Characteristics Of Cluster Formulas For Bulk Metallic Glasses

Bulk metallic glasses combine exceptional glass-forming ability with many desirable properties,such as high strength and hardness,superior wear-and corrosion-resistance,and the flexibility of processing in the supercooled liquid regime.Although during the past decades,a great number of compositions capable of forming bulk metallic glass have been spotted.The development,however,still relies on the time-consuming,labor-intensive and expensive trial-and-error.The composition rules of bulk metallic glasses remain unclear.Since composition is closely related to structure,this task hinges on insights into the structure of metallic glasses.Although the atomic structure of metallic glasses lacks long-range periodicity,which is the defining structural feature of crystals,certain short-range and medium-range orders do exist.The former is often represented by the nearest-neighbor polyhedral cluster,while the latter refers to the organization of clusters.Our group's cluster-plus-glue-atom model views the structure of metallic glasses as being composed of clusters and glue atoms situated outside the clusters,thus a chemical building block,expressed by the so-called cluster formula – [cluster](glue atom)x,is extracted,where x is the number of glue atoms per cluster,and equals to one or three for good glass formers.Furthermore,our group has proposed that a eutectic composition can be interpreted by a dual-cluster formula – [clustera+clusterb](glue atom)2,4 or 6,based on the assumption that the structure of a eutectic liquid can be treated as the mixing product of two relatively stable liquids,each can be expressed by [cluster](glue atom)1 or 3.Inspired by this work,the present thesis covers the following aspects:(1)Clusters centered by each non-equivalent atomic site of any alloy phase are defined by employing the radial atomic density and Friedel oscillation method.In this method,the cutoff distance should be set at the furthest shell position within 1.5rS,where rS is the innermost shell distance relative to the center,whose corresponding atomic density hasn't drop to below the average one.(2)Principal clusters are selected by using the cluster isolation and close packing criteria.A principal cluster should have a large isolation degree,manifested by a large ratio between the number of atoms inside an effective cluster and the cluster.A principal cluster should also display atomic dense packing,reflected by the efficient packing index ?c being close to one,or a larger atomic density.(3)The cluster formulas for binary bulk and non-bulk metallic glasses are constructed by dissociating the dual-cluster formulas for eutectics.The cluster formulas for multicomponent bulk metallic glasses are established by similar elements substitution on the basic cluster formulas for binary bulk metallic glasses.(4)By comparing the cluster formulas for bulk and non-bulk metallic glasses in terms of structural dense packing and degree of order,the characteristics of the cluster formulas for bulk metallic glasses are revealed as follows: the close packing index of cluster formula ? ? 1,the number of electrons per cluster formula e/u ? 24,and cluster formula of the form [M-shell]M1 or 3,i.e.the center atom and glue atom sites contain the same species.These characteristics can provide practical help for the composition design of bulk metallic glasses.