Dandan Dong

Composition formulas of binary eutectics

The present paper addresses the long-standing composition puzzle of eutectic points by introducing a new structural tool for the description of short-range-order structural unit, the cluster-plus-glue-atom model. In this model, any structure is dissociated into a 1st-neighbor cluster and a few glue atoms between the clusters, expressed by a cluster formula [cluster]gluex. This model is applied here to establish the structural model for eutectic liquids, assuming that a eutectic liquid consist of two subunits issued from the relevant eutectic phases, each being expressed by the cluster formula for ideal metallic glasses, i.e., [cluster](glue atom)1 or 3. A structural unit is then composed of two clusters from the relevant eutectic phases plus 2, 4, or 6 glue atoms. Such a dual cluster formulism is well validated in all boron-containing (except those located by the extreme phase diagram ends) and in some commonly-encountered binary eutectics, within accuracies below 1 at.%. The dual cluster formulas vary extensively and are rarely identical even for eutectics of close compositions. They are generally formed with two distinctly different cluster types, with special cluster matching rules such as cuboctahedron plus capped trigonal prism and rhombidodecahedron plus octahedral antiprism.

Nearest-neighbor coordination polyhedral clusters in metallic phases defined using Friedel oscillation and atomic dense packing

It is generally difficult to define coordination polyhedral clusters with multi-shelled nearest neighbors. The present work defines their cutoff distances in typical metallic phase structure types via Friedel oscillation and atomic dense packing. The nearest neighbors of a cluster are considered to be confined within the first effective Friedel minimum in the pair potential, with the radial distance ratio of the outermost shell to the innermost shell being rL/rS = 1.5. This ratio is further correlated to atomic radius ratio, facet-capping configuration and cluster coordination number after considering hard-sphere atomic dense packing. The cluster cutoff distance is obtained by multiplying the rL/rS ratio with the measured innermost shell distance, as is well validated in common cluster types.

Composition formulas of Fe-based transition metals-metalloid bulk metallic glasses derived from dual-cluster model of binary eutectics

It is known that bulk metallic glasses follow simple composition formulas [cluster](glue atom)1 or 3 with 24 valence electrons within the framework of the cluster-plus-glue-atom model. Though the relevant nearest-neighbor cluster can be readily identified from a devitrification phase, the glue atoms remains poorly defined. The present work is devoted to understanding the composition rule of Fe-(B,P,C) based multi-component bulk metallic glasses, by introducing a cluster-based eutectic liquid model. This model regards a eutectic liquid to be composed of two stable liquids formulated respectively by cluster formulas for ideal metallic glasses from the two eutectic phases. The dual cluster formulas are first established for binary Fe-(B,C,P) eutectics: [Fe-Fe14]B2Fe + [B-B2Fe8]Fe ≈ Fe83.3B16.7 for eutectic Fe83B17, [P-Fe14]P + [P-Fe9]P2Fe≈Fe82.8P17.2 for Fe83P17, and [C-Fe6]Fe3 + [C-Fe9]C2Fe ≈ Fe82.6C17.4 for Fe82.7C17.3. The second formulas in these dual-cluster formulas, being respectively relevant to devitrification phases Fe2B, Fe3P, and Fe3C, well explain the compositions of existing Fe-based transition metals-metalloid bulk metallic glasses. These formulas also satisfy the 24-electron rule. The proposition of the composition formulas for good glass formers, directly from known eutectic points, constitutes a new route towards understanding and eventual designing metallic glasses of high glass forming abilities.

Spherical periodicity as structural homology of crystalline and amorphous states

It has been widely accepted that spherical periodicity generally dominates liquid and amorphous structure formation, where atoms tend to gather near spherically periodic shells according to Friedel oscillation. Here we revealed that the same order is just hidden in the atomic global packing modes of the crystalline phases relevant to bulk metallic glasses. Among the nearest-neighbor clusters developed from all the non-equivalent atomic sites in a given phase, there always exists a principal a principal cluster, centered by which the spherical periodicity, both topologically and chemically, is the most distinct. Then the principal clusters plus specific glue atoms just constitute the cluster-plus-glue-atom structural units shared by both metallic glasses and the corresponding crystalline phases. It is further pointed out that the spherical periodicity order represents a common structural homology of crystalline and amorphous states in the medium-range through scrutinizing all binary bulk-glass-relevant phases in Cu-(Zr, Hf), Ni-(Nb, Ta), Al-Ca, and Pd-Si systems.摘要球周期在液体与非晶的结构形成过程中占有主要地位, 根据Friedel振荡理论, 原子倾向于聚集在球周期壳层上. 本文提出在非晶晶体相结构中依然隐藏着球周期序列. 在一个给定的相中, 所有非等效原子占位皆衍生出相应的最近邻团簇, 其中必然存在一个具有代表性的主团簇, 以其为中心时, 球周期最明显. 该主团簇加上特定的连接原子组成了对应非晶态的团簇加连接原子结构单元. 本文通过全面分析Cu-(Zr, Hf), Ni-(Nb, Ta), Al-Ca与Pd-Si二元块体非晶形成体系中的晶化相, 进一步指出球周期序代表了晶态与非晶态在中程序的结构同源性.