An isotropic glass phase in Al-Fe-Si formed by a first order transition
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An isotropic glass phase in Al-Fe-Si formed by a first order transition John W. Cahn and Leonid A. Bendersky Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899 USA
ABSTRACT We present evidence of the nucleation and growth of a metallic glass phase from the melt, as if by a first-order transition. Microstructures of a number of rapidly solidified Al-Fe-Si alloys demonstrate that this glassy phase, which we term q-glass, is not a kinetically frozen liquid. It is the first phase to form from the melt as isolated nuclei that grow and deplete the melt of iron and silicon. From the nucleation behavior and the compositional partitioning, we infer an interface between the q-glass and the melt, and that, in a narrow composition range, the q-glass has a lower energy and entropy (is more ordered) than a conventional glass.
AN UNUSUAL MICROSTRUCTURE Figure 1a shows an unpublished microstructure taken by transmission electron microscope (TEM) from a 1987 NIST study [1,2] of an Al91Fe7Si2 alloy after e-beam surface melting at a scan velocity of 50 cm/sec and solidification. Similar microstructures form by melt spinning at 4000 rpm on a 10 cm copper disk. The figure shows a microstructure of nodules which, because they are surrounded by a radiating duplex structure, must have been the first to nucleate and grow. The diffraction pattern taken from a nodule shows it to be an isotropic glassy phase. Dark field and high-resolution TEM imaging also support the isotropic, but highly
Figure 1. (a) Microstructure of an Al91Fe7Si2 alloy after electron-beam surface melting with scan velocity of 50 cm/sec. (b) Dark field image of a nodule and the glassy phase in the monotectic, obtained by positioning a SAD aperture on the strongest diffuse ring in (c).
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speckled, nature of the phase. The duplex two-phase microstructure surrounding each nodule is composed of crystalline fcc aluminum and this glassy phase. (We call it a monotectic; it would be a eutectic if the interspersed glassy phase were crystalline.) The monotectic radiating from different nodules meet along what appear to be ordinary impingement zones. These glassy nodules formed from the undercooled melt before the monotectic, not everywhere, but only at isolated sites where the glass nucleated, and then grew. This implies that there was a barrier to the nucleation of the glass. Presumably glass and melt are distinct phases with an interface with a positive surface free energy between them. After the monotectic solidification of the residual melt, the interface would have become the interface between the glass and monotectic seen in the micrograph of Figure 1a. Further indication that there was an interphase interface between the glassy phase and the melt comes from chemical EDS analyses of the nodule and monotectic regions, which show that the nodule is enriched in both Fe and Si, and that the two-phase monotectic is enriched in Al.
Figure 2. Microstructure of an Al70Fe13Si17 alloy prepared by e-b
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