Medium-Range Order in High Al-content Amorphous Alloys Measured by Fluctuation Electron Microscopy
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Medium-Range Order in High Al-content Amorphous Alloys Measured by Fluctuation Electron Microscopy W.G. Stratton, J. Hamann, J.H. Perepezko, P.M. Voyles Department of Materials Science and Engineering, University of Wisconsin – Madison 1509 University Avenue, Madison, WI 53706, U.S.A. ABSTRACT We have used fluctuation electron microscopy (FEM) to measure nanoscale mediumrange order in amorphous Al92Sm8. Samples of this amorphous alloy formed by rapid quenching (melt-spinning) show a high density of pure Al nanocrystals (>1020 m-3) after low temperature (< 250 ºC) devitrification. In samples amorphized by deformation (cold-rolling), primary Alcrystallization does not occur. This difference in devitrification behavior suggests an underlying structural difference in the amorphous state. FEM is a quantitative microscopy technique for determining nanoscale medium-range order in amorphous materials. Our measurements show that amorphous alloys formed by melt-spinning and cold-rolling have significant structural differences, and that annealing melt-spun alloy under conditions previously shown to modify the devitrification thermodynamics also changes the medium-range structure.
INTRODUCTION High-Al content amorphous alloys [1, 2] exhibit very high tensile strength and unusual devitrification microstructure, the origin of which is not well understood. These amorphous alloys can be produced by rapid quenching from the melt or by mechanical deformation. The very high cooling rates required for rapid quenching are most often achieving by melt-spinning. A bulk ingot of the desired composition, often produced by arc-melting, is melted and ejected under pressure onto a metal wheel spinning at high speed in an inert atmosphere, producing a ribbon of amorphous alloy. As solidified high Al-content amorphous alloys have tensile strengths as high as ~1000 MPa [1]. Upon devitrification, these amorphous alloys exhibit primary crystallization of Al nanocrystals at an extremely high density in a amorphous matrix [3]. In melt-spun amorphous Al92Sm8, annealing at < 250 °C produces pure Al nanocrystals at concentrations >1020 m-3 [4, 5]. These nanocrystals form from material with a higher solute concentration than the eutectic alloy of the system [6]. In other high-Al content amorphous alloys, partial crystallization increases the tensile strength to ~1500 MPa [1, 7]. Deformation-induced amorphization involves intense mechanical working of polycrystalline starting materials. This is often achieved by repeated cold rolling, which involves stacking elemental foils into a multi-layer sandwich with the desired composition and rolling it repeatedly at room temperature until it amorphizes. (See Ref [8] for a discussion of cold-rolling Al92Sm8.) In cold-rolled Al92Sm8 amorphous alloy, devitrification does not proceed by primary Al-crystallization [5]. While the high density of nanocrystals formed from the melt-spun alloy is attractive, the structural precursor of nanocrystal formation in the amorphous phase is uncertain. Large impurity conc
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