The effect of composition and milling conditions on the structure of mechanically alloyed Mg-Al based alloys
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I. INTRODUCTION
THERE has recently been growing interest in Mg-containing amorphous[1,2,3] or crystalline[4,5] alloys prepared by mechanical alloying (MA), as well as both amorphous[6,7,8] and crystalline[9,10] Mg alloys prepared by rapid solidification. Due to the density advantages of Mg alloys for structural applications, it was deemed worthwhile to see if mechanical alloying could produce amorphous alloys with enhanced mechanical and high-temperature properties, and improved corrosion resistance if possible. An amorphous phase has been reported near Mg38Al57Ca5[1] and now has been found with other elements in place of Ca. II. EXPERIMENTAL PROCEDURES AND ANALYSIS Many of the experimental details have been described previously.[1] Three 5/8-in. steel balls were normally used for milling, though initially three 1/2-in. balls and two 1/4-in. balls were used. Larger vials were also made from the same heat-treated S5 tool steel to permit use of three 13/16-in. balls for more intense processing. Powders were loaded into the vial in an argon glovebox, usually at around 10 to 20 ppm O2. Run times ranged from 0.25 to 100 hours. Early Mg-AlCa runs were done at ⫺100 ⬚C or ⫺150 ⬚C. The vial was cooled by a liquid nitrogen stream while shaking, using a temperature controller with 36 gage thermocouples mounted on the vial. The exterior vial temperature was controlled to within 2 ⬚C at the thermocouple and 10 ⬚C over the vial’s exterior. The temperature inside the vial was found to be within 3 ⬚C to 5 ⬚C of the control temperature. O-ring seals leaked at low temperature and were replaced by copper-sealed vacuum flanges. For runs at ambient, the vial temperature was around 40 ⬚C to 60 ⬚C. The total amount of powder milled ranged from 0.5 to 2.5 g in standard vials and 5 to 7.5 g in the large vials. Powder masses larger than these resulted in inhomogeneous processing. A thin adherent layer was always present on the
LOWELL E. HAZELTON was formerly Project Leader with the Central Research Labs, The Dow Chemical Co., Walnut Creek, CA 94598. Manuscript submitted August 29, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
walls and media, in addition to some free powder. The powder thickness on the balls and vial was found to be about equal. The ratio of free to total mass was constant at 36 ⫾ 1 pct for 0.5 to 2.5 g powder mass. The structure was determined by X-ray diffraction, using CuK␣ radiation from a PHILIPS* PW1729 generator with a Norelco (Philips *PHILIPS is a trademark of Philip Electronic Instruments Corp., Mahwah, NJ.
Analytical, Natick, MA) diffractometer equipped with a graphite exit beam monochromator and a Nicolet L11 powder diffraction system. X-ray diffraction patterns of free and adherent powders were identical for powder masses up to about 2.0 to 2.5 g in a standard vial using three 5/8-in. dia. balls, so 2 g was chosen as a maximum for further work. Powder density was measured to better than 0.1 pct accuracy using a Micromeritics (Norcross, GA) Accupyc 1330 helium pycnometer. Vickers microhardness measurements
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