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Liu S.,Central South University | Liu S.,Key Laboratory of Non ferrous Metal Materials | Li C.,Central South University | Li C.,Key Laboratory of Non ferrous Metal Materials | And 6 more authors.
Metals and Materials International

The precipitation behavior in an Al-6.8Zn-1.9Mg-1.0Cu-0.12Zr alloy after direct quenching from solution heat treatment temperature of 470°C to 205-355°C was investigated by means of hardness tests, electrical conductivity tests, and transmission electron microscopy. At temperatures below 265°C, the hardness increased gradually to a peak value and then decreased rapidly with time. At 265 °C, the hardness was almost unchanged within the initial 2000 s and then decreased gradually. At higher temperatures, the hardness decreased slowly with time. The electrical conductivity started to increase after a certain period of time and then tended to maintain a constant value at all temperatures. Microstructure examination indicated heterogeneous precipitation of the η phase at grain boundaries and inside grains during holding at 205 °C and 325 °C. Based on the electrical conductivity data, the precipitation kinetics could be described quite well by the Johnson-Mehl-Avrami-Komolgorov relationship with a n value varying between 0.78 and 1.33. The activation energy was estimated to be about 44.9 kJ/mol, which is close to that expected for a dislocation diffusion mechanism. Time-temperature-transformation diagrams were constructed and the nose temperature ranged from 295 °C to 325 °C. © 2014 The Korean Institute of Metals and Materials and Springer Science+Business Media Dordrecht. Source

Liu S.D.,Central South University | Liu S.D.,Key Laboratory of Non ferrous Metal Materials | Dai Y.,Central South University | Dai Y.,Key Laboratory of Non ferrous Metal Materials | And 6 more authors.
Applied Mechanics and Materials

The microstructure and mechanical properties of friction stir welded Al-Zn-Mg-Cu alloy sheet were investigated by means of hardness and tensile tests, optical microscope and scanning electron microscope. The hardness profile of the weld exhibits a W shape with the lowest value in the thermo-mechanically affected zone on the advancing side. The tensile strength and elongation of the weld are about 71% and 72% that of the base material. In the nugget zone, there are a number of fine recrystallized grains and dispersed precipitates at grain boundaries. In the thermo-mechanically affected zone, the grain size is not uniform and there are a number of precipitates at grain boundaries. In the heat affected zone, the grain structure is similar to the base material. © (2014) Trans Tech Publications, Switzerland. Source

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