Metallic-glass-relevant Alloy Phases Interpreted By Cluster Formulas

In both crystalline and non-crystalline alloys, dissimilar atoms tend to form first-shell coordination polyhedra, called clusters for simplicity, and these clusters constitute the characteristic local atomic features of the formed phases. However, the cluster information is usually submerged in the unit cell plus periodicity scheme of crystallography. This is why the compositions of alloy phases are in general unrelated to their structure features.Based on previously proposed "cluser-plus-glum-atom" model, the present work was engaged in developing cluster formulas to describe the composition and structure of alloy phases. This new approach, mimicking the molecula formulas for chemical substances, correlates the macro-scale compositions with atomic-scale structural features of any alloy phases. The background on the "cluser-plus-glum-atom" model and relevant "cluster line" and cluster formula, originally developed for quasicrystals and metallic glasses, were first summarized. The key point lies in the dissociation of any structure into two parts, a cluster part and a glue atom part, expressed by cluster formula as [cluster]1(glue atom)x. The present thesis was then devoted to solving some key issues in this model, such as defining precisely the clusters, determining the cluster formulas of the Al-Ni-Zr and B-Co-Si alloy phases, defining the principal clusters characterizing local atomic short-range orders, and applying the principal clusters in interpretating bulk metallic glass formation. Through a thoughrough analysis on the cluster geometries and packings, cluster formulas were shown to be able to present both composition and structural information of alloy phases including metallic glasses. The same approach can also be extended to explaining covalent compounds. The main results are listed below.First, the difficult problem for defining the clusters was resolved. The nearest neighbor atoms are often distributed on multiple shells, leading to ambiguities in defining the clusters. Here radial distribution of atomic density (number of atoms in the sphere enclosed by certain radial distance) was used to define the clusters. The spherical shell enclosing a high atomic density and having dense-packed triangular facets is selected as the nearest neighbors of a cluster.The clusters and cluster formulas of Al-Ni-Zr and B-Co-Si alloy phases were then analyzed. Composition and structure of alloy phase can be expressed by effective cluster and glue atoms because of atoms sharing, and the resultant cluster formulas are expressed as [effective cluster](glue atom)x. For example, using cluster formula, AINiZr (Fe2P) can be written as [NiAl3Zr3]Ni2. The cluster formulas similar to molecular formulas for chemicals, contain both structure and composition information of the alloy phases. Two types of phases are classified, cluster phases with glue atoms, such as AlNiZr (structural type Fe2P), BCo5Si2(Si3W5), and non-cluster phases without glue atoms, such as AlNi2Zr (BiFs), BCo5Si2(Si3W5).To select a unique clusters among multiple ones existing in alloy phases, the principal cluster concept was put forward, considering close-packing, degree of cluster overlapping, and glue atoms. The principal cluster should represent the characteristic local atomic features of the formed phases. For instance, Ni3Zr9is the principal cluster of Al2NiZr6(InMg2) and B3Co7is the principal cluster of BCo (BFe). In addition, a new method is proposed to classify the alloy phases according to the symmtry of their principal clusters. As examples Al-Ni-Zr and B-Co-Si alloy phases are analyzed with the objective to reveal the relationships of alloy phses and their principal clusters. Different from traditional crystallography, this classification gives the structure information of short-range-order of alloy phases, so it can provide a new view for alloy composition design.Furthermore, the principal clusters were applied in the explanation of bulk metallic glasses and were verified by experiments. Two examples were shown:the best glass former in the Al-Ni-Zr system can be explained by Al13.3Ni26.7Zr60=[Ni3Zr9](NiAl2), where Ni3Zr9is the principal cluster of a devitrification phase Al2NiZr6; using the B3Co7and B3Co8principal clusters, the best glass forming composition in B-Co-Si-Ta system can be expressed by the summary of two cluster formulas B26.1Co60.9Si6.5Ta6.5=[B3(Co7Ta)](Si0.5Ta0.5)+[B3Co7]Si.Finally, the cluster formulas method was extended to describing a few covalent compounds such as oxide, nitride and carbide in order to illustrate the universality of cluster formulas. In sharp contrast to alloy phases, the structures of clusters in the strong covalent compounds are generally simple, either tetrahedron or octahedron.