Three-dimensional topological structures and formation processes of dislocations in Au nanowire under tension loading

Dislocation is an important microstructural configuration in most crystalline materials, but it is still difficult for experiments to detect its local atomic structures and formation processes. Molecular dynamics simulation provides a powerful tool to investigate the dislocation by means of some geometrical structural analysis methods, but most of these methods cannot accurately distinguish the nuances of local configurations. Herein, we propose a new microstructural analysis method of cluster-type-index method (CTIM). The three-dimensional (3D) topological structures and formation processes of dislocations during tensioning Au nanowire are illuminated by means of CTIM. It is found that the favourite local atomic structures in dislocations are the defective FCC clusters represented by the CTIM indexes (13, 3/1421 2/1431 2/1441 4/1541 2/1661). The vector analysis method based on the accumulation of lattice distortions cannot identified the aggregation of disordered atoms in the initial nucleation of dislocation. In the formation processes of dislocation atoms, three trajectories of T1: FCC→(13, 3/1421 2/1431 2/1441 4/1541 2/1661), T2: FCC→(12, 2/1311 1/1411 9/1421) and T3: FCC→(11, 4/1311 7/1421) are favorite. The dislocation atoms following the T1 trajectory are mainly located at the core of dislocations, while that following the T2 and T3 trajectories is at the front of dislocations. CTIM would provide an effective tool to investigate the defective structures and their interactions in nanocrystalline structures

A new method for analyzing the local structures of disordered systems

We present a novel method to identify local structures in disordered systems according to topological criteria. Its effectiveness is demonstrated in the analysis of the atomic structures in the rapid cooling of silver liquid. The method is parameter free and scale independent, and can generally be used for structural analysis of amorphous systems involving atoms or particles at different length scales

The preference of synthesis modes and routes of stable Aln+m (n + m ≤ 13) clusters

By using linear synchronous transit (LST) and quadratic synchronous transit (QST) methods, the formation routes of stable Aln (n = 2-13) clusters assembled by two small clusters have been investigated in the framework of first-principles calculation. The addition of one sole atom to a cluster, i.e., the growth process, is generally automatic exothermic reaction, except for the growth of non-crystal configurations on the basis of crystal clusters. For the association of one cluster with another, i.e., the coalescence process, there usually exists reaction energy barrier ΔER-T. Comparison of the reaction heats ΔHR-P and activation energy ΔER-T suggests that the coalescence processes are more favorable than the growth processes for Aln (n = 2-13) clusters. In the coalescence processes, the clusters with typical crystal symmetry elements, i.e., the crystal clusters, have higher formation ability than those with fivefold or tenfold symmetry axes, i.e., the non-crystal clusters. The formation with non-crystal Al7 cluster as a precursor, i.e., Alm + Al7 → Alm+7, is most preferable in energetics among the coalescence routes considered

Formation mechanism of bulk nanocrystalline aluminium with multiply twinned grains by liquid quenching : a molecular dynamics simulation study

The formation mechanism of bulk nanocrystalline aluminium with multiply twinned grains has been investigated by a large-scale molecular dynamics simulation. The results show that bulk nanocrystalline aluminium can be obtained directly by quenching liquid at an appropriate cooling rate window. Most nanograins do not merge with each other in the coarsening stage due to the fast cooling rate, but are separated by high-angle grain boundaries. The nanograins exhibit a narrow grain-size range with an average diameter of 6.4 nm and random crystallographic orientations. These microstructure features are consistent with some of the bulk nanocrystalline alloys prepared by melt casting in experiments. The nanograins display various twinned morphologies, and they can be described by three twin elements of parallel, cross and fivefold twinning in varying amounts. The multiply twinned nanograins come from the successive formation of twin HCP planes during the layer-by-layer growth of FCC stacking blocks, and the growth kinetics plays a critical role in the grain morphology. This formation mechanism is different from that of fivefold deformation and annealing twins in the nanocrystalline metals

Simulation study of size distributions and magic number sequences of clusters during the solidification process in liquid metal Na

To investigate the size distributions of various clusters formed during solidification processes, a molecular dynamics (MD) simulation study has been performed for a system consisting of 106 liquid metal Na atoms. With the cluster-type index method (CTIM), it is demonstrated that the basic clusters of (13 3 6 4), (13 1 10 2), (14 2 8 4), (14 4 4 6) and (12 0 12 0) and their combinations play a critical role in the microstructure transitions. Also, using a new method to classify all the clusters in the system, the size distributions of various clusters clearly reveal magic number characteristics. The total magic number sequence can be regarded as the superposition of all partial magic numbers corresponding to the related group levels of clusters. The first 10 magic numbers are present in order 14, 22, 28, 34, 41(43), 46(48), 52(54), 57(59), 61(66), 70(74),... (the numbers in the parentheses are the second magic numbers corresponding to the same group level of clusters). This magic number sequence is compared with the experimental and computational results of other authors

The effect of cooling rates on hereditary characteristics of icosahedral clusters in rapid solidification of liquid Cu56Zr44 alloys

A molecular dynamics simulation is performed to investigate the influence of cooling rates γ on the hereditary characteristics of icosahedral clusters during the rapid solidification of liquid Cu56Zr44 alloys. The analysis from an extended cluster-type index method based on the H–A bond-type index shows (12 0 12 0 0 0 0 0 0 0) standard icosahedra and (12 0 8 0 0 0 2 2 0 0) as well as (12 2 8 2 0 0 0 0 0 0) defective icosahedra play a key role in the formation of Cu56Zr44 glassy alloys. With the increase of cooling rates γ, the glass transformation temperature Tg rises, but the clustering degree towards icosahedra descends in the rapidly solidified solid. An inverse tracking of atom traces reveals the icosahedral clusters at 300 K mainly originate from the configuration heredity below Tg, and the perfect heredity is dominant among perfect, core and segmental heredity modes. Relative to (12 0 8 0 0 0 2 2 0 0) and (12 2 8 2 0 0 0 0 0 0) defective icosahedra, (12 0 12 0 0 0 0 0 0 0) standard icosahedra are of high structural stability and large heredity ability, and their descendible efficiencies and heredity behaviors vary with γ. In the case of high γ, not only the descendible fractions fi of icosahedral clusters above Tg are enlarged, but also their initial descendible temperatures Tonset in the super-cooled liquid region are elevated. As a result, icosahedral medium-range orders (IMROs) can be easily formed and grown in the super-cooled liquid. Therefore, a big Trg = Tg/Tm of Cu56Zr44 alloys at high γ can be attributed to the ascent of Tonset caused by increasing γ to some extent

Cooling rate dependence of solidification for liquid aluminium : a large-scale molecular dynamics simulation study

The effect of the cooling rate on the solidification process of liquid aluminium is studied using a large scale molecular dynamics method. It is found that there are various types of short-range order (SRO) structures in the liquid, among which the icosahedral (ICO)-like structures are dominant. These SRO structures are in dynamic fluctuation and transform each other. The effect of the cooling rate on the microstructure is very weak at high temperatures and in supercooled liquids, and it appears only below the liquid–solid transition temperature. Fast cooling rates favour the formation of amorphous structures with ICO-like features, while slow cooling rates favour the formation of FCC crystalline structures. Furthermore, FCC and HCP structures can coexist in crystalline structures. It is also found that nanocrystalline aluminium can be achieved at appropriate cooling rates, and its formation mechanism is thoroughly investigated by tracing the evolution of nanoclusters. The arrangement of FCC and HCP atoms in the nanograins displays various twinned structures as observed using visualization analysis, which is different from the layering or phase separation structures observed in the solidification of Lennard-Jones fluids and some metal liquids

Freezing structures of free silver nanodroplets : a molecular dynamics simulation study

Freezing structures of free silver nanodroplets containing different numbers of atoms have been studied by using MD simulation and adopting quantum Sutton-Chen (QSC) potential. It is demonstrated that during most of the solidification processes there are a first order and a continuous phase transitions. By means of the cluster-type index method (CTIM-2) and three dimension graphic techniques, the internal structures of final nanoparticles have been investigated intensively. Besides regular crystalline, decahedral and icosahedral nanoparticles, some very interesting novelty morphologies have also been found. From geometrical views, these novelty structures can be called surface-isomers, because they can be constructed on the base of regular crystalline, decahedral or icosahedral nanoparticles by adding few layers with specific atomic arrangement. These new morphologies have well global (three- or five-fold) symmetry and lower energy than other structures, with same size and one of them is in agreement with the observation in experiment [Phys. Rev. Lett. 92 (2004) 196102]

Atomic dynamics of grain boundaries in bulk nanocrystalline aluminium : a molecular dynamics simulation study

Dynamics of grain boundary (GB) atoms in the bulk nanocrystalline aluminium is investigated by means of a large-scale molecular dynamics simulation. It is found that the GB atoms in the nanocrystalline aluminium display the glassy dynamics. Their dynamic features are, on one hand, similar to those in the glass systems and bicrystal GBs, in terms of the time-correlation functions of mean-square displacement (MSD) and non-Gaussian parameter (NGP). But on the other hand, these GB atoms also demonstrate some different dynamic behaviors due to the more complicated microstructure. In particular, the immobile GB atoms are localized into their equilibrium positions due to the strong cage effect, and they are mainly at the surfaces of grains, especially the large grains, while the mobile GB atoms are active and can easily hop away from the cage around them. These mobile GB atoms gather together at the surfaces of small grains and the triple junction regions of GBs, and they form some abnormally big clusters. The size distribution of these clusters significantly deviates from the power law

Competition between TCP and crystalline clusters during phase transition of rapidly super-cooled aluminum

Undercooling is usually a necessary condition for the starting of crystallization of molten metal under cooling. What structure hampered crystallization is however still unclear yet. The rapid cooling of pure aluminum has been conducted by a large-scale molecular dynamics simulation at different cooling rates. The evolution of local structures and phase transition mechanism are investigated in terms of the atom energy and the largest standard clusters. It is found that with high local density and low average potential energy (APE), the topologically close-packed (TCP) clusters in liquids increase exponentially with decreasing temperature, and have a number saturation stage whose survival time is related to cooling rate. When the crystalline clusters’ APE is lower than that of the system, crystal nucleation does not begin until the TCP clusters’ APE become higher than the system. Thus the competition between TCP and crystalline clusters results in the saturation of TCP clusters. These findings will provide new understandings on solidification and nucleation theory