An observation on microstructure of a casting Zn-40 Wt pct al alloy
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INTRODUCTION
THE family of Zn-Al alloys has been widely used as a bearing material in industry to meet the need for higher strength and good friction and wear properties. It has been shown that these alloys have a number of advantages over traditional bearing materials.[1,2] However, serious problems may arise with these casting alloys after aging at ambient and slightly elevated temperatures. These include the decrease in hardness and the gradual, irreversible expansion due to the decomposition of the metastable phase a', which is an fcc solid solution of Al supersaturated with Zn.[2] It is well known that the mechanical properties depend on the microstructure. The microstructure of the Zn-Al alloys is complex, due to their special solid-state transformation and mechanism.[3,4] Thus, much work has been done on the microstructure of the ZnAl alloys, especially on alloys such as ZnAl8 (Al 5 8 to 8.8, Cu 5 0.8 to 1.3, Mg 5 0.015 to 0.03, and the balance Zn, in wt pct), ZnAl12 (Al 5 10.5 to 11.5, Cu 5 0.5 to 1.25, Mg 5 0.015 to 0.03, and the balance Zn, in wt pct), and ZnAl27 (Al 5 25 to 27, Cu 5 2.0 to 2.5, Mg 5 0.01 to 0.02, and the balance Zn, in wt pct). The studies were mainly concentrated on the changes in microstructure during the quench aging,[5,6,7] and there are disagreements on the precipitation sequence and the mechanism of the phase transformation in decomposition of the metastable phases, etc. In practice, Zn-Al alloys are usually used as castings, and it is necessary to understand the cast microstructure to guide the industrial application of Zn-Al casting alloys. In this article, the microstructure for the ZnAl40 casting alloy has been observed and analyzed by transmission electron microscopy (TEM). XIAOLEI XU, ZHIWEI YU, and YAQIN SHI, Associate Professors, and YONGQING MA and ZUKUN HEI, Professors, are with the Institute of Metal and Technology, Dalian Maritime University, Dalian 116024, People’s Republic of China. Manuscript submitted November 4, 1997.
METALLURGICAL AND MATERIALS TRANSACTIONS A
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EXPERIMENTAL METHODS
High-purity zinc (99.99 pct), aluminum (99.99 pct), and magnesium (99.99 pct), as original materials, and aluminum plus copper as a prealloyed addition, were melted in a resistance furnace at 923 to 953 K, and a specimen 20 mm in diameter and 30 mm in height was cast in a metal mold. The composition of the alloy was analyzed by electron probe microanalysis (EPMA), with the average value of the ten points listed in wt pct, as follows: Al 5 40.29, Cu 5 1.29, Mg 5 0.005, and the balance Zn. The samples for TEM and scanning electron microscopy (SEM) were taken from a cast specimen. The preparation for TEM foil samples was performed by wet grinding on silicon carbide papers, with a grit size of 600 to 1000, to produce a plate with a thickness of 40 to 50 mm; then, discs 3 mm in diameter were punched. Final thinning of the discs was performed by electropolishing in a double-jet Tenupol electropolisher using a 10 pct perchloric acid and 90 pct alcohol electrolyte at a voltage of 20 V a
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