Effect of long-term room-temperature storage on the structure and properties of glassy melt-spun Mg-Al-Ca alloys
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I. INTRODUCTION
ALUMINUM-BASED alloys with ultra high strength and usable ductility have been obtained with a nanoscale dispersion of fcc-Al based crystallites in a glassy matrix, either by controlling the quench rate of the liquid[1] or by annealing the precursor amorphous alloys.[2,3,4] For even lower density magnesium-based alloys, corresponding observations have been made for the Mg-Zn-Ce,[5] Mg-ZnLa,[6] and Mg-Al-Ca[7] systems. In the previous work on the Mg-Al-Ca system,[7] microhardness measurements obtained for Mg-Al-Ca alloys showed no significant decrease in hardness with decrease in alloying content from Mg71Al19Ca10 to Mg84Al11Ca5 (at. pct). This was interpreted as offsetting, by an increased contribution from the aMg crystallites, of any reduction in hardness of the amorphous matrix arising from reduced alloying content. The aim of the present work is to report the effect of long-term (48 months) storage at room temperature on the structure and properties of glassy melt-spun Mg-Al-Ca alloys. II. EXPERIMENTAL Mg-Al-Ca alloys with compositions containing 15, 23, and 28 vol pct Al2Ca at equilibrium (Table I) were melt spun by chill block melt spinning in an argon environment.[7,8] The peripheral wheel speed for the melt spinning was 31 m/s and ribbons produced were between 35 and 55 mm in thickness. The amorphous state of the ribbons as melt-spun was verified by X-ray diffraction (XRD) and transmission electron microscopy (TEM), as reported previously.[7,8] After a room-temperature storage period of 48 months, the as-spun samples were mounted edge-on in cold setting resin, followed by metallographic preparation and study by optical M.S. ONG, Student, and Y. LI, Senior Lecturer, are with the Department of Materials Science, National University of Singapore, Singapore 119260. W.M. RAINFORTH, Reader, and H. JONES, Professor, are with the Department of Engineering Materials, University of Sheffield, Sheffield S1 3JD, United Kingdom. Manuscript submitted November 2, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
microscopy. Microhardness testing was carried out using a Knoop indenter with a 50 g load applied for 10 seconds on transverse sections and on the free and chill faces of the ribbon. Each resulting measurement was the average of a minimum of ten indentations on the same sample. X-ray diffraction using Co Ka radiation was carried out for phase identification and for aMg lattice parameter measurements on both the free and chill sides of the ribbons. Differential scanning calorimetry (DSC) was performed with heating rates of 20, 40, and 80 K/min. The TEM samples were prepared in the standard manner using a Gatan Duomill fitted with a cold stage to minimize ion beam heating. The general microstructure was determined on a JEOL* 200CX *JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.
operating at 200 kV. High resolution structural and chemical analysis was performed on a JEOL 2010F field emission gun transmission electron microscope fitted with an Oxford energy-dispersive spectroscopy (EDS) sys
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