Heredity of icosahedrons: a kinetic parameter related to glass-forming abilities of rapidly solidified Cu56Zr44 alloys

Cu-Zr alloy system,as a representative of transition metal-transition metal (TM-TM) metallic glass (MG),has attracted considerable attention due to its high glass-forming ability in a wide range of compositions.Many researchers have realized that the GFA of Cu-Zr alloy is intimately related to Cu-centered Cu-Zr icosahedral atomic cluster in supercooled liquid and rapidly solidified into amorphous solid.And lots of molecular dynamics simulations have shown that Cu-centered Cu-Zr icosahedral clusters not only affect the thermo-dynamical properties of metal or alloy melts,but also exhibit excellent structural stability and configuration heredity ability during the rapid solidification.Hereof a model of the metallic glass structure based on like icosahedron has become widely accepted,which plays an important role in the glass transition and its strong kinetic constraint on nucleation.However,though more and more standard and distorted Cu-Zr icosahedral clusters have been found and reported in Cu-Zr metallic glass,the fundamental understanding of these Cu-Zr icosahedral clusters of MGs is still lacking.More essential properties of Cu-centered Cu-Zr icosahedral cluster, especially on the electronic structure are still unclear.Based on this,as a further step towards in depth understanding the electronic structures of those icosahedral clusters,we will investigate the electronic structures of the stable Cucentered CunZr13-n (n=6,7,8,9) icosahedral clusters in this work,and consider all the possible atomic configurations for given chemical composition in view of originate in theory And a DMol3 molecular orbital package based on density functional theory (DFT) is adopted to calculate the energetics and electronic structures of Cu-centered Cu-Zr icosahedral clusters.During optimization and total energy calculation,electronic exchange-correlation energy functions in reciprocal space with the Perdew-Burke-Emzerhof type under general gradient approximate are used.A double-numerical basis set together with d-polarization functions (DNP) is chosen to describe the electronic wave functions of Cu and Zr atoms. And only core electrons described by the DFT Semi-core Pseudopots are calculated.All atomic positions in Cu-centered CunZr13-n (n=6,7,8,9) icosahedral clusters are relaxed by geometry optimization under a root mean square (RMS) force of 0.002 Ha/ and RMS displacement of 0.005 .The calculations of total energy and electronic structure are followed by the geometry optimization with self-consistent field tolerance of 110-5 Ha.It is found that homogeneous atoms in the shell of clusters with low binding energy prefer to bond to each other.In this case,the results of electronic structures reveal this segregation at low energy and stable configurations can be attributed to their low N (EF) at EF to some extent.A further analysis of Mulliken'population shows that these 4s and 4p of shell Cu atoms are all donees in the formation of icosahedral cluster,different from the donations of 3d and 4s of core Cu atoms and 5s of shell Zr atoms, and this charge transfer tendency does not change with order parameter nor chemical composition of Cu-centered Cu-Zr icosahedral cluster.In addition,calculating the infrared vibration spectrum of Cu-Zr icosahedral cluster is a new idea for accurately characterizing the cluster structure.

Electronic structures of stable Cu-centered Cu-Zr icosahedral clusters studied by density functional theory

Jiang¹,

Peng²

2018

Identification and tracking of different types of crystalline nucleiduring isothermal crystallization of amorphous Ag

Li¹,

Peng²

2019

The isothermal crystallization of amorphous Ag is investigated by a molecular dynamics (MD) simulation, and the heredity and evolution of different types of crystalline clusters aretracked and analyzed by a reverse tracking method of atom trajectories with the help of cluster-type index method (CTIM) based on Honeycutt-Anderson (H-A) bond-type index. According to the difference in the type of crystalline cluster and the linkage mode, i.e., vertex-sharing (VS), edge-sharing (ES), face-sharing (FS) and intercross-sharing (IS), a cluster analysis method which can efficiently characterize fcc single-crystal, fcc poly-crystal and fcc hydrid-crystal, is proposed. That is, the IS-linkage of fcc basic clusters, i.e., a fcc medium range order, is defined as a fcc single-crystal cluster. The extended cluster of fccbasic clusterslinked by ISand FS modes is named fcc poly-crystal clusters. In the case of IS-linkages, if the majority of core atoms arefcc atoms, the extended cluster composed of fcc, hcp and bcc basic clusters will be regarded as a fcc hydrid-crystal cluster. Moreover, a structural analysis method of critical nuclei distinguishing embryosis also developed in terms of the hereditary characteristics of various crystalline clusters. In this scheme, the extended cluster which has only transient heredity and no continuous heredity is defined as an embryo, while it will be named nuclei if part of core atoms in extended clusters can keep cluster type of atoms unchanged and be continuously passed down in the early stage of crystallization. Thus, corresponding to the onset time/temperature of continuous heredity, the critical nuclei of fcc singe-crystals, fcc poly-crystals and fcc hybrid-crystals can be identified and characterized. It is found that the nuclei of fcc crystalsemerge after the steep drop of total energy of system and before the abrupt increase of sizesof tracked clusters. And regardless of critical sizes or geometric configurations, an evident difference exists among fcc singe-crystal, hybrid-crystal clusters and fcc poly-crystal clusters, of which the fcc single-crystal nucleus is the smallest (~1.6 nm ×1.0 nm × 1.1 nm), followed by poly-crystal nucleus (~1.7 nm × 1.0 nm × 1.6 nm) and hydrid-crystal nucleus (~2.3 nm × 2.0 nm × 2.4 nm) in sequence. There are a few hcp and bcc atoms at surfaces, i.e. shells, of single-crystal and poly-crystal nucleus, but neither hcp nor bcc atom can be detected at the shell of fcc hydrid-crystal nucleus. And theconfiguration of fcc single-crystal, poly-crystal and hydrid-crystal critical nuclei are all non-spherical.

Evolution characteristics and hereditary mechanisms of clusters in rapidly solidified Pd82Si18 alloy

高明¹,

邓永和²,

文大东³

et al. 2020

Molecular dynamics (MD) simulation and first-principles calculation were used to study the heredity characteristics, evolution trend and structural stability of basic clusters during the rapid solidification of Pd82Si18 alloy. The local atomic structures were characterized by the pair distribution function g(r) and the extended cluster-type index method (CTIM). The MD simulations reveal that the number of bi-cap Archimedes anti-prism (BSAP) clusters with CTIM index (10 2/1441 8/1551) is dominant in the amorphous solids rather than three-cap triangular prism(TTP) with CTIM index (9 3/1441 6/1551), which is identified be the most popular basic units in Pd82Si18 alloys analyzed by Voronoi index Relative to other basic clusters, the Si-centered BSAP possesses much larger fraction in the glassy state of Pd82Si18 alloys. Different from the findings in Cu-Zr alloys, the Si-centered BSAP instead of icosahedra has a larger hereditary fraction than any other Kasper clusters. During the solidification, it was found that most of the other Si-centered basic clusters are transferred into BSAP. Via the DFT calculations, it is observed that the Si-centered basic clusters with higher fraction of heredity and possesses lower binding energy. Among of them, BSAP always keeps lower binding energy than any other Si-centered Kasper clusters during the rapid solidification, resulting in its highest structural stability and the largest heredity fraction.