberg 2011
Abstract: The mechanical properties and stress corrosion cracking (SCC) resistance of an Al-Zn-Cu-Mg-Sc-Zr alloy under different aging conditions were investigated. The dependence of microstructure and mechanical properties on aging parameters was evaluated by tensile test, hardness test and conductivity measurement. The results show that for the alloys with retrogression and re-aging treatment (RRA), the conductivity increases with the retrogression time and temperature, while the tensile strength decreases. The transmission electron microscopy (TEM) results show that the precipitates η(MgZn2) at grain boundary aggregate apparently with retrogression time and the precipitates inside the matrix exhibit the similar distribution to T6 temper, which comprises fine GP zones, large η′(MgZn2) and η(MgZn2) phases. According to the mechanical properties and microstructure observations, the optimal RRA regime is recommended to be 120 °C, 24 h + 180 °C, 30 min + 120 °C, 24 h. The strength level of the alloy after the optimum RRA treatment is similar to that in T6 condition and the SCC resistance is improved obviously in contrast to T6 condition.
Key words: Al-Zn-Cu-Mg-Sc-Zr alloy; retrogression and re-aging treatment; microstructure; mechanical properties; stress corrosion cracking
1 Introduction
Ultra-high strength aluminum alloys based on Al-Zn-Cu-Mg are typical structural materials with applications in the aerospace and other fields. These alloys usually show high strength, high ductility and high susceptibility to stress corrosion cracking (SCC). The minor addition of Mn, Cr, Sc, Zr and Ag as modifiers has been widely used to increase the mechanical properties [1-2]. As an effective modifier, Sc has the most significant effect in improving the mechanical properties of the Al-Zn-Cu-Mg alloys [3-4]. In these Sc-containing alloys, the main strengthening phase is Al3Sc, which has a L12 structure and is coherent to the matrix. Sc can also form Al3(Sc,Zr) dispersoids together with Zr, which reduces the requirement of Sc in the Al-Zn-Cu-Mg-Sc alloys [5-7].
The Al-Zn-Cu-Mg-Sc-Zr alloys can easily achieve peak strength through T6 treatment. However, these alloys usually exhibit poor SCC resistance. Although the difficulty in SCC resistance of the Al-Zn-Cu-Mg alloys has been overcome through over-aging treatment, the corresponding strength is reduced by about 10%-15% compared with that of the T6 temper [8]. Therefore, it is necessary to explore a new aging treatment to decrease the susceptibility of SCC and to maintain the high strength. A new technology known as retrogression and re-aging (RRA) was proposed to reduce the SCC susceptibility and at the same time to obtain the strength level of T6 temper in the AA7075 aluminum alloys [9-10]. It is found that the SCC susceptibility of Al-Zn-Cu-Mg-Sc-Zr alloy largely depends upon its microstructure. It has been observed that the main differences between the T6 temper and the over-aging condition are related to the precipitates distribution [11]. The alloy after T6 temper presents high densities of GP zones and h′(MgZn2) precipitates in the matrix, and the over-aged alloy shows an increase in the amount of equilibrium precipitates h(MgZn2) at grain boundaries [12]. The RRA treatment consists of the first aging and the re-aging, with a retrogression step between them. The first aging corresponds to the T6 temper. The retrogression consists of a partial annealing at temperatures usually between 180 and 240 °C for 5- 2 400 s, followed by a water quenching. Finally, the alloy is re-aged at conditions similar to T6 temper. It is reported that the main micro- structural changes during retrogression are the partial dissolution of GP zones and fine h′(MgZn2) precipitates in the matrix which re-precipitate during the re-aging process, and the formation and growth of h(MgZn2) precipitates along the grain boundaries [12]. Such microstructural evolutions are beneficial to both SCC resistance and mechanical strength.
The purpose of the present study is to examine in detail the relations between the longitudinal mechanical properties and parameters of RRA treatments. Obtaining the optimal RRA regime is another objective of this work. The transverse mechanical properties and SCC resistance of the Al-Zn-Cu-Mg-Sc-Zr alloy under T6 temper and the optimal RRA condition are investigated for the comparison, which is the main difference in contrast to other studies. The previous work [13-14] done on the same material is also compared with the results of the current work.
2 Experimental
The material used in this work is Al-8.1%Zn- 2.3%Cu-2.05%Mg-0.21%Sc-0.12%Zr (mass fraction). The alloy was prepared with pure Al, Zn, Mg and Al-Cu, Al-Sc, Al-Zr master alloys by ingot metallurgy in a crucible and then poured into a copper book-shaped mould (260 mm × 150 mm × 30 mm). The ingots were homogenized at 460 °C for 24 h and then hot-rolled and cold-rolled to 2.3 mm-thick plates. The specimens were first solution treated at 465 °C for 40 min and then at 490 °C for 30 min followed by water quenching. The T6 temper was carried out at 120 °C for 24 h. The over-aged sample was treated at 160 °C for 24 h. The double aging was carried out at 120 °C for 6 h and then 150 °C for 16 h. The first aging of RRA was conducted at 120 °C for 24 h, then followed by a retrogression at temperatures ranging from 160 to 200 °C for time ranging from 15 to 60 min, and the re-aging was also performed at 120 °C for 24 h. All of these aging treatments were carried out in a box-type furnace.
The stress corrosion cracking (SCC) measurement was carried out through the slow strain rate test (SSRT) according to GB/T 15970.7—2000. The specimens of SSC were cut and prepared from the transverse direction and immersed into a solution of 10% NaOH (volume fraction) for 15 min and then washed with 30% HNO3 (volume fraction). The SSC tests were accomplished in air and solution of 3.5% NaCl (volume fraction) with a strain rate of 1.33×10-6 s-1.
All the tensile specimens were prepared according to GB/T228—2002. The mechanical properties were measured on a CSS-44100 tensile machine with a crosshead speed of 2 mm/min, with the tensile axis parallel to the rolling direction. The hardness was tested on a 401MVD MICRO-VICKERS. The conductivity was measured on a Model7501 vortex flow conductor. Thin foils for transmission electron microscopy (TEM) analysis were prepared by twin-jet polishing with an electrolyte solution consisting of 25% HNO3 and 75% methanol (volume fraction) below -20 °C. The foils were examined on Tecnai G2 20 at 200 kV.
3 Results and discussion
3.1 Mechanical properties
Fig.1 shows the results of longitudinal mechanical properties through RRA treatments at different retrogression temperatures for 30 min. A proper retrogression temperature can be concluded from the curves. Fig.1(a) shows the variations of hardness and conductivity with retrogression temperature. It is evident that the conductivity increases with increasing the temperature. However, the hardness has a sharp decrease at 180 °C, which is the cross point of the two curves. Fig.1(b) shows the variations of mechanical properties with retrogression temperature. It can be seen that the strength decreases significantly with elevating the temperature of retrogression from 180 °C. In order to make a compromise for achieving high strength and high conductivity, the desirable retrogression temperature for RRA treatment is recommended to be 180 °C.
Fig.1 Variations of mechanical and physical properties with retrogression temperature in RRA treatment
Fig.2 reveals the relations between longitudinal mechanical properties and retrogression time of the alloy in RRA treatment at 180 °C. There is also a cross point between the two curves of hardness and conductivity in Fig.2(a). The corresponding time to this point is 30 min. Fig.2(b) shows the variations of mechanical properties with retrogression time in RRA treatment. Compared with other retrogression time, the samples with a retrogression time of 30 min under RRA condition show the best combination of high strength and high conductivity. From the overall consideration of Fig.1 and Fig.2, the desirable parameters for RRA treatment can be concluded. Such parameters are composed of two aging stages at 120 °C for 24 h and a retrogression at 180 °C for 30 min followed by water quenching.
Values of longitudinal mechanical properties and conductivity of the studied alloy under different aging regimes are listed in Table 1. It can be seen that the alloy in T6 condition has the highest strength and the lowest conductivity. The samples under over-aging and double aging conditions exhibit higher conductivity but lower strength. The sample after the optimal RRA treatment exhibits both high strength and high conductivity, which shows great benefits to structural parts under complicated environment.
The transverse mechanical properties under stress corrosion cracking (SCC) of the alloy at T6 and the optimal RRA aging regimes are listed in Table 2. The results show that the alloy under RRA condition has higher strength and elongation than the samples after RRA treatment. It can be seen that after both T6 and RRA treatment the differences between ultimate strength and yield strength become much smaller compared with those of longitudinal results listed in Table 1. The transverse mechanical properties under stress corrosion cracking (SCC) of the alloy in Table 2 exhibit important values of allowable design stress in corrosive environment.
3.2 Microstructure
Fig.3 shows transmission electron micrographs (TEM) of the alloys under T6 temper, double aging and over-aging conditions. From Figs.3(a) and (b), we can see fine dispersed precipitates, η′(MgZn2) and Al3(Sc,Zr), distributing inside the grains, which are accompanied with continuous precipitates along the grain boundary. The image of Al3(Sc,Zr) particles with its diffraction pattern is shown in Fig.3(a). The precipitates in Fig.3(b) grow up continuously at grain boundary, which are identified as the equilibrium η(MgZn2) [15]. The precipitates inside the grains enhance the resistance of dislocation movement, therefore increase the strength of the alloy. The continuous precipitates at grain boundaries act as the anodic tunnels of intergranular corrosion and accelerate the stress corrosion cracking (SCC) of the alloy [16].
Fig.2 Variations of mechanical and physical properties with retrogression time in RRA treatment
Table 1 Longitudinal mechanical properties and conductivity of aged alloys
Table 2 Transverse mechanical properties under stress corrosion cracking (SCC) of alloy
Fig.3 TEM images under typical aging conditions: (a) and (b)120 °C, 24h (Insert: diffraction pattern of Al3(Sc,Zr)); (c) 120 °C, 6 h + 150 °C, 16 h; (d) 160 °C, 24 h
Figs.3(c) and (d) show the TEM images of the alloy at double aging and over-aging states, respectively. The obvious features in these two images are uncontinuous coarsened precipitates η(MgZn2) along the grain boundaries and large dispersed precipitates, η′(MgZn2), η(MgZn2) and S(Al2CuMg), distributing inside the grains. Compared with fine dispersed precipitates, the strengthening of large precipitates inside the grains are less effective, which corresponds to the sharp decrease in strength. Precipitates are coarsened aggregatively along the grain boundaries, which decreases the scattering of electron and results in an increment of the conductivity. The coarsened discontinuous precipitates at grain boundaries cannot play a role of anodic tunnels of intergranular corrosion and improve the stress corrosion cracking (SCC) resistance of the alloy[17].
Typical TEM images of specimens after different RRA treatments are shown in Fig.4. The precipitates η(MgZn2) at grain boundaries aggregate obviously when the retrogression time is increased. The precipitates inside the matrix exhibit similar distribution under T6 temper which comprises fine GP zones, large η′(MgZn2) and η(MgZn2) phases. These hardenable phases with similar sizes and types inside the matrix make the specimens under RRA condition have high strength similar to T6 temper. The aggregated discontinuous η(MgZn2) phases on grain boundary are incoherent with the matrix, which results in a reduction of lattice distortion and an increment of the conductivity [18]. It can be seen in Fig.2(b) that the strength decreases with the increment of retrogression time from 30 min, and the conductivity increases gradually.
When the retrogression temperature remains at 180 °C, the precipitates at grain boundaries agglomerate and coarsen with the extension of retrogression time (Figs.4(c) and (d)). As a result, the better relaxation of internal stress inside the specimens can be obtained, which enhances the conductivity. The precipitates in the matrix are uniformly distributed and accompanied by a lot of Al3(Sc,Zr) particles, which are the common features of microstructure under T6 temper and RRA treatment [12].
Fig.4 TEM images under different RRA conditions: (a) and (b) Retrogression at 180 °C for 30 min; (c) Retrogression at 180 °C for 15 min; (d) Retrogression at 180 °C for 60 min
The best combination of strength and conductivity is achieved through a RRA treatment of two T6 aging treatments and a retrogression of 180 °C, 30 min. The precipitates inside the grain interior of Figs.4(a) and (b) are more and smaller than those under T6 temper of Fig.3(a), whereas the precipitates at grain boundaries become more discontinuous and larger compared with those of T6 temper. The yield strength and ultimate strength decline by only 3 MPa and 7 MPa, respectively, which is an useful compromise compared with the significant increase of 5.7% of conductivity on the basis of 24.5% (vs IACS) under T6 condition. Excessive retrogression would make the dissolved hardenable phases re-precipitate to form large η′ and η particles after re-aging, which decreases the strength greatly.
4 Conclusions
1) The RRA treatment can obviously improve the stress corrosion cracking (SCC) resistance of the Al-Zn-Cu-Mg-Sc-Zr alloy and remains the high strength under T6 condition. The agglomerated precipitates along the grain boundaries and the fine dispersed precipitates, such as GP zones, η′(MgZn2) and Al3(Sc,Zr), distributing inside the grains, are the microstructural characteristics of the alloy.
2) The retrogression temperature and time have significant influence on strength and conductivity of the Al-Zn-Cu-Mg-Sc-Zr alloy in RRA treatment. The conductivity is enhanced monotonically with the increment of retrogression temperature and time.
3) The best combination of strength and conductivity is obtained through a RRA treatment with a retrogression at 180 °C for 30 min. The yield strength and ultimate strength are 648 MPa and 687 MPa, respectively, and the elongation is 7.4%, which are similar to the results of T6 temper. The conductivity is 30.2% (vs IACS), which is close to that of the over-aging condition.
4) The transverse mechanical properties under stress corrosion cracking (SCC) of the alloy after the optimal RRA aging regimes are improved. The transverse yield strength would be an important value for designing the structural parts.
References
[1] KIM K T, KIM J M, SUNG K D, JUN J H, JUNG W J. Effect of alloying elements on the strength and casting characteristics of high strength Al-Zn-Mg-Cu alloys [J]. Materials Science Forum, 2005, 475/476/477/478/479(3): 2539-2542.
[2] YOU Jiang-hai, LIU Sheng-dan, ZHANG Xin-ming, ZHANG Xiao-yan. Influence of quench transfer time on microstructure and mechanical properties of 7055 aluminum alloy [J]. Journal of Central South University of Technology, 2008, 15(2): 153-158.
[3] NING Ai-lin, LIU Zhi-yi, PENG Bai-shan, ZENG Su-min. Redistribution and re-precipitation of solute atom during retrogression and reaging of Al-Zn-Mg-Cu alloys [J]. Transactions of Nonferrous Metals Society of China, 2007, 17(5): 1005-1011.
[4] SENKOV O N, BHAT R B, SENKOVA S V, SCHLOZ J D. Microstructure and properties of cast ingots of Al-Zn-Mg-Cu alloys modified with Sc and Zr [J]. Metallurgical and Materials Transactions A, 2005, 36(8): 2115-2126.
[5] FENG Chun, LIU Zhi-yi, NING Ai-lin, LIU Yan-bin, ZENG Su-min. Retrogression and re-aging treatment of A1-9.99%Zn-1.72%Cu- 2.5%Mg-O.13%Zr aluminum alloy [J]. Transactions of Nonferrous Metals Society of China, 2006, 16(5): 1163-1170.
[6] HE Yong-dong, ZHANG Xin-ming, YOU Jiang-hai. Effect of minor Sc and Zr on microstructure and mechanical properties of Al-Zn-Mg- Cu alloy [J]. Transactions of Nonferrous Metals Society of China, 2006, 16(5): 1228-1235.
[7] COSTELLO F A, ROBSON J D, PRANGNELL P B. The effect of small scandium additions to AA7050 on the as-cast and homogenized microstructure [J]. Materials Science Forum, 396/402: 757-762.
[8] YUAN Zhi-shan, LU Zheng, XIE You-hua, DAI Sheng-long, LIU Chang-sheng. Effects of RRA treatments on microstructures and properties of a new high-strength aluminum-lithium alloy—2A97 [J]. Chinese Journal of Aeronautics, 2007, 20(2): 187-192.
[9] CINA B. Reducing the susceptibility of alloy, particular aluminum alloys to stress corrosion cracking: US Patent 3856584 [P]. 1974.
[10] NORMAN A F, HYDE K, COSTELLO F, THOMPSON S, BIRLEY S, PANGNELL P B. Examination of the effect of Sc on 2000 and 7000 series aluminium alloy castings: For improvements in fusion welding [J]. Materials Science and Engineering A, 2003, 354(1/2): 188-198.
[11] LI Hong-ying, GENG Jin-feng, DONG Xian-juan, WANG Chang-jian, ZHENG Feng. Effect of aging on fracture toughness and stress corrosion cracking resistance of forged 7475 aluminum alloy [J]. Journal of Wuhan University of Technology: Materials Science Edition, 2007, 22(2): 191-195.
[12] OLIVEIRA A F, DE BARROS M C, CARDOSO K R, TRAVESSA D N. The effect of RRA on the strength and SCC resistance on AA7050 and AA7150 aluminum alloys [J]. Materials Science and Engineering A, 2004, 379(1/2): 321-326.
[13] LI Wen-bin, PAN Qing-lin, ZOU Liang, LIANG Wen-jie, HE Yun-bin, LIU Jun-sheng. Effects of minor Sc on the microstructure and mechanical properties of Al-Zn-Mg-Cu-Zr based alloys [J]. Rare Metals, 2009, 28(1): 102-106.
[14] LI Wen-bin, PAN Qing-lin, LIU Jun-sheng, LIU Xiao-yan, GUO Yun-shu, ZHANG Xin-ming. Optimum retrogression and reaging heat treatment of super-high strength Al-Zn-Mg-Cu-Zr alloy containing Sc [J]. The Chinese Journal of Nonferrous Metals, 2009, 19(9): 1533-1538. (in Chinese)
[15] BUHA J, LUMLEY R N, CROSKY A G. Secondary ageing in an aluminium alloy 7050 [J]. Materials Science and Engineering A, 2008, 492(1/2): 1-10.
[16] FENG Chun, LIU Zhi-yi, NING Ai-lin, ZENG Su-min. Effect of retrogression and reaging treatment on stress corrosion cracking resistance of super high strength aluminum alloy [J]. Journal of Central South University: Science and Technology, 2006, 37(6): 1054-1059. (in Chinese)
[17] REDA Y, ABDEL-KARIM R, ELMAHALLAWI I. Improvements in mechanical and stress corrosion cracking properties in Al-alloy 7075 via retrogression and reaging [J]. Materials Science and Engineering A, 2008, 485(1/2): 468-475.
[18] ZHENG Zi-qiao, LI Hong-ying, MO Zhi-min. Retrogression and reaging treatment of a 7055 type aluminum alloy [J]. Transactions of Nonferrous Metals Society of China, 2001, 11(5): 771-776.
(Edited by YANG Bing)
Foundation item: Project(2006AA03Z523) supported by the National High-tech Research and Development Program of China
Received date: 2010-03-10; Accepted date: 2010-04-27
Corresponding author: PAN Qing-lin, Professor, PhD; Tel: +86-731-88830933; E-mail: pql@mail.csu.edu.cn