Effect of the Supercooled Liquid Region on Al 85 Ni 7 Gd 8 Metallic Glass Crystallization Products
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Effect of the Supercooled Liquid Region on Al85Ni7Gd8 Metallic Glass Crystallization Products F. Q. Guo1, S. J Poon1 and G. J. Shiflet2 1 Department of Physics, 2Department of Materials Science and Engineering, University of Virginia, Charlottesville, Va 22903, U.S.A. ABSTRACT The effect of the supercooled liquid region on the primary crystallization of Al85Ni7Gd8 metallic glass, which exhibits a clear glass transition before its primary crystallization, was evaluated systematically under different annealing conditions, including isothermal annealing at different temperatures and non-isothermal annealing employing different heating rates. It was found that isothermal annealing within the supercooled liquid region and non-isothermal annealing at small heating rates (≤ 5 0C/min) result in the co-precipitation of fcc-Al and Al compound(s). When isothermal annealing is done at temperatures where partial crystallization is involved, or non-isothermal annealing is carried out at a larger heating rate, the primary crystallization product is a single phase of fcc-Al. The effect of the supercooled liquid region on the crystallization product is discussed in detail. INTRODUCTION Al-based metallic glasses are considered to be promising structural materials because of their combination of high specific strength, relatively good ductility and good corrosion resistance [13]. Introduction of nano-sized crystals into the Al amorphous phase matrix yields an even higher tensile strength, as high as 1.5 GPa, which is comparable to steel and about 50% higher than the corresponding fully amorphous alloys [4,5]. The introduction of nano-sized crystals can be realized by either mechanical deformation or thermal annealing of the amorphous alloys. The latter route is easier to control and easier to obtain a homogenous microstructure. Hence, thermal crystallization of Al-based metallic glasses is currently an active subject in materials research. Until now, crystallization studies of the Al-TM-RE type metallic glasses (here TM represents later transition metals, in most cases Fe, Co, Ni separately or combination, and RE refers to rare earth elements) are unclear in many aspects. In addition to the different proposed crystallization mechanisms of Al-based metallic glasses [6-9], there exist some disparate views as to what phase or phases will occur during crystallization. Inoue et al. [10] reported that fcc-Al and fcc-Al + an unknown compound come out corresponding to the primary crystallization of Al87Ni10Ce3 and Al85Ni10Ce5, respectively. Johnson [11] reported that only fcc-Al forms during the primary crystallization of Al90Y10 and only an Al compound of Al9(Fe,Y)2 was identified after the first crystallization reaction of Al85Fe7.5Y7.5. In Calin and Koster’s study [7], only fcc-Al was recognized during the primary crystallization of both Al90Ni6Nd4 and Al85Ni5Y10. Kelton et al. [12] studied the crystallization of Al88Ni4RE8 metallic glasses, where they mixed various rare earth elements to obtain different average atomic sizes. Th
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