Devitrification Mechanisms in Al-Y-Ni Glasses
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Devitrification Mechanisms in Al-Y-Ni Glasses A.L.Vasiliev, M.Aindow, M.J.Blackburn, 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 & Process Engineering, Structural Alloys & Processes, 400 Main Street, Mail Stop 114-40, East Hartford, CT 06108, USA. ABSTRACT Crystallization of gas atomized Al-Y-Ni alloy powder during consolidation has been studied ex-situ using high-resolution lattice imaging, diffraction and energy-dispersive X-ray spectrometry experiments in a transmission electron microscope. In the as-atomized powder amorphous particles occur but others show some evidence of decomposition. On the application of heat and pressure two types of decomposition product are formed initially; equiaxed nanoscale α-Al grains embedded in an amorphous matrix, and dendritic aluminum structures with veins of amorphous and micro-crystalline phases between the aluminum-rich regions. Complex ordered structures were identified in the α-Al: thin sheets of solute rich material were formed on the {100} and {110} aluminum planes with ordered cubic symmetry. Precursors for the Al19Ni5Y3 and Al3Y phases are formed in the vein regions. The second and third stages of crystallization involve the conversion of these ordered phases and embryonic precipitates to the better-known binary and ternary compounds. INTRODUCTION Devitrification of Al / rare earth / transition metal alloy glasses has been studied by a number of techniques including differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), and Mössbauer spectroscopy. The vast majority of these studies have been performed on melt-spun ribbons [1-8] and a small number on gas atomized powder [9,10]. It has been shown that the crystallization process depends on the alloy composition and heating rate. Continuous heating DSC scans revealed multiple stages of devitrification and typically three peaks are observed. The first is associated with the formation of Al either as nanoscale particles or dendritic crystals embedded in an amorphous matrix. Subtle features in XRD patterns [1,11] and streaking in selected area diffraction patterns (SADPs) [10] have been interpreted as evidence for chemical ordering in the α-Al. The transformations associated with the second and third peaks are less certain; the formation of meta-stable intermediate structures may account for the second exotherm and the equilibrium phases for the third. The specifics of devitrification in gas atomized powder is a function of starting particle size. In general, finer powders experience faster cooling rates and have high amorphous content. Larger particles can show the beginning of precipitation. Since a potential route for the manufacture of improved Al alloys is consolidation of the powder by hot pressing and/or warm extrusion, the identification of the nature and kinetics of phase formation is important. Only through understanding a
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