It is tempting to surmise that the excess solute atoms might be located at the

It is tempting to surmise that the excess solute

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atoms. It is tempting to surmise that the excess solute atoms might be located at the interstitial ‘vacancies’ of the densely packed clusters 9 . But we found instead that a different kind of building unit emerges. In our ab initio calculated systems, the connection between solute atoms takes the form of ‘strings’. For instance, two neighbouring solute atoms form an isolated pair surrounded by the solvent atoms, and the resulting configuration may be termed ‘extended clusters’. In the Zr 80 Pt 20 metallic glass (Fig. 6a) a solute pair is seen to have a total CN of , 17. Similarly, string-like solute atoms are apparent in systems with higher solute concentrations (for example, Fig. 6b for amorphous Al 75 Ni 25 ). The dense packing of the extended clusters, via edge-, vertex-, or face-sharing, constitutes the second type of MRO. This type of packing becomes significant when the solute concen- tration reaches roughly 20–30%, depending on the R * ratio. As the number of the direct bonds among solutes further increases with enriched solute concentration, more and more solute–solute contacts become unavoidable. A pronounced first-neighbour solute– solute pair correlation peak in the partial RRDFs emerges, as measured before for glasses such as Ni–Nb (ref. 43; Supplementary Fig. S7). When the solute interconnection percolation threshold is eventually reached, a network of the solute atoms takes form (Fig. 6c for amorphous Ni 63 Nb 37 ). The string- or ring-like interconnection reduces the number of like bonds (increases the number of unlike bonds), when compared to aggregated solute atoms, leading to energy reduction. This is illustrated in Supplementary Fig. S9: when the Zr 70 Pd 30 glass 44 is cooled from 1,800 to 300 K, the solute–solute connection becomes more network-like. The acute (solute–solute–solute) bond angles observed at high temperatures disappear, reflecting the tendency to order the solute atoms in string- like and ring-like medium-range arrangements that reduce the number of like bonds. The solute–solute CN in this open and extended connection is generally no more than 3 to 7. This net- work-type arrangement of the solute atoms leads to a much-involved third type of MRO. The concept of connected random network has previously been proposed in ref. 45. We have thus uncovered a whole spectrum of atomic packing schemes, which vary not only with R * but also with solute concentration. Limitations of models We should briefly discuss the inherent limitations of the modelling techniques. The rapid quenching used for the ab initio systems did Figure 6 | Configurations of solute atoms at increasing solute concentrations. a , Extended clusters in the form of ‘lone pairs’ (here, two atom-sharing icosahedra with CN ¼ 17) in the Zr 80 Pt 20 MG. b , Extended clusters in the form of ‘strings’ in the Al 75 Ni 25 MG. The dense packing of such ‘extended’ clusters constitutes the second type of MRO. c , The solute network formed in the Ni 63 Nb 37 MG, in which the solute concentration is high. The network-like arrangement of the solute atoms gives rise to a
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  • Summer '19
  • Amorphous metal

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