Highly Ordered Noncrystalline Metallic Phase

We report the characterization of a unique metallic glass that, during rapid cooling of an Al-Fe-Si melt, forms by nucleation, followed by growth normal to a moving interface between the solid and melt with partitioning of the chemical elements. We determine experimentally that this is not a polycrystalline composite with nanometer-sized grains, and conclude that this may be a new kind of structure: an atomically ordered, isotropic, noncrystalline solid, possessing no long-range translational symmetry.

Figure 1

Overview of the metallic alloy glass. (a) Microstructure of an ${\mathrm{Al}}_{91}{\mathrm{Fe}}_7{\mathrm{Si}}_2$ alloy after electron beam surface melting with a scan velocity of $50\mathrm{cm}/\mathrm{s}$. The nodules at the center of radiating patterns of crystallization are glass. This morphology indicates that the glass was first to solidify from the melt. (b) High resolution TEM image near a nodule-Al interface showing isotropic nature of the nodule. (c) Electron diffraction from the nodule showing glasslike continuous diffuse rings. (d) SEM image of a cross section of a melt-spun ribbon of the Al-15Fe-20Si at. % alloy showing a uniform glassy structure throughout the thickness.

Figure 2

$G(r)$ of the three phases overlaid for comparison. To improve the overlay of the general features of the PDFs and to account for the different lattice parameters of the different phases, the data have been scaled (by $1.024r$) for the glass and (by $1.041r$) for the icosahedral phase.

Figure 3

Peak intensities from the crystalline phases as a function of time during the $305°\mathrm{C}$ and (inset) $330°\mathrm{C}$ thermal anneals show the increase of the $\beta$ phase and increase and later decrease in the fcc Al phase at $305°\mathrm{C}$. As a result of a declining x-ray beam intensity, the plateaus that start at 600 min. are actually flat and indicate that equilibrium has been reached.