Crystallization of Glassy Powder from Aluminum-Rare Earth- Transition Metal Alloys
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Crystallization of Glassy Powder from Aluminum-Rare Earth- Transition Metal Alloys A.L. Vasiliev, M. Aindow, M.J. Blackburn and T.J. Watson1 Department of Metallurgy and Materials Engineering, Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, USA. 1 Pratt & Whitney, Materials & Processes Engineering, Structural Alloys & Processes, 400 Main Street, Mail Stop 114-40, East Hartford, CT 06108, USA ABSTRACT The microstructures exhibited by gas atomized powders of two alloys, Al-6Gd-6Ni-1Fe and Al-5Y-10Ni (at. %), have been analyzed using electron microscopy. It was found that the microstructure depends critically on the particle size obtained during atomization. Small particles (30µm) are often fully crystalline, consisting of fine interspersed Al and intermetallic grains. INTRODUCTION Metallic glasses can be produced from certain aluminum alloys containing rare earths (RE) and transition metals (TM), provided they are cooled rapidly enough from the melt. In most cases, the glasses are metastable and crystallization will occur during subsequent processing that involves thermal exposure. The devitrification products can result in extremely fine and homogeneous microstructures and these are coupled with attractive properties suggesting that the processing of such materials via the amorphous state could be a path to improved materials. In fact, it has been shown that Al-(Y,Gd)-(Ni,Fe) alloys processed in this manner exhibit very high strengths with reasonable ductilities [1-3]. The devitrification of Al-RE-TM alloys have been studied by a number of techniques including X-ray diffraction, transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and Mössbauer spectroscopy. The vast majority of these studies have been performed on melt-spun ribbons [4-10]. It has been shown that the first stage of crystallization depends critically on the alloy composition, and two main types of decomposition products have been observed: spherical nanoscale α-Al grains embedded in an amorphous matrix, or mixtures of crystalline phases with a dendritic morphology. In general, with increasing aging exposure the morphology of the Al crystals changed progressively from spherical to dendritic while the intermetallic phases formed as equiaxed single crystal grains before forming large heavily twinned crystal assemblies or spherulites. A potential route for the manufacture of bulk amorphous Al alloys is to first produce powder by gas atomization followed by consolidation via hot pressing and/or warm extrusion. Conventional gas atomization yields powder with a range of sizes, in general the finer the particle the faster the cooling rate and thus the better the chance for retaining the amorphous state. An understanding of the structural condition, including the nature of devitrification products, as a function of particle size is necessary to determine the viability of the powder
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processing route [11]. In this paper we describe preliminary microstructural data obtained in scan
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