Advisor : Tzu-Yao Tai Advisee : Sin-Bo Wang Date : 2015/12/29 The rapid age strengthening induced by Ag additions in 7075 aluminum alloy Xiaofeng Xu, Yuguang Zhao, Xudong Wang, Yangyang Zhang, Yuheng Ning Advisor : Tzu-Yao Tai Advisee : Sin-Bo Wang Date : 2015/12/29

Outline Introduction Experimental Results and discussion Conclusions 2019/1/12

Introduction - 1 High strength Al–Zn–Mg–Cu alloys (7xxx series) are widely used in aircraft structures due to their high strength-to-density ratio . To obtain the superior mechanical properties, these alloys are usually serviced in the so-called T6 condition, which is gained by performing artificial ageing (AA) at peak hardness after a solid-solution treatment (SST). During aging, the usual precipitation sequence in Al–Zn–Mg–Cu alloys can be summarized as: supersaturated solid solution→GP-zones → η′ → stable η phase. 2019/1/12

Introduction - 2 Polmear stated that increasing the alloy Ag content from 0.3 to 1 could improve the strength of Al–Zn–Mg–Cu alloys. Furthermore, Hunsicker et al.'s work proposed that the Ag additions decreased the strength loss at the elevated temperature. However, previous works always focused on the changed mechanical properties, but the influence of the Ag additions on the precipitation process has been little reported in the previous studies. In this paper, the effect of various Ag additions on the microstructure and mechanical behavior of 7075 Al alloy will be investigated. 2019/1/12

Experimental The experimental material alloys were prepared with commercial 7075 Al alloy (5.63 wt% Zn, 2.35 wt% Mg, 1.64 wt% Cu, and balance Al) by ingot metallurgy in crucible furnace. Different Ag content (0 wt% Ag, 0.2 wt% Ag and 0.4 wt% Ag) were added into the prepared alloys. After casting, the materials were homogenized at 460 °C for 24 h, and the sheets in thickness were hot rolled (HR) from 5.9 mm to 2 mm thick strip with a reduction thickness of <30% per pass at 350 °C. The samples were solution-treated at 475 °C for 1 h, water quenched and then immediately aged at 120 °C to 30 h. 2019/1/12

Vickers microhardness was measured with a load of 50 g for a holding time of 10 s. At least 10 points were measured for each sample. The tensile tests were conducted on a servo-hydraulic materials testing system (MTS, MTS810, USA) at the strain rate of 10-3 s-1 at the room temperature, and at least 3 specimens were tested. The specimens for transmission electron microscope (TEM) observation were prepared by the standard twin-jet electropolishing method with a voltage of 10–15 V in 80% ethanol and 20% perchloric acid at - 30 °C. The TEM observations were carried out on a JEM–2100F and operated at 200 kV. 2019/1/12

Results and discussion - 1 183 12 h 16 h 179 26 h 178 Increasing Ag additions remarkably accelerate the age hardening process, and further increasing the ageing time results in a decrease in microhardness, indicating the occurrence of over-ageing of the SST sample. It is indicated that the increasing Ag additions could not only accelerate the aging process but also is beneficial for the age hardening level. 2019/1/12

Results and discussion - 2 0 Ag 26h (7075) 0.4 wt% Ag 12h Yield Strength (YS) 425 Mpa 447 MPa Ultimate Tensile Strength (UTS) 528 MPa 555 MPa elongation to failure close to 16.7% 18.3 % 2019/1/12

Results and discussion - 3 With increasing the Ag content, the residual phases dispersed more uniformly after heat-treatment, implying that Ag addition can decrease the phase clusters, which will be beneficial for the ductility of the alloys. The elevated temperature accelerates the dissolving process and recrystallization of the alloys. 2019/1/12

Results and discussion - 4 2019/1/12

Results and discussion – 4-1 ∆σ Or =M 0.4Gb π 1−v ln 2 𝑟 /𝑏 𝜆 𝑝 Typical conditions were as follows : M = 3.06 for the fcc polycrystalline matrix; G = 26.9 GPa for Al 7075; b = 0.286 for fcc metals; v = 0.33 for Al 7075; r0Ag = 6.5 and r0.4Ag = 3.7 nm; λp-0Ag = 31.6 and λp-0.4Ag = 24.2 nm. Based on the parameters, the calculated values of ΔσOr are 419 and 462 MPa for 7075 Al alloy and 0.4 wt% Ag alloy. 2019/1/12

Results and discussion – 4-2 7075 Al alloy (0 wt %). 18h A great amount of precipitates are observed after T6 treatment, and the primary secondary phases are η at the grain boundary, inter-grain GP zones and η′ phase in the under-aged 7075 Al alloy. T6處理後觀察到大量的析出物，主二次相是在晶界的η 相，跨晶粒的GP區和在上述7075 Al 合金的 η'相。 2019/1/12

Results and discussion – 4-3 Increasing the aging time, the sample obtains the peak hardness, and it can be seen that there exist some coarse η phases (black arrows in Fig. 4b) in the grains, implying the prolonged aging makes parts of precipitates transform and grow, which is beneficial for the ductility of the specimen. 7075 Al alloy (0 wt %). 26h 意味著長期時間老化實驗使析出物的部分 改變和生長，這能增加材料的延展性。 2019/1/12

Results and discussion – 4-4 0.4 wt %. 12h It is indicated that the degree of the precipitation is nearly same to the peak hardness 7075 Al alloy. As is known, Ag element can make a large amount of Mg–Ag clusters, which can be the nucleus of the precipitates and will be beneficial for the precipitation process. 2019/1/12

Results and discussion – 4-5 0.4 wt %. 22h Therefore, the precipitation is accelerated and the number of precipitates is improved in the alloy with Ag addition. Continuing increasing the aging time to 22 h in the alloy with 0.4 wt% Ag (approaching to the peak aging time in 7075 Al alloy), most of the fine precipitates change to the coarse η and η′ phases, so the strength and hardness will decrease. 2019/1/12

Conclusions Investigation on the influence of Ag additions on the mechanical properties and precipitation process was performed in this study. The results show that trace amount of Ag addition accelerates the precipitation process in 7075 Al alloy, and the remarkably accelerated peak age strengthening was achieved in the alloy with Ag element, moreover, both the strength and ductility are improved in the alloy with Ag additions. The peak-aged alloy is attributed to the combined strengthening effect of the η′ and η phases. Ag addition can make the residual phase distribute uniformly, and during artificial aging, a large amount of Mg–Ag clusters play an effective role in promoting the precipitation process. Hence, Ag addition is a promising method to obtain the superior mechanical properties in a short time in 7075 Al alloy. 在本研究中 觀察 添加銀對機械性能與凝結過程中 造成的影響。 結果表明，在7075 鋁合金中加入銀能加速沉澱的過程，並且顯著地發現增加銀含量能加速 材料達到強化峰值，而且 合金中添加銀能改善的強度和延展性。   合金老化的峰值 歸因於η‘和η相結合強化的效果。 加入銀可以使殘留相位均勻分佈，並且在老化實驗 沉澱過程中 大量的鎂銀原子簇 扮演重要角色。 因此，在7075 鋁合金裡 添加銀 可以在短時間內獲得優越的機械性 能。 2019/1/12

Thank you for your attention. 2019/1/12