Characterization and Modeling of Precipitation Kinetics in Aluminium 7000 Alloys
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LTPCM / ENSEEG, Domaine Universitaire de Grenoble, 38402 St. Martin d'Heres, France
ABSTRACT The precipitation kinetics of 7108.70 aluminum alloy has been investigated in a wide range of temperatures by in situ Small Angle X-ray Scattering (SAXS) and Transmission Electron Microscopy (TEM), and computer modeled by use of an internal-state variable model which predicts the evolution of microstructural parameters. The modeling and experiments were done for isothermal heat treatment at 120, 140, 150, 160 and 170°C. The industrial T6 and T7 treatments have also been investigated. INTRODUCTION The 7108.70 alloy is widely used in automotive applications, mainly in car bumpers. due to its high mechanical properties. The process route for these alloys includes a solution treatment, a stretch forming operation followed by some natural aging and a two step artificial aging heat treatment. The first step ranges from 100°C to 120'C which is within the stability range of GPzones [1], and the second step from 140 0 C to 170'C which is the temperature range for ri' and rl precipitation [2]. In order to improve and control the heat treatments in these alloys it is important to know both which phases are present and the kinetics of the precipitation reaction at different times and temperatures. The first step to reach this goal is to measure, in a quantitative manner the precipitation kinetics at the different temperatures involved and then to apply a modeling approach to the experimental results.
In the present work the precipitation kinetics for several isothermal heat treatments between 100 and 170'C has been investigated experimentally and modeled by an internal state variable model concerning two regimes, nucleation and growth and growth and coarsening. EXPERIMENTS The 7108.70 alloy was provided by Hydro-Raufoss Automotive Research Center as extruded plates. Alloy composition is 5.5%Zn, 1.2%Mg, 0.16%Zr and 0.15%Fe (all in wt%). The solution treatment was 30 minutes at 480'C followed by water quenching, and resulted in a fully fibrous structure, with an average sub-grain size of approximately 3.5 pm. The SAXS experiments were performed in situ during heat treatment, measuring scattering vectors in the range from 0.02 to 0.5A-1 from 80gm thick samples with Cu K,, radiation. Scattering curves were corrected for background, fluorescence and absorption effects. Isothermal heat treatments were performed at 100, 120, 140, 150, 160 and 170°C with holding times up to 24 hours. A fast heating ramp of 320°C/hr was used, close to industrial heating ramps. The industrial T6 treatment was also investigated by SAXS: 6 hours at 100°C and 6 hours at 150'C with the same heating ramps, 320°C/hr. 481 Mat. Res. Soc. Symp. Proc. Vol. 578 © 2000 Materials Research Society
The particle dimensions were calculated using the Guinier approximation, which gives the gyration radius of the particles [3]. The nature and distribution of precipitates were investigated by Transmission Electron Microscopy (TEM) for the industrial T6 and T7 treatments. The T7
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