A Study of the Kinetic of Precipitation and the Morphology of Precipitates Induced by Plastic Deformation in Al-Zn Alloy
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A STUDY OF THE KINETIC OF PRECIPITATION AND THE MORPHOLOGY OF PRECIPITATES INDUCED BY PLASTIC DEFORMATION IN Al-Zn ALLOYS
, F. LIVET C. MAI 2, P. MERLE ' , J. MERLIN I.N.S.A. - BAT 502 - 69621 - VILLEURBANNE - FRANCE x GEMPPM (LA.341) E.N.S.E.E.G. - BP 44 - ST MARTIN D'HERE - FRANCE xx LTPCM (LA.29)
ABSTRACT The influence of plastic deformation on the kinetic of precipitation and the morphology of precipitates in an Al-Zn 4.4 % at. studied by T.E.P. and SAXS is reported. Some results can be pointed out ; after a given amount plastic deformation at room temperature by tensile strength : i - the precipitates have a flat shape (instead of the spherical or ellipsoidal shape usualy observed in this alloys. ii - the precipitate volume fraction is a function of strain (e) and strain rate (W). For a given A, the higher precipitate volume fraction is obtained with the higher E. For a given C, the lower t the higher the volume fraction of precipitates. iii - the scattering intensities at small angle for three orientations of the sample (a = 0,45%, 900 where a is the angle between the axis of tensile and the scattering vector- are different. A calculation for an anisotropic orientation of flat discs shows that precipitates have a preferential texture. The orientation is about 500 from the tensile axis.
INTRODUCTION The first stages of solid solution decomposition in Al-Zn alloys after quenching or irradiation have been the subject of a lot of experimental and theoritical works LI-5) and the phenomenon is nowadays rather well known. Nevertheless, the effect of plastic deformation has been less taken into account. Generally, it has previously been shown that plastic deformation immediately after quenching introduces an important slowing down of the kinetic of the precipitation of Guinier-Preston zones in respect to those obtained directly after quench [6-8). This effect is due to the elimination of vacancies by dislocations inducing a decrease of the diffusion rate of solute atoms. Recently, somes studies have been done [9J using thermoelectrical power (TEP) technique to observe the influence of plastic deformation on the kinetic of the precipitation in reverted samples of Al-Cu alloys. In this case, it should be observed that plastic deformation induced an acceleration of the precipitation phenomenon. The purpose of this study is to determine the influence of plastic deformation on the kinetic of the precipitation and on the morphology of precipitates in Al-Zn alloys. EXPERIMENTAL In the following, all the results are obtained in an Al-Zn 4.4 % at. alloy. Polycrystalline samples in the form of thin plates C0 mm thickness) 0 are homogenised 24 h at 250 C and then quenched in oil at 100°C. They are maintained 1 h at this temperature and quenched again in cold water. This thermal treatment give a metastable solid solution whose natural evolution at room temperature is negligeable (preliminaries studies in 4.4 %, 5.2 % and 6.8 % at. Zinc in Al-Zn alloys have shown that it is more easy to obtain stabilized samples without precip
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