Search

A molecular dynamics simulation is performed to investigate the icosahedron in Mg 70 Zn 30 metallic glass during solidification at the cooling rate of 1 × 10 13  K/s. It is found that the 1551, 1541 and 1431 bond-types are closely related to 13-atom icosahedron-like clusters. Two kinds of clusters are very similar to full icosahedron in geometry. The three clusters can transform into each other by changing one bond between a pair of outer atoms. One distorted icosahedron act as the intermediate between the two others. This work provides an insight into the glass formation in alloy from the cluster-level structural viewpoint and will shed light on developing more metallic glasses. [ABSTRACT FROM AUTHOR]

GaN is a promising third-generation semiconductor material for fabricating nanodevices with nanoscale dimensions. In particular, thick-film sensors fabricated with GaN nanoparticles (NPs) have good gas-sensing characteristics. This study used molecular dynamics simulation to investigate the effect of temperature and size on the structural morphology and surface recrystallization of GaN NPs during sintering. The diffusion behavior and changes in the structural morphology of atoms were analyzed based on the shrinkage ratio, mean square displacement, potential energy, and other methods. The results show that during sintering, the surface of two GaN NPs experienced recrystallization, which expanded with increasing temperature; however, the obvious sintering phenomenon occurred at 3000 K. Interestingly, the surface recrystallization of NPs exhibited a lamellar structure formed through a specific stacking method of the crystal structure. However, when the size of one of the two NPs was reduced, the small NP rapidly melted and was induced to crystallize into a complete NP (having the same crystal orientation as the large NP) by the large NP. Moreover, NPs underwent rigid-body rotation and relative rotation during sintering; however, plastic deformation and dislocations resulting from rotation were only found in the neck of NPs of the same size. The study results provide valuable insights at the atomic scale regarding the sintering of GaN NPs and a useful reference for preparing and processing nanodevices. [ABSTRACT FROM AUTHOR]

The Hall–Petch relationship and deformation process in Ti3Al nano-polycrystalline alloys are investigated using molecular dynamics simulations in this research. The impact of different grain sizes and temperatures on mechanical characteristics was considered. Different Hall–Petch relations were detected for samples with the grain sizes ranging from 2 to 20 nm, inverse Hall–Petch relations in big grain sizes ranges, and positive Hall–Petch relations in the medium grain size region, respectively. A transition zone formed at the interception of positive and inverse Hall–Petch relations, which will vanish as the temperature rises. The critical grain size of the positive Hall–Petch relation decreases as the temperature rises from 10 to 600 K. With the grain size increases in the smaller grain size range, the yield stress increases, which is made by the grain boundary motion and rotation. Besides, temperatures affect mechanical characteristics. The rising temperatures lead to the decreasing dislocation density and a high grain boundary fraction. [ABSTRACT FROM AUTHOR]

In the realm of semiconductor materials, the integration of graphene (Gr) and gallium nitride (GaN) has emerged as a promising avenue for enhancing device performance. In this study, a Gr/GaN layered nanocomposite was constructed using molecular dynamics simulation and the LAMMPS code. Diamond abrasive was pressed into the composite at a speed of 50 m/s at 300 K, followed by ultrasonic vibration-assisted polishing on the (0001) plane along the [ 0 1 ‾ 10 ] direction. By rigorously exploring various factors such as number of Gr layers, vibration amplitudes, vibration frequencies and grinding speeds, the study elucidates the intricate mechanisms governing material removal and subsurface damage. Key findings reveal optimal processing conditions, including three layers of Gr, an amplitude of 5 Å, a vibration frequency of 80 GHz and a grinding speed of 100 m/s, which result in enhanced material removal rates and improved surface quality. Additionally, the insertion of Gr layers within the GaN matrix demonstrates a remarkable reduction in friction and wear rates, highlighting the potential for enhancing device reliability and performance. This comprehensive investigation not only advances our understanding of ultrasonic vibration-assisted grinding processes but also underscores the transformative impact of Gr/GaN composites in semiconductor device fabrication. [Display omitted] • Graphene-containing composites exhibit enhanced wear resistance and reduced wear rate. • Temperature and friction variations in UVAP resemble sinusoidal patterns. • The graphene layer presence results in nearly uniform thickness of the subsurface damage layer. • Higher amplitudes result in increased material removal, albeit with higher severity of subsurface damage layers. • Elevated velocities raise regional temperature, affecting atom removal quantity and subsurface quality. [ABSTRACT FROM AUTHOR]

• The topologically close–packed (TCP) atoms dominate in undercooled Zr liquid. • The TCP atoms are energetic and structural favourable and then stabilize super-cooled liquid. • Three stages between TCP and BCC competition process can be observed. • Crystallization cannot occur until the TCP atoms with T 5 > 1/3 drops in number and interconnect. The crystallization of undercooled liquid zirconium (Zr) during isothermal relaxation was investigated by molecular dynamics simulation. In particular, the dynamic competition mechanism between the topologically close–packed (TCP) and body–centered cubic (BCC) structures during crystallization was deeply studied using the atomic energy, the pair distribution function and the largest standard cluster analysis (LaSCA). The study found that TCP clusters, rather than icosahedral clusters, play an important role in hindering the crystallization of the undercooled liquid. The TCP clusters with T 5 = n 5 /(n 4 + n 5 + n 6) > 1/3 (where n 4 , n 5 and n 6 represent the number of common neighbor sub-cluster: 444, 555 and 666 respectively) stabilize the undercooled liquid and prevent crystallization; while the other TCPs exhibits an evolution trend similar to that of BCC-like clusters. Furthermore, three stages of the competition process between TCP and BCC structures can be observed, and crystallization does not happen until the number of major TCP clusters with T 5 > 1/3 decreases and the average degree of interconnection among them weakens. [ABSTRACT FROM AUTHOR]

• With a crystallization-like E-T curve Ni 50 Ag 50 amorphization includes a LLT and a LST. • The average cutoff distance exported by LaSCA is useful in identifying state transitions. • LLT can be identified by the evolution of SRO and MRO structures, where MRO shows the glass feature. • In LST, TCP-based MRO and TLRO structures grow rapidly. • TCP structures reflect the essential nature of amorphous systems more effectively than ICOs. A molecular dynamics simulation of rapid cooling at a cooling rate of 1011 K/s for Ni 50 Ag 50 alloys was conducted. The rapid cooling process was analyzed by using the system energy, the pair distribution function, and the largest standard cluster analysis. It is found that with a crystallization-like energy curve the solidification of Ni 50 Ag 50 alloy experiences a liquid-liquid and a liquid-solid transition (LLT and LST), finally result in an amorphous solid. In LLT stage the topologically close-packed (TCP) short-range order structures of both crystal (S666) and glass (S555) increase; whereas only glass medium-range order structures are formed. TCP-based long-range order (LRO) structures of glass do not increase until the beginning of the LST. Even in solid (T < T g) the LRO parameter based on ICO structures fluctuates violently, while that based on TCP structures is quite stable. Therefore, TCP not only is the basic structural characteristics of disordered system but also reasonably represents the metastability of metallic glasses. These findings not only clarify the importance of LLT for the glass forming ability of alloys, but also fully demonstrate the effectiveness and necessity of TCP structures for describing the structure of amorphous systems. [ABSTRACT FROM AUTHOR]

Two-phase Îł -TiAl/ α 2-Ti3Al lamellar intermetallics have attracted considerable attention because of their excellent strength and plasticity. However, the exact deformation mechanisms remain to be investigated. In this paper, a solidified lamellar Tiâ€"Al alloy with lamellar orientation at 0°, 17°, and 73° with respect to the loading direction was stretched by utilizing molecular dynamics (MD) simulations. The results show that the mechanical properties of the sample are considerably influenced by solidified defects and tensile directions. The structure deformation and fracture were primarily attributed to an intrinsic stacking fault (ISF) accompanied by the nucleated Shockley dislocation, and the adjacent extrinsic stacking fault (ESF) and ISF formed by solidification tend to form large HCP structures during the tensile process loading at 73°. Moreover, cleavage cracking easily occurs on the Îł / α 2 interface under tensile deformation. The fracture loading mechanism at 17° is grain boundary slide whereas, at 73° and 0°, the dislocation piles up to form a dislocation junction. [ABSTRACT FROM AUTHOR]

High‐quality TiAl alloy is used to fabricate aerospace equipment with excellent mechanical properties because of its interesting properties, including low density, high specific yield strength, and better corrosion resistance. However, the fabrication of high‐quality TiAl alloy is challenging because the connection mechanism between different types of clusters in TiAl alloy during the rapid cooling process is not clear. This study uses molecular dynamics (MD) simulation methods to study the connection models of icosahedral (ICO) structures and other defective ICO structures at different cooling rates. The ICO structures represent the short‐range ordered structure in the system. Moreover, the complex connections between the ICO and its defective structures constitute the basic structural features of TiAl metallic glass. The results show that the connections between ICO and its defective structures differ significantly with the cooling rate. It is easier to form a more complex nanostructure connected by the ICO structures with higher proportion of Al central atoms at the lower cooling rate. As the temperature drops at the same rate, the number of Al central atoms with smaller atomic radius in the icosahedrons increases gradually, giving them an absolute advantage. [ABSTRACT FROM AUTHOR]

[Display omitted] • Graphene enables copper nanodroplets to form hexagonal close-packed layers. • The TCP atoms in copper nanodroplets hinders the formation of crystal atoms. • Graphene improves the temperature and life-time of the liquid–solid transition. • Graphene affects the nucleation work and crystallization pathway of nanodroplets. • Wettability can tune the temperature and lifetime of solid–liquid coexistence. The microstructure determines the macro properties, so the chemical and physical properties of nanoparticles can be tuned by changing their structures. A molecular dynamics simulation for the rapid solidification of Cu nanodroplets on the graphene sheet is conducted to investigate the effect of substrate on nanoparticles. The phase transition, structure evolution, and crystallinity are quantified by the potential energy, the largest standard cluster analysis, and Bond-orientation order parameters. It is found that the solidification is inhomogeneous and stepwise crystallization. Interestingly, originating from the graphene wettability, different solid–liquid coexistence states are obtained that may have the potential to modify the chemical and physical reactions on nanoparticles. The temperature of this solid–liquid coexistence state depends strongly on the structural evolution of crystallization and thus can be regulated by the wetting properties of the graphene substrate. These results significantly extend the fundamental understanding of heterogeneous nucleation in nanodroplets, providing important insights for designing and fabricating composite materials with multifunctional properties. [ABSTRACT FROM AUTHOR]

The use of high-energy ions or clusters to bombard graphene is emerging as a new method for the theoretical study of graphene properties. In this study, using molecular dynamics simulations and empirical potentials, the behaviors of graphene after bombardment by C60 under different initial velocities from 13.7 to 15.7 km/s were investigated. The simulations showed four types of defects: Stone–Wales defects, single vacancy defects, multiple vacancy defects, and out-of-plane carbon adatoms. The self-healing phenomenon of defective graphene was observed. In the low-speed region (< 14.5 km/s), the self-healing ability of graphene is enhanced at higher temperature. However, the effect of temperature is not obvious in the high-speed region, where velocity dominates. To repair vacancy defects in graphene, a physical method was proposed. The initial positions of lost atoms were traced, and then, the atoms were slowly dropped into the vacancy defects to effect repair. The simulations provide a fundamental understanding of bombardment between graphene and C60 and propose a new method for repairing vacancy defects in graphene. [ABSTRACT FROM AUTHOR]

Quadruple Ti particles microstructure evolution by sintering. [Display omitted] • The sintering process of quadruple Ti particles is studied utilizing the MD method. • The initial arrangement of the particles affects the microstructure of the quadruple Ti particles. • The higher the sintering temperature, the easier the transformation of HCP-Ti into BCC-Ti. • A new calculation method of multiparticles fusion ratio is proposed. The sintering dynamics of binary particles have been extensively studied; however, the sintering behavior of multiparticles still needs further investigation because it plays a key role in many industrial processes and advanced technologies. Herein, we used molecular dynamics simulations to investigate the sintering dynamics of quadruple titanium (Ti) particles. We investigated two particle arrangement configurations of Ti particles, ααββ and αβαβ, at three sintering temperatures (900 K, 1300 K, and 2000 K) to explore the microstructural changes of Ti particles during sintering. Furthermore, we proposed a formula for calculating the fusion ratio of multiple particles. The results show that ααββ-Ti could retain its HCP structure when sintered at 1300 K and 900 K, whereas only BCC-Ti was observed after sintering at high temperatures. The effect of diffusion mechanisms was more substantial at high temperatures, and the higher the sintering temperature, the better the particle fusion effect. In addition to the effect of temperature on atomic migration, different particle arrangement configurations can impact atomic migration. [ABSTRACT FROM AUTHOR]

The growth of GaN crystals at different pressures was studied by molecular dynamics simulation employing the Stillinger–Weber potential, and their structural properties and defects were characterized using the radial distribution function, the Voronoi polyhedron index method, and a suitable visualization technology. Crystal structures formed at 0, 1, 5, 10, and 20 GPa featured an overwhelming number of <4 0 0 0> Voronoi polyhedra, whereas amorphous structures comprising numerous disordered polyhedra were produced at 50 GPa. During quenching, coherent twin boundaries were easily formed between zinc-blende and wurtzite crystal structures in GaN. Notably, point defects usually appeared at low pressure, whereas dislocations were observed at high pressure, since the simultaneous growth of two crystal grains with different crystal orientations and their boundary expansion was hindered in the latter case, resulting in the formation of a dislocation between these grains. [ABSTRACT FROM AUTHOR]

• The void defects significantly affect the deformation and plasticity in FeNiCrCoCu HEAs. • The deformation behavior reveals that the void is the main cause of initial strain. • The effects of different temperatures and strain rates on FeNiCrCoCu HEAs voids were explored. • The tensile strength significantly increases at a strain rate of 1010 s−1 compared to 109 and 108 s−1. While preparing and using high entropy alloys (HEAs), many defects such as voids are inevitably formed. The effects of voids on the mechanical properties of FeNiCrCoCu HEAs are investigated using molecular dynamics simulations. The evolution of voids is examined by using models with one or two voids considering different void sizes, applied strain rates, and temperatures. The results demonstrate that the existence of the voids does influence the mechanical properties of HEAs. The tensile strength of the single-void model is higher than that of double-void model. The stress–strain curves demonstrate that a larger initial void size could reduce the tensile strength. During the tensile deformation, all dislocation emission occurs initially from the surface of the void. However, it occurs between the two voids in double-void models, which causes the deformation of the voids. With temperature increases, the tensile strength decreases. Under various strain rates, the tensile strength slightly rises at strain rates ranging from 108to 109 s−1. However, it significantly increases at a strain rate of 109 to 1010 s−1. [Display omitted] The development of void defect in FeNiCrCoCu HEAs under the tensile process at 300 K and a strain rate of 109 s−1. (a) single-void model; (b) double-void model. [ABSTRACT FROM AUTHOR]

Recently, SiC has gained considerable interest owing to its avant-garde nature and relevance in future applications. Molecular dynamics simulation was performed to investigate nanopolishing of three-body diamond abrasives on workpieces that had undergone cluster deposition and annealing processes. The coordination number, temperature, atomic distribution, kinetic energy, friction coefficient, crystallization, and average height of the workpiece were determined. Further, the effects of different polishing depths and polishing speeds on the physical properties of the workpiece were determined. Results show that polishing depth considerably affects the coordination number, surface morphology, friction coefficient, crystallization, and temperature of the workpiece. The surface of the workpiece was polished while maintaining the crystallization rate at a polishing depth of 20 Å. With an increase in the polishing rate, the coefficient of friction decreased, the temperature of the workpiece increased, and the polishing trajectory became clearer, with a better crystallization at 1 Å/ps. [Display omitted] • The workpiece undergoes the process of deposition and annealed. • Three three-body diamond abrasives are parallel to each other, more like a "brush". • Friction coefficient of the abrasive is related to the temperature of the workpiece. • The height of SDL region is closely related to polishing depth and speed. [ABSTRACT FROM AUTHOR]

• High cooling rate 10 K/ps results in vitrification while two lower ones lead to crystal. • The topologically close-packed (TCP) atoms can better describe the structure of liquid and super-cooled liquid. • Crystallization cannot happen until TCP atoms drops off in number at the end of saturation stage. • The competition between TCP and crystalline atoms persists long enough will result in vitrification. 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. [ABSTRACT FROM AUTHOR]

Dislocation and twin structures often found in many metal alloys significantly affect their mechanical properties. However, observing the evolution processes of dislocation and twin structures using traditional experimental methods is challenging. In this paper, molecular dynamics was used to study the formation mechanism of dislocation and twin structures in Ti 3 Al during rapid solidification. Two microstructure types were obtained: nanocrystalline, composed of multiple grains, and lamellar. The lamellar structure is dominated by stacking fault and coherent twin boundary. According to the synthesis and decomposition of different types of dislocations, the formation and evolution of crystal defects under different models are analyzed. This provides a reference for understanding the mechanical properties of Ti 3 Al alloys with different microstructures. • The quenching process of Ti 3 Al with different relaxation time has been investigated and different mechanical properties have been obtained. •The dislocation reaction of crystallization process has been studied and a variety of twin structures have been found. •The tensile strength had a maximum value which has the large-angle twin and parallel TBs. [ABSTRACT FROM AUTHOR]

Gallium nitride (GaN) is a promising material for high-frequency and high-power electronics owing to its high thermal stability, wide bandgap, and high breakdown voltage. Monocrystalline GaN films are usually produced by metal organic chemical vapor phase deposition using Si, SiC, or Al 2 O 3 as the substrate. However, the difference in lattice constants and thermal expansion coefficients causes many defects in GaN epilayers. In particular, the formation of twins will strongly influence the electronic properties of GaN-based devices. In this study, the solidification process of GaN was simulated by molecular dynamics at a cooling rate of 1012 K/s, and the twin structures in GaN were discussed. The results of this simulation showed two types of twins, including coherent twin and rotating twin, which were analyzed by visualization technology. The formation of twins was related to the coexistence of zinc blende and wurtzite phases. Furthermore, the types of twins were closely associated to the stacking ways of zinc blende and wurtzite structures. In general, it is essential to study the microstructure and the structural evolution of twin GaN, in order to give references to experiments about the growth and characteristics of twins. • Zinc blende and wurtzite nucleated randomly and formed lots of complex structures at the beginning of crystallization. • Coherent and rotating twins were formed according to different stacking methods of zinc blende and wurtzite structures. • The rules for the combination of twin structures in GaN crystals have been summarized and it can be generalized. • An error in the stacking sequence can cause the metastable phase c-GaN to start seeding the h-GaN phase. [ABSTRACT FROM AUTHOR]

The two-layer nanotubes consisted of carbon atoms on the outside layer and silicon atoms on the inside layer (CNT@SiNT) show a series of diversity in the shape transitions, for instance transforming from a circle through an oval to a rectangle. In this paper, we investigate this geometric change from three perspectives. In the first aspect, we stationary time, followed by quantize in the three-dimensional Z-axis of nanotubes. In the second aspect, we stationary Z-axis, followed by quantize in the time. Finally, we tracked distance of nanotubes flattest section and roundest section. At the stationary time, the overall image of different Z-axis distance distributions is similar to a plan view of multiple ice creams, regardless of whether CNT or SiNT are on the same Z-axis, their slice plans are circle or rectangle of the projection of the Z-axis section on the XOY plane. In the stationary Z-axis, the nanotubes periodically change from a circle to an oval, and then from an oval to a rectangle at different times. Most remarkably, the distance value of deformation which we track the flattest and roundest is a constant value, and in the same distance period, there is only one roundest circle and one longest rectangle at different section and different time. The geometric analysis provided theoretical reference for the preparation of various devices and semiconductor nano-heterojunctions. [ABSTRACT FROM AUTHOR]

In this study, the solidification processes of liquid GaAs were investigated by molecular dynamics simulation at different cooling rates of 1010 K/s, 2 × 1010 K/s, 5 × 1010 K/s, 1011 K/s, 1012 K/s, and 1013 K/s. The structural evolution of GaAs was investigated in detail by the radial distribution function, average atomic energy, average coordination number, and visualization technology. The results show that segregation phenomena occur to different extents during the quenching of GaAs at the six cooling rates. The segregation areas of As tend to be simple cubic structures that are considerably influenced by the cooling rate. For a cooling rate of 5 × 1010 K/s, the system forms a large number of segregation structures at a crystallization temperature of approximately 690 K. When the temperature is higher than the crystallization temperature, the As atoms of the segregation areas are in a disordered state, whereas at a lower temperature, some of the As atoms in the segregation rapidly crystallize to form the simple cubic structure. [ABSTRACT FROM AUTHOR]

Medium-range order structure is the one of the important local structural units in the field of bulk metallic glasses. The rapid solidification of Cu 50 Zr 40 Al 10 ternary alloy is investigated by molecular dynamics simulation. Simultaneously, the connections between the full icosahedral and defective icosahedral clusters in medium-range order structures of Cu 50 Zr 40 Al 10 alloy are characterized systematically by the quantitative method and visualization analysis. The results reveal that the icosahedral clusters as the main structures with high structural stability are bonded with each other by interpenetrating connection, and form the triangle, double triangle, triple triangle, tetrahedron and chain connected icosahedral medium-range order clusters. Moreover, the icosahedral and defective icosahedral clusters prefer to link with each other and form the bigger medium-range order clusters with different configurations in this system. These structures keep the high structural stability of icosahedral medium-range order section and the structural variability of chain-like medium-range order structures. • The medium-range order clusters connected by short-range order structures with various modes are investigated. • <0 0 12 0> preferred to connect with itself and defective icosahedral clusters to form medium-range order clusters. • Icosahedral clusters are bonded with each other in different modes to form the icosahedral medium-range order clusters. [ABSTRACT FROM AUTHOR]