Precipitation Hardening of Mg-Zn and Mg-Zn-RE alloys
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I.
INTRODUCTION
g number of commercial magnesium casting alloys are based on the Mg-Zn binary alloy system ClJwith small additions of rare earth elements. The rare earth additions, usually made by adding misch metal (MM) are known to improve the casting characteristics and elevated temperature strength, t2~ One example is the sandcasting alloy ZE41 (3.5 to 5.0 wtpct Zn, 0.75 to 1.75 wt pct MM, and 0.4 to 1.0 wt pct Zr), which was developed to meet the needs of medium strength applications and which has good castability. 13m Another alloy with good castability and creep strength, EZ33, has approximately equal amounts of rare earth elements and zinc. 15~ Mg-Zn alloys have a pronounced response to age hardening. Investigations of their age-hardening behavior have shown that the age-hardening results from the precipitation of a transition phase (/3'0. I6-14j In the present work, the microstructures and the agehardening characteristics of permanent mold cast Mg-Zn-RE pseudoternary alloys have been studied and compared with those of a binary Mg-Zn alloy in order to examine the effects of rare earth additions on precipitation and aging characteristics of this kind of alloy. The rare earth additions were made in the form of MM, since commercially pure MM is significantly cheaper than individual rare earth elements. II.
EXPERIMENTAL
The following permanent mold cast alloys were investigated (wt pct): Mg-SZn-I.5RE, Mg-4Zn-I.5RE, and Mg-9Zn. The rare earth additions were made using MM, which is a mixture of approximately (wt pct) 50Ce, 25La, 20Nd, and 3Pr. Specimens were solution treated for 4 hours at 315 ~ followed by water quenching. They were subsequently age hardened for various times at 200 ~ In order to avoid burning or severe oxidation, the specimens were embedded in MgO powder during these heat treatments. L.Y. WEI, Senior Research Scientist, is with the Department of Engineering Materials, Lule~ University of Technology, S 971 87 Lule~, Sweden, G.L. DUNLOP, Professor, Department of Mining and Metallurgical Engineering, and Director, CRC for Alloy and Solidification Technology (CAST), is with the University of Queensland, Z 4072 Australia. H. WESTENGEN, Section Manager, is with the Department of Magnesium Materials Technology, Norsk Hydro a. s., Research Centre Porsgrunn, N-3901 Porsgrunn, Norway. Manuscript submitted August 24, 1994. METALLURGICALAND MATERIALSTRANSACTIONSA
Both macro- and microhardness measurements were made on the age-hardened specimens. Specimens for scanning electron microscopy (SEM) were mechanically polished followed by etching with 1/3HNO3 in ethanol and investigated using a JEOL 733 SEM. Thin foils for transmission electron microscopy (TEM) were prepared by electropolishing in 1/3HNO3 in ethanol at 8 to 15 V and ~0 ~ and subsequently ion beam thinning for about 1 hour with an incidence angle of - 1 5 deg in order to remove the oxide layer that formed during electropolishing. The thin foil specimens were then examined in a JEOL 2000 FX TEM/STEM instrument operating at 200 keV. III.
RESUL
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