Relation between the Electrical Resistivity and the First Stages of Precipitation
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RELATION BETWEEN THE ELECTRICAL RESISTIVITY AND THE FIRST STAGES OF PRECIPITATION
G. VIGIER, J.M. PELLETIER, J. MERLIN Groupe d'Etudes de Mdtallurgie Physique et de Physique des Matdriaux LA 341 INSA de LYON Bat. 502 69621 Villeurbanne FRANCE
ABSTRACT Recent theories make it possible to think that electrical resistivity measurements could provide quantitative informations about the microstructural evolution. These theories are tested by studying the influence of the measuring temperature T . Our experiments show that when spherical G.P. zones appear in AI-Zn and Al-kg no influence of TM is observed, while the tormation of plate-like G.P zones in Al-Cu and Cu-Be alloys induce electrical variations function of I . This morphology effect is roughly predicted by the theoretical models buv supplementary parameters must be taken into account to obtain a quantitative explanation of experimental results.
INTRODUCTION Electrical resistivity measurements are often performed for the study of the first stages of solid solution decomposition. However it is necessary to connect precisely the resistivity variations with the evolution of the microstructure, in order to exploit correctly the results. That has been the purpose of many theoretical models since the early work of Mott [1] and now two physical mechanisms have been proposed : - the one given by Hillel et al. [2,3,4] (to be collectively referred to as HEW) based on Bragg diffraction of electrons by the atoms within the precipitates. - the one given by Rossiter and Wells [5] (to be referred to us RW) based on the fact that coherent scattering cannot occur for distances between scattering centers greater than the electron mean free path. effect But their validity has not been positively proved by experiments ;in only qualitative comparisons are mainly performed and the prediction of the occurrence of a maximum is often the only criterion retained. The influence of the measurement temperature is taken into account in RW and HEW mechanisms, but no detailed experimental work has been performed on this influence and therefore this is the purpose of the present work. This could provide a test for theoretical predictions and so resistivity variations may become a quantitative way for precipitation studies. Four alloys have been chosen from amongst systems where a resistivity maximum is observed during G.P. zone formation : - spherical G.P zones are observed in Al-Zn and Al-Ag alloys [6,7,8]. - plate-like zones are observed in Cu-Be and Al-Cu alloys [9,10,11,12]. EXPERIMENTAL METHOD The compositions of the different alloys are as follows (where ni is impurity content).
the
Al-5.3 at%Zn (ni < 50 ppm) - Al-l.6 at%Ag (ni < 50 ppm) Al-1.72 at%Cu (ni00C the resistivity peak is cancelled. For spherical precipitates our experimental results show that T has no influence on the magnitude of the electrical resistivity variation observed during the G.P. zone formation. In contrast to what is observed in alloys with plate-like precipitates the RW mechanism may coexist with the HEW one si
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