| Numerous molecular dynamics (MD) simulations for rapid solidifications of liquid metals and alloys and the tracking study upon the micro-structural evolution reveal those icosahedral clusters in the super-cooled liquid exhibit excellent configuration heredity as the cooling rate y is high enough to the critical value yc where nucleation is suppressed. However, few works aiming at their heredity and evolution mechanism are reported up to now. In this work, several typical clusters in simple metal Ag and binary Cu-Zr alloy, i.e., Agn(n=13,19,23,24,25) and CunZr13-n(n=6,7,8,9) icosahedral clusters and Agn(n= 13,20) fcc-clusters, are firstly identified from present MD simulation results and selected as candidates. And then their structural stability and formation capability are further investigated by the first-principle calculation method. A special attention is paid to the micro-structural origin of inheritable icosahedons as well as the effect of chemical orders on configuration stabilities of Cu-Zr icosahedral clusters.The major works and primary conclusions are as follows:(1) Using linear synchronous transit (LST) and quadratic synchronous transit (QST) technique based on the minimum energy path (MEP) with the aid of the climbing image nudged elastic band (CI-NEB) method, a new transition state (TS) search technique for stable configurations of clusters is developed, in which the stable configuration is independent from the initial configurations. As applied this technique to search the stable configurations of Agn(n=13,19,23,24,25) clusters, it is confirmed the low energy stable configuration of the isolated Ag13 is not icosahedron but a plate-shape layered structure C2-Ag13 including a pentagonal bipyramid segment. And the ground state of Ag19 is a spherical geometry C1-Ag19 consisting of 6 atoms around a deformed Ag13 icosahedron. In addition, all the stable configurations of Ag23, Ag24 and Ag25 clusters have a deformed Ag13 icosahedral nucleus as reported in literatures.(2) A density-function-theory (DFT) calculation conjoining with MD simulation is performed to investigate the structural stability and formation capability of isolated icosahedral Ag13 and extended icosahedral Agn(n=19,23,24,25) clusters linked by vertex-shared(VS), edge-shared (ES), face-shared (FS) and intercross-shared (IS) atoms in the rapid solidification. It is found that the high structural stability of the inheritable Ih-Ag13 short-range orders (ISROs) and the medium-range orders (IMROs) constructed by IS-linkages can be attributed to their large chemical hardness η relative to FS-, ES- and VS-linkages, and to some extent a large released calorie ΔHR-P in the formation of D5h-Ag19 clusters should be responsible for the coalescence preference of IS-linkages in super-cooled liquids and subsequently intensive emergence in rapidly solidified solids. In this case, a strong electronic interaction between core and shared atoms is demonstrated to play a key role in the aggregation of inheritable icosahedral basic clusters.(3) Based on MD simulations of liquid silver at two cooling rates (5×1013K/s and 5×1011K/s), the energetics, chemical stability and the electronic structures of ISROs, IMROs, fcc short-range orders (FSROs) and fcc medium-range orders (FMROs) at high and low temperatures are investigated by a DFT calculation. TS search shows that the evolution of ISRO to FSRO in supercooled liquid must overcome an energy barrier and this transformation is an endothermic process. A high chemical hardness should be responsible for the large population of ISROs and IMROs over FSROs and FMROs in supercooled liquid and the dominant FMROs at room temperature under low cooling rate, and their high chemical stabilities can be attributed to larger energy gaps, more low highest occupied molecular orbital (HOMO) energy levels, less electrons at Femi level (N(EF)) and more perfect full fill of electrons in the HOMO.(4) Using a first-principles calculation, the atomic arrangement of CunZr13-n(n=6,7,8,9) icosahedral clusters centred by Cu atom is investigated. It is found that homogeneous atoms in shell of clusters with low binding energy and high chemical stability prefer to bond each other. In this case, the Cu-Cu interaction is demonstrated to play a key role in the formation of stable Cu-centered icosahedrons. Compared with the icosahedrons with disorder distribution of Cu atoms, both of ionic bondings between core and shell Cu atoms and covalent bondings between shell and shell Cu atoms in high stable icosahedrons are strong. The results of electronic structures reveal this segregation in 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 these shell Cu atoms are all donees in the formation of icosahedral cluster, different from the donation of core Cu atoms and shell Zr atoms, and this charge transfer tendency does not change with order parameter and chemical composition of Cu-centered icosahedral clusters. |
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