Rapid chemical and topological ordering in supercooled liquid Cu${}_{46}$Zr${}_{54}$

Evidence for rapid ordering in a supercooled Cu${}_{46}$Zr${}_{54}$ liquid, obtained from high-energy x-ray diffraction in a containerless processing environment, is presented. Relatively sudden changes were observed in the topological and chemical short-range order near 850 °C, a temperature that is 75 °C below the liquidus temperature and 465 °C above the glass transition temperature. A peak in the specific heat was observed with supercooling, with an onset near 850 °C (the same temperature as the onset of ordering) and a maximum near 700 °C, consistent with the prediction of a molecular-dynamics calculation using embedded atom potentials. The chemical and topological ordering measured here are in agreement with predictions of a rapid development of chemically ordered icosahedral clusters in the supercooled liquid.

Figure 1

Temperature measured as a function of time for Cu${}_{(100-x)}$Zr${}_x$ liquids during free cooling for (a) $x$ = 36 and (b) $x$ = 54. Note the different scales on the time (abscissa) axes; the temperature (ordinate) axes are of the same scale.

Figure 2

Total static structure factors (left-hand side) and pair distribution functions (right-hand side), with changing temperature, for Cu${}_{100-x}$Zr${}_x$ liquids with $x$ = 36 (top), 54 (middle), and 70 (bottom).

Figure 3

The first peak in $g$($r$) for Cu${}_{(100-x)}$Zr${}_x$ liquids for (top) $x$ = 36 at 777, 927, and 1127 °C, (middle) $x$ = 54 at 650, 900, and 1100 °C, and (bottom) $x$ = 70 at 777, 1027, and 1327 °C; in all cases the lowest magnitude corresponds to the highest temperature. For each composition, the dashed curve is the $g$($r$) obtained from the rapidly quenched glass. The vertical lines indicate the positions of the partial pair separation distances at room temperature, taken from EXAFS and neutron-scattering data (Refs. 34 and 35) and MD simulations (Ref. 36).

Figure 4

(a) The differences in the pair distribution functions $\Delta g(r)=g_T(r)-g_{{1100}^{°}\mathrm{C}}(r)$ for Cu${}_{46}$Zr${}_{54}$ at $T=1000$, 900, 800, 750, 700, and 650°C, showing a splitting of the first peak that becomes more distinct as the temperature is reduced. The curve with the lowest magnitude at the first peak corresponds to the highest temperature ($T=1000$°C), and the arrow indicates the increasing magnitude of $\Delta g(r)$ with supercooling. (b) The ratio of the heights of the two distinct components of the first peak in $\Delta g(r)$, with supercooling (error bars are smaller than the symbols), showing a rapid increase near 850°C (1123 K).

Figure 5

An example of the quality of the RMC fits to the measured $S$($q$) data for supercooled liquid Cu${}_{46}$Zr${}_{54}$; the data at 650 °C are shown by the solid circles and the fit is indicated by the red line.

Figure 6

The numbers of HA pairs with various local symmetries for Cu${}_{46}$Zr${}_{54}$ liquids as a function of temperature; the ICOS (1551 •) is the dominant order, and both it and the DICOS (1431$▴$, 1541$▾$) show sharp changes near the transition temperature at 850 °C (dashed vertical line), while hcp-fcc (1421-1422 $\blacksquare$) and bcc (1661$▸$) change more gradually.

Figure 7

(a) The specific heat for liquid Cu${}_{46}$Zr${}_{54}$ as a function of temperature [$C_p(T)$] for two cooling cycles [symbols: red filled circle (•) and $\blacksquare$]. The scatter between the two data sets reflects the experimental uncertainty; (b) The $C_p(T)$of a Cu${}_{50}$Zr${}_{50}$ liquid and glass calculated from an MD simulation using an embedded atom potential (dash-dot line), and the results of an HA index analysis of the resulting MD configurations (ICOS (1551•), DICOS (1431$▴$, 1541$▾$), HCP/FCC (1421/1422 $\blacksquare$) and BCC (1661$▸$)).

Figure 8

The measured density for liquid Cu${}_{46}$Zr${}_{54}$; no sudden changes can be seen across the temperature range studied.

Figure 9

A Rietveld refinement of the diffraction data for Cu${}_{46}$Zr${}_{54}$ after liquid solidification. The fits indicate a mixture of Cu10Zr7 and Zr${}_2$Cu; the input data are shown with red symbols ($○$), the prediction from the refined structure in a green line, and the difference in a purple line.