EFFECTS OF AGE HARDENING ON MAGNETIC AND TRANSPORT PROPERTIES OF Mg±1.3 WT% Ce ALLOYS T. YAMASHITA 1,2 , P. M. KELLY 3 , P. CAVALLARO 1 and M. HISA 4 1 Centre for Microscopy and Microanalysis, 2 Advanced Ceramics Development, UniQuest Ltd., 3 Department of Mining, Minerals and Materials Engineering, and 4 CRC for Alloy and Solidi®cation Technology (CAST), Department of Mining, Minerals and Materials Engineering, University of Queensland, St. Lucia, Queensland 4072, Australia (Received 18 December 1997; accepted 29 December 1997) AbstractÐA Mg±1.3 wt% Ce alloy has been isothermally heat treated at 423 K and the transport and mag- netic properties are investigated. This alloy is known to have distinct age hardening behavior and its age hardened microstructure has been studied in detail. The transport properties depend on the early stage of precipitation which is dicult to de®ne by transmission electron microscopy. The scattering sites of elec- trons are not identical to precipitates, but consist of strain ®elds induced by the precipitates, solute atoms, dislocations and vacancies. The resistivity was found to increase initially with aging time and then decrease. The highest resistivity was obtained from a specimen aged for 3.6 ks. This aging time is far less than that of 1800 ks which gives the maximum hardness. On the other hand, magnetic properties correlate with the later stages of the precipitation. In particular, the imaginary part of the magnetic susceptibility is related to macroscopic formation of precipitates. The imaginary part of the magnetic susceptibility of the alloys seems to be generated by eddy current loss. The imaginary part of the magnetic susceptibility increases monotonically with aging time but it may decrease for extensive aging treatments beyond 3600 ks. 1. INTRODUCTION Magnesium is the lightest structural metal. It has advantages over many other materials in terms of speci®c strength, machinability and shock absorp- tion. Improvements in magnesium alloy design and more stringent requirements to reduce fuel con- sumption and air pollution, have recently focussed attention on the use of magnesium alloys for auto- motive components. It has been shown that the properties and beha- vior of magnesium alloys are strongly dependent on small quantities of other elements Ð either as alloy- ing elements or impurities. It is possible that these elements will aect the magnetic and transport properties of Mg alloys. For automobile appli- cations, two types of Mg alloys, i.e. Mg±Al±Zn and Mg±Al±Mn, are currently in use [1]. Microstructure, hardness and transport properties of AZ91 (Mg±8.5 wt% Al±0.5 wt% Zn) have been studied [2]. However, it is well known, for example, that the creep strength of AZ91 deteriorates below an acceptable level for practical use at 1308C and higher [3]. It has been shown that additions of rare earth elements can improve the mechanical proper- ties of Mg alloys, especially creep strength at elev- ated temperature [4]. The alloy selected for the present investigation, Mg±1.3 wt% Ce, is known to have well de®ned age hardening behavior [5±7] and its microstructures have also been recently studied [5, 6]. In the present work, the transport and magnetic properties of Mg±1.3 wt% Ce are investigated and attempts are made to correlate these properties with the mechan- ical properties and microstructures. 2. EXPERIMENTAL PROCEDURES The Mg±1.3 wt% Ce alloy was ®rstly melted under Ar atmosphere. It was solution treated at 843 21K in an inert atmosphere and then quenched into iced water at 276 K. Finally the alloy was isothermally aged at 423 23 K in a salt bath. Vickers hardness of the heat treated alloys was measured using a load of 49 N. Microstructures of the samples were analyzed using a JEOL JEM-1210 transmission electron microscope with an accelerat- ing voltage of 120 kV. Samples heat treated at 423 23 K and 473 23 K are used for microstruc- tural analysis. The details of the results of the microstructural study are given in Refs [5, 6]. Electrical resistivity and magnetic susceptibility were measured for the samples isothermally aged for 1.8, 3.6, 36, 108 and 3600 ks, (0.5, 1, 10, 30 and 1000 h, respectively), as a function of temperature between 77 K and 300 K. Because of the require- ments of sample size for a Model 7000 AC Susceptometer (LakeShore), which was used for transport and magnetic measurements, samples had average dimensions of 1.5 mm  1.5 mm  10 mm. Some diculties were experienced with transport measurements due to sample preparation. Erratic results in the early stages of the investigation were Acta mater. Vol. 46, No. 9, pp. 2977±2981, 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd Printed in Great Britain 1359-6454/98 $19.00 + 0.00 PII: S1359-6454(98)00018-4 2977