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I. INTRODUCTION

MAGNESIUM-BASED alloys have been increasingly used for the past decade because of their superior strength-toweight ratios and high impact resistance.[1] The application of magnesium alloys that is increasing most significantly is within the commercial automotive sectors; this type of application requires an incentive for decreasing weight in order to maximize fuel economy.[2,3] Significant inroads have been made in automotive interior and other room-temperature applications, such as instrument panel beams, steering components, and various housings.[4] Another use within the automotive industry to which magnesium alloys are increasingly put is in high-temperature applications, including those involving engine and transmission components.[5] Significantly, these applications require marked improvement in the high-temperature creep resistance of magnesium alloys.[6,7,8] Diecasting is a cost-effective processing route for producing magnesium components due to its excellent productivity, precision, and surface quality.[1,9] Although the creep properties of diecast magnesium alloys have been extensively investigated and discussed,[10–19] little research has been carried out on the creep mechanisms of the alloys.[20–23] The present work aims to investigate the creep behavior of a diecast AM50 alloy and to discuss the mechanism controlling the creep of the alloy in the temperature range between 423 and 498 K. Among the conventional commercial magnesium alloys, AM50 is endowed with a good combination of castability, ductility and fracture toughness.[24] It has been demonstrated that a considerable improvement of the creep resistance has been achieved for the AM50 alloy by the addition of calcium and/or strontium.[19,23] II. EXPERIMENTAL The high-purity magnesium alloy of AM50 (Mg-5.42 mass pct Al-0.28 mass pct Mn) was produced using a cold-chamNAOYA ISHIMATSU, Graduate Student, YOSHIHIRO TERADA, Associate Professor, and TATSUO SATO, Professor, are with the Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo 152-8552, Japan. Contact e-mail: [email protected] KOICHI OHORI, Chief Research Metallurgist Manager, is with the Technical Development Center, Mitsubishi Aluminum Co., Ltd., Shizuoka 410-1127, Japan. Manuscript submitted August 23, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A

ber diecasting machine, at the casting and die temperatures of 993 and 473 K. The materials were obtained in the form of plates 150 mm in length, 70 mm in width, and with thicknesses that varied in steps from 1 to 3 mm. Specimens for the creep tests with a gage length of 28 mm and a rectangular cross-section of 6  3 mm (Figure 1) were taken from the 3-mm-thickness sections of the castings. The axes of the specimens were parallel to the 70-mm direction of the plates. Constant-load tensile creep tests were carried out in air, using lever-arm creep machines at 423, 448, 473, and 498 K, under initial applied stresses that varied between 20 and 120 MPa. Two chromel-almel thermocouples were att