CALCULATION OF THE EFFECT OF ALLOY CHARACTERISTICS ON THE PERMANENT EXPANSION OF COLD COMPACTED HYPEREUTECTIC Al-Si-Fe-X POWDER AFTER THERMAL TREATMENT G. Timmermans and L. Froyen Departement Metaalkunde en Toegepaste Materiaalkunde (MTM), Katholieke Universiteit Leuven, de Croylaan 2, B-3001 Leuven, Belgium (Received October 26, 1998) (Accepted in revised form November 27, 1998) 1. Introduction When a metastable atomised Al-Si-Fe-X powder is exposed to elevated temperatures, the alloy evolves to a thermal stable condition by decomposition of the supersaturated Al-matrix, resulting in the precipitation of new phases and by transformation of metastable phases into stable ones. Many of these phenomena result in a remaining expansion of the material. This is theoretically and experimentally verified for binary and for commercial Al alloys by Hunsicker (1). Van Mourik et al. studied the basic effect on the lattice parameter of the Al-matrix (2). It is important to predict the permanent dimensional change because high alloyed Al alloys are used for accurate parts (e.g. automobile) and they are very often heat treated after shaping so that a final sizing operation might be necessary. For the atomised powder Al-17Si-5Fe-3.5Cu-1.1Mg-0.6Zr (wt%), investigated in this work, a permanent elongation of about 0.8% of a bar shape sample of a green product (cold compacted powder) is measured by Thermo Mechanical Analysis after a first thermal cycle up to 470 °C, when a heating rate of 5 °C/min or 1 °C/min is applied (Fig. 1). This 0.8% is the average of 10 measurements. In subsequent cycles, there is no extra permanent expansion anymore, indicating the mainly irreversible character (Fig. 1). The irreversible part of the expansion mainly occurs between 200 and 300 °C, where Si is precipitating, as detected by X-Ray Diffractometry (XRD) and Differential Scanning Calorimetry (DSC) (3). This Si-precipitation from supersaturated Al solid solutions is observed by many researchers, using also XRD Al line shift analysis and microscopy techniques (2,4 –11). 2. Calculation In the present calculation of the permanent expansion, the following effects are taken into account: the solid solution of Si, Cu and Mg in the Al-matrix, the accompanying precipitations during heat treatment, the difference between the thermal expansion coefficient of the dispersed Si-particles and the Al-matrix, the transformation of the metastable phase (Al 9 FeSi 3 ) into the thermally stable (Al 5 FeSi). The solid solution of Fe in Al is neglected because the Fe solubility in Al is very low. The difference between the thermal expansion coefficient of the dispersed (Al,Fe,Si) phases and the Al-matrix is neither considered, Pergamon Scripta Materialia, Vol. 40, No. 6, pp. 743–750, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/99/$–see front matter PII S1359-6462(98)00453-9 743