The intergranular microstructure of cast Mg-Zn and Mg-Zn-rare earth alloys
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I.
INTRODUCTION
A number of commercial magnesium alloys contain rare earth additions that apparently give improved castability and high-temperature creep resistance. However, little is known about the detailed influence on microstructure that such additions might have. This article addresses this issue for a Mg-Zn alloy containing a small addition of misch metal (MM), which is a relatively cheap source of rare earth elements for the purpose of alloying. Previous work on Mg-Zn-RE (rare earth) systems has mainly been concerned with their phase diagrams, e.g., MgLa-Zn,m Mg-Nd-Zn, and Mg_Pr_Zn.t2,31 In the present work, the microstructure of a cast Mg-Zn-MM alloy has been compared to a Mg-Zn alloy that has approximately the same Zn content in order to determine the influence of MM additions on the phase constitution and the microstructure. This article concentrates on the interdendritic phases which arise during solidification and solution treatment of these two alloys. Special attention is given to identification of the various phases and their crystal structure, morphology, and composition. II.
EXPERIMENTAL
Two alloys of the following compositions (weight percent) were gravity cast in a permanent mold: Mg-9Zn and Mg-8Zn-I.5MM. The rare earth additions were made as MM with the approximate composition (weight percent) 50Ce, 25La, 20Nd, and 3Pr. Measurement of solidification curves was carried out separately on both alloys during solidification at a cooling rate of - 1 ~ The as-cast alloys were heat-treated at 315 ~ for 4 hours followed by a water quench. This treatment corresponds to a solution treatment prior to age hardening. Specimens for optical metallography were mechanically
L.Y. WEI, Senior Research Scientist, is with the Department of Engineering Materials, Luleh University of Technology, S 971 87 Lule~, Sweden. G.L DUNLOP, Professor and Director of CRC for Alloy and Solidification Technology (CAST), is with the Department of Mining and Metallurgical Engineering, The University of Queensland, Q 4072, Australia. H. WESTENGEN, Section Manager, is with Magnesium Materials Technology, Norsk Hydro a.s, Research Centre Porsgrunn, N3901 Porsgrunn, Norway. Manuscript submitted August 17, 1994. METALLURGICALAND MATERIALSTRANSACTIONS A
polished using standard techniques and subsequently etched in 1/3HNO3 in ethanol. These specimens were also studied by scanning electron microscopy (SEM) in a JEOL* 733 *JEOL is a trademark of Japan Electron Optics Ltd., Tokyo, Japan.
scanning electron microscope. The average size of the dendrite cells in the cast alloys was measured by the linear intersect method, and the volume fraction of the intermetallic phases was obtained by point counting on polished surfaces using optical microscopy. Thin foils for transmission electron microscopy (TEM) were prepared by jet electropolishing in 1/3HNO3 in ethanol at 8 to 15 V and - 0 ~ The thin foils were subjected to a short ion beam thinning operation of about 1 hour with an incidence angle of - 1 5 deg. This removed the oxide layers which
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