Short- and medium-range order in amorphous ${\text{Zr}}_2\text{Ni}$ metallic alloy

Icosahedral clusters are commonly believed to be the key building blocks in many metallic glasses. Here we propose a structural model for ${\text{Zr}}_2\text{Ni}$ metallic glass which has a small fraction of icosahedral clusters. By analyzing the correlation between the local structure and dynamics in the undercooled liquid and glass, we show that the formation of metallic glass in this system can be attributed to the slow dynamics of $⟨0,2,8,1⟩$ and $⟨0,2,8,2⟩$ Ni-centered Voronoi polyhedra. There is a high proportion of less mobile $⟨0,2,8,4⟩$, $⟨0,2,8,5⟩$, $⟨0,1,10,4⟩$, and $⟨0,1,10,3⟩$ Zr-centered clusters on the first shell of the two types of Ni-centered clusters. These special Ni- and Zr-centered clusters arrange together to form a stringlike backbone network in the metallic glass.

Figure 1

(Color online) The structure model of ${\text{Zr}}_2\text{Ni}$ undercooled liquid at 1173 K from the constrained RMC simulation reproduce well the structure factor $S\left(q\right)$ from experimental XRD (a) and partial PCFs from ab initio MD [(b), (c), and (d)].

Figure 2

(Color online) Same as in Fig. 1 for ${\text{Zr}}_2\text{Ni}$ metallic glass at 300 K.

Figure 3

(Color online) (a) Scatter plot of the total fractions of various VP types in different initial configurations vs that in the random initial configurations. (b) Scatter plot of the total fractions of various VP types from different initial configurations vs that from the random initial configuration. The empty diamond and solid circle correspond to the fraction distribution resulting from the liquid ${\text{Zr}}_2\text{Ni}$ and an ISRO-rich model, respectively. Identical distribution should lie on the dashed line.

Figure 4

(Color online) Fractions of the 20 most frequent VPs in (a) ${\text{Zr}}_2\text{Ni}$ undercooled liquid at 1173 K and (b) glasses at 300 K, develop upon undercooling from the liquid due to their thermodynamic stability, develop upon undercooling from the liquid due to their thermodynamic stability, normalized by total Zr and Ni atoms for Zr- and Ni-centered VPs, respectively.

Figure 5

(Color online) (a) The population changes from undercooled liquid to glass $\left(P_{glass}-P_{liquid}\right)$ vs the population difference between the 10% of slowest diffusing atoms and the whole liquid sample $\left(\Delta P\right)$ for the most prevalent VPs. Red circles and black squares correspond to Zr- and Ni-centered VP indices, respectively. Only the top six are labeled. (b) The population differences of the various VP types on the first shell of the $⟨0,2,8,1⟩$ and $⟨0,2,8,2⟩$ clusters and in the whole glass sample.

Figure 6

(Color online) Spatial connectivity of the slow VPs. (a) shows the distribution of the number of nearest-neighbor connections among the Ni central atoms of $⟨0,2,8,1⟩$ and $⟨0,2,8,2⟩$ VPs (grided bars) and that plus their neighboring Zr atoms belonging to the center atoms of $⟨0,2,8,4⟩$, $⟨0,2,8,5⟩$, $⟨0,1,10,3⟩$, and $⟨0,1,10,4⟩$ VPs (solid bars). The purple and green colors denote the portion of atoms belonging to dimers or at the end of chains and branches, respectively. (b) Spatial distribution of the Ni central atoms of $⟨0,2,8,1⟩$ and $⟨0,2,8,2⟩$ VPs (red balls) with their neighboring Zr atoms belonging to the center atoms of $⟨0,2,8,4⟩$, $⟨0,2,8,5⟩$, $⟨0,1,10,3⟩$, and $⟨0,1,10,4⟩$ VPs (green balls).

Figure 7

(Color online) ${\text{log}}_{10}\left(N_d\right)$ versus ${\text{log}}_{10}\left(L\right)$ of the network consisting of Ni central atoms of $⟨0,2,8,1⟩$ and $⟨0,2,8,2⟩$ VPs and their neighboring Zr atoms belonging to the center atoms of $⟨0,2,8,4⟩$, $⟨0,2,8,5⟩$, $⟨0,1,10,3⟩$, and $⟨0,1,10,4⟩$ VPs. Topological dimensionality analysis is carried out for each group of connected center atoms. $N_d$ is the total number of nodes in each connected network and $L$ is the characteristic length of the network and defined as the maximum among the topologically shortest path between all node pairs in the network.