CORRELATION BETWEEN THE MICROSTRUCTURAL EVOLUTION OF A 6061 ALUMINIUM ALLOY AND THE EVOLUTION OF ITS THERMOELECTRIC POWER V. MASSARDIER{, T. EPICIER and P. MERLE Groupe d'Etudes de MeÂtallurgie Physique et de Physique des MateÂriaux, UMR CNRS 5510, INSA Lyon, BaÃt. 502, F-69621 Villeurbanne Cedex, France (Received 11 October 1999; accepted 15 March 2000) AbstractÐThe thermoelectric power (TEP) of a 6061 Al alloy was measured during isothermal ageing per- formed between 200 and 5008C. A correlation between the change in TEP and the precipitation kinetics of various metastable phases was established using TEM and HRTEM observations and resistivity measure- ments. It was demonstrated that the TEP kinetics allow an assessment of the residual concentration of solute in the ®nal equilibrium state. Furthermore, it was shown that the non-monotonous TEP changes re¯ect the microstructural transformations occurring in the alloy. In particular, the transition between two phases can be detected, owing to the change of the intrinsic eect or of the chemical composition of the precipitates. 7 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Re sume ÐLe pouvoir thermoeÂlectrique (PTE) d'un alliage 6061 a eÂte mesure pendant des vieillissements iso- thermes eectueÂs entre 200 et 5008C. Une correÂlation a eÂte eÂtablie entre les eÂvolutions de PTE mesureÂes et les cineÂtiques de preÂcipitation des dieÂrentes phases meÂtastables, en utilisant des observations en MET et METHR ainsi que des mesures de reÂsistiviteÂ. Il a eÂte eÂtabli que les cineÂtiques de PTE permettent d'eÂvaluer la concentration reÂsiduelle en eÂleÂments d'alliage lorsque l'eÂtat d'eÂquilibre microstructural de l'alliage est atteint. Par ailleurs, il a eÂte montre que les eÂvolutions non-monotones de PTE, caracteÂriseÂes par dieÂrents stades de deÂcroissance et croissance, traduisent les eÂvolutions microstructurales ayant lieu dans l'alliage. En particulier, la transition entre deux phases peut eÃtre mise en eÂvidence, en raison de l'eÂvolution de l'eet intrinseÁque ou de la composition chimique des preÂcipiteÂs pendant cette transition. 7 2000 Acta Metallur- gica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Aluminium; Aging; Phase transformations; Thermoelectric eects 1. INTRODUCTION In order to optimize the microstructure of Al±Mg± Si ingots prior to their extrusion, it is necessary to understand the high-temperature precipitation of these alloys during the cooling from the solutioniz- ing temperature. Indeed, this precipitation is expected to govern the Mg and Si content in solid solution before extrusion. This content has to be minimized, in order to obtain good extrudability. In this context, it is important to know the microstruc- tural evolution and the time±temperature±precipi- tation characteristics of these alloys. In the case of the ternary Al±Mg±Si alloys with- out excess of Si, previous TEM works [1±3] showed that the precipitation sequence is rather well estab- lished and can be described as follows: supersatu- rated solid solution 4 GP zones 4 needle-shaped precipitates aligned along h100i directions of the matrix and coherent with the matrix along their major axes (b0 phase) 4 rod-shaped precipitates semi-coherent with the matrix (b ' phase) 4 plate- shaped equilibrium precipitates (b phase of Mg 2 Si composition). However, when the alloy contains a small ad- dition of Cu, this sequence is more complicated, as was demonstrated in a previous work using conven- tional and high-resolution transmission electron mi- croscopy (CTEM and HRTEM) [4]. In an alloy containing 0.25 wt% Cu, it was shown that two dierent types of metastable b ' precipitates, desig- nated b 0 I and b 0 II , may coexist during ageing at ap- proximately 3008C. The b 0 I precipitate possesses a hexagonal structure a  0:705 nm, c  0:405 nm and forms typically in the pseudo-binary Al±Mg±Si alloys, as was ®rst shown by Jacobs [3] and later con®rmed by other authors [5, 6]. The b 0 II precipi- tate also possesses a hexagonal structure Acta mater. 48 (2000) 2911±2924 1359-6454/00/$20.00 7 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6454(00)00085-9 www.elsevier.com/locate/actamat { To whom all correspondence should be addressed.