First X-Ray Scattering Studies on Electrostatically Levitated Metallic Liquids: Demonstrated Influence of Local Icosahedral Order on the Nucleation Barrier

To explain the unusual stability of undercooled liquids against crystallization, Frank hypothesized that the local structures of undercooled liquids contain a significant degree of icosahedral short-range order, which is incompatible with long-range periodicity. We present here the first direct experimental demonstration of Frank’s complete hypothesis, showing a correlation between the nucleation barrier and a growing icosahedral short-range order with decreasing temperature in a ${\mathrm{T}\mathrm{i}}_{39.5}{\mathrm{Z}\mathrm{r}}_{39.5}{\mathrm{N}\mathrm{i}}_{21}$ liquid. A new experimental facility, BESL (Beamline Electrostatic Levitation), was developed to enable the synchrotron x-ray structural studies on deeply undercooled, reactive liquids.

Figure 1

(a) Cooling curves for electrostatically levitated 2.5 mm droplet of ${\mathrm{T}\mathrm{i}}_{39.5}{\mathrm{Z}\mathrm{r}}_{39.5}{\mathrm{N}\mathrm{i}}_{21}$ as a function of temperature showing two recalescence events (indicated by arrows). (b) X-ray diffraction pattern as a function of momentum transfer, $q$ ($=4\pi \sin \theta /\lambda$), for the undercooled ${\mathrm{T}\mathrm{i}}_{39.5}{\mathrm{Z}\mathrm{r}}_{39.5}{\mathrm{N}\mathrm{i}}_{21}$ alloy liquid at 1029 K, during the 1st recalescence to the $i$ phase, and during the 2nd recalescence to the C14 phase.

Figure 2

$S(q)$ for the ${\mathrm{T}\mathrm{i}}_{39.5}{\mathrm{Z}\mathrm{r}}_{39.5}{\mathrm{N}\mathrm{i}}_{21}$ liquid as a function of temperature. An increase in the intensity of the shoulder on the second peak (indicated by the arrow) is observed as the temperature is lowered below the liquidus temperature (1083 K).

Figure 3

Comparison between the experimental data (circles) for $S(q)$ for a ${\mathrm{T}\mathrm{i}}_{39.5}{\mathrm{Z}\mathrm{r}}_{39.5}{\mathrm{N}\mathrm{i}}_{21}$ alloy liquid at 1473 (a) and 1029 K (b), and the fit to a 13 atom icosahedral cluster decorated by a Ni atom at the center and Ti atoms at the 12 vertices (solid line). Such short-range order is reflected in the second peak (shown), and higher order peaks (not shown).