Initial Stages of Precipitation in the Cu-Be Alloys
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INITIAL STAGES OF PRECIPITATION IN THE Cu-Be ALLOYS J.M. PELLETIER, G. VIGIER and C. MAI Groupe d'Etudes de Mdtallurgie Physique et de Physique des Matdriaux LA 341 INSA de LYON Bat. 502 69621 Villeurbanne FRANCE ABSTRACT The clustering in Cu-Be alloys lead to large modifications of the X-ray scattering curves and to increases of both electrical resistivity and thermopower. The microstructure is investigated in the initial stages of precipitation : small clusters exist in the quenched state, but no spherical precursor precipitates are observed before the formation of plate-like G.P. zones. The evolution of the transport properties is qualitatively analyzed and the deformation effect induced by anisotropic particles is considered.
INTRODUCTION The study of the initial stages of precipitation has been the purpose of many papers : experimental investigations as well as theoretical models (including numerical simulations) have been reported. If various alloys have been considered, most works are concerned with aluminium alloys : Al-Zn, Al-Cu, Al-Ag in particular. Only a few works deal with copper -base alloys . Therefore the purpose of the present paper is to study these initial stages of precipitation in a Cu-Be alloy, where it is known that G.P. zone formation for an aging induces a resistivity maximum [1]. The precipitation sequence in this alloy is [2,3,4] : solid solution a -> G.P. zones -> y" phase -> y' phase -> y phase. G.P. zones are plate-like particles ; but Rioja and Laughlin (4) think, from their electron micrographs, that a precursor phase (equiaxed clusters) forms during aging at room temperature. However, due to the small strains induced by these very small precipitates, the contrast on micrographs is very weak ; therefore electron microscopy does not appear like the best method for initial stage study. On the opposite, as copper and beryllium atoms have a very different atomic number, the small angle (and even diffuse) scattering of X-rays look like a more adequate experimental method and thus this technique is used in the present work to detect the clustering. Electrical resistivity is very sensitive to any microstructural change and then this property is a precise detector of any solid solution decomposition and is presently used. Thermopower measurements (TEP) are more scarcely performed, but are also very sensitive to clustering [5]. In previous work [6] a theoretical model has been proposed, which predicted a proportionality between resistivity and TEP evolutions during the initial stages of precipitation ; therefore the Cu-Be system appears like an additive test for this model. EXPERIMENTAL PROCEDURE High purity Cu-13 at. % Be alloys alloys were solution treated in a tube furnace with a pure argon atmosphere for 1 h at 800OC; samples are quenched to water at room temperature and then reheated to the aging temperature 0 TA ; T is chosen between 20 C and 200°C and thq treatments are performed in oil baths. The mean grain size is about 0.01 mm . Electrical resistivity measurements are carried out w
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