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Precipitation Behavior in Al-Zn-Mg-Cu Alloy After Direct Quenching
Shengdan Liu,Chengbo Li,Yunlai Deng,Xinming Zhang,Qiming Zhong 대한금속·재료학회 2014 METALS AND MATERIALS International Vol.20 No.2
The precipitation behavior in an Al-6.8Zn-1.9Mg-1.0Cu-0.12Zr alloy after direct quenching from solutionheat treatment temperature of 470 °C to 205-355 °C was investigated by means of hardness tests, electricalconductivity tests, and transmission electron microscopy. At temperatures below 265 °C, the hardnessincreased gradually to a peak value and then decreased rapidly with time. At 265 °C, the hardness wasalmost unchanged within the initial 2000 s and then decreased gradually. At higher temperatures, the hardnessdecreased slowly with time. The electrical conductivity started to increase after a certain period oftime and then tended to maintain a constant value at all temperatures. Microstructure examination indicatedheterogeneous precipitation of the η phase at grain boundaries and inside grains during holding at205 °C and 325 °C. Based on the electrical conductivity data, the precipitation kinetics could be describedquite well by the Johnson-Mehl-Avrami-Komolgorov relationship with a n value varying between 0.78 and1.33. The activation energy was estimated to be about 44.9 kJ/mol, which is close to that expected for a dislocationdiffusion mechanism. Time-temperature-transformation diagrams were constructed and the nosetemperature ranged from 295 °C to 325 °C.
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Lingying Ye,Yu Dong,Shengdan Liu,Haichun Jiang,Daxiang Sun,Xinming Zhang 대한금속·재료학회 2018 METALS AND MATERIALS International Vol.24 No.5
The effects of pre-ageing on the microstructure and mechanical properties of T9I6 treated 2519A aluminium alloy wereinvestigated by hardness testing, tensile testing, transmission electron microscope, high resolution transmission electronmicroscope and differential scanning calorimetry. With the increase of pre-ageing time from 40 min to 4 h, the strength tendsto increase fi rst and then decrease; the pre-ageing time of 80 min results in the highest 0.2% proof strength and ultimatetensile strength of 491.9 and 513.8 MPa, respectively, and a high elongation of 14.6%. Pre-ageing is favorable for the formationof a number of primary GP zones, which can greatly improve the distribution of dislocations introduced by subsequentpre-deformation and secondary GP zones introduced by subsequent interrupted ageing. As a result, the number of precursorsfor θ′ hardening precipitates is maximized and the mechanical properties is improved after fi nal re-ageing.
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Dongfeng Li,Bangwen Yin,Yue Lei,Shengdan Liu,Yunlai Deng,Xinming Zhang 대한금속·재료학회 2016 METALS AND MATERIALS International Vol.22 No.2
By means of the end-quenching technique, we investigated the relationship between quenching rate and hardness as well as exfoliation corrosion rating for Al-2.21 Zn-3.59 Mg-0.45 Cu-0.038 Zr (at%) alloy plate. In order to achieve an exfoliation corrosion rating of P or EA, the quenching rate must be greater than approximately 460 °C/min and 300 °C/min, respectively, and the drop degree in hardness should simultaneously be lower than approximately 2.0% and 3.5%, respectively. The results of microstructural and microchemical examination using a scanning transmission electron microscope indicate that a lower quenching rate leads to a higher content of Zn, Mg, and Cu in the grain-boundary particles and a greater width of precipitate-free zones near grain boundaries; therefore, grain-boundary particles with Zn and Mg contents less than approximately 13.39% and 10.23% (at%), respectively, and precipitate-free zones near grain boundaries with widths less than about 107 nm can contribute to an exfoliation corrosion rating better than EA. The amount of quench-induced η- phase particles, which lead to lower hardness, increases with decreasing quenching rate, and the area fraction of these particles is approximately 2.9% at a quenching rate of 300 °C/min.
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