Papers by Author: Lin Zhang

Abstract: A numerical model describing the liquid-liquid phase transformation of the nucleation, the diffusional growth, Ostwald coarsening and macro-transport phenomena was established based on Euler-Euler method. The microstructure development of Al-Bi alloys in different solidification rate has been simulated by coupling the calculated temperature and velocity fields with the kinetic equation which controls the microstructure evolution. The results showed that average diameter difference of L₂ phase droplets between top and bottom of samples in low cooling rate increased by 151 % than in high cooling speed and the maximum volume fraction of the droplets at the bottom of the sample in low cooling rate much higher than in high cooling rate. Analysis that the bigger cooling rate can shorten the action time which caused by gravity settling and collisions coagulation of the droplets, and then improves the macrosegregation of solidification structure in favor of uniform distribution of the solidification structure.

Abstract: The solidification experiments about macrosegregation formation of the Al-10%Bi hyper monotectic alloys under gravity conditions have been carried out. The results showed that the average diameter of the Bi-rich droplets linearly increases and the number density of the Bi-rich droplets exponentially decreases with solidification time under the gravity condition. Because of gravity settling and collisions coagulation between the droplets, area fraction of Bi-rich increased rapidly in the bottom of samples during early solidification. It’s easy to form Bi-rich layer at the bottom of the sample. The analysis demonstrates that nucleation and diffusion growth of drops are the dominant factors influencing the solidification microstructure during the early solidification and the same distribution of Bi-rich in different locations of sample. As the solidification process, gravity migration and collision coagulation beginning to play the leading role, lead to the difference in the distribution of Bi-rich droplets in different locations of sample 90%e5%a2%9e%e5%a4%a7&tjType=sentence&style=&t=increases+gradually" increases gradually in the same time. It caused macrosegregation of the final solidification microstructure under the gravity condition.

Abstract: Silicene is a single atomic layer of silicon thin film structure, its structure is similar to the graphene, the hexagonal lattice structure with pleats. In recent years it has aroused widespread concern because of its unique physical properties. In this paper, use the tight-binding method (DFTB) of density functional theory calculation of the variation of silicene structure and structural changes in the bond lengths and bond angles. The results show that for small sized silicene its structure is unstable and relatively large changes. With the increase of the size of silicene, the structure tends to be stable and some structures appear symmetric. When the difference between the X coordinate values of the left and right borders silicene initial structure reaches 38 Å, the structure is completely symmetrical on both sides.

Abstract: In an effort to develop into a practical application of TiAl alloys for aerospace materials, Researchers at home and abroad in the use of alloying and thermal processing methods to improve the brittleness and other areas a lot of work.The existence of cracks which led to the kinds of material degradation of the major causative factor. However, it is difficult for us to observed the change of the local atom structure of the material by experiments in the limitation from our experimental conditions, nevertheless, computational research provide us the possibility that we can observe the evolution of the structure. Molecular dynamics calculation is considered to be well suited to describe the change of potential energy of the atoms in such a system. Embedded atom method (EAM) and canonical ensemble (NVT) molecular dynamics simulations have been carried out to obtain pre-cracks of different lengths of the structure of TiAl alloy. According to the average energy of atoms and the radial density distribution function, with increasing temperature the changes, analysis of TiAl alloy films of micro-cracks in the heating process, the structure changes with temperature.

Abstract: In this paper, structures and electronic properties of atomic chains with 5 to 20 silicon atoms and different atomic distances (d = 1.652 ~ 2.752Å) were calculated by the tight-binding method based on density functional theory. The results showed that the majority of the silicon atomic chains were symmetrical structures. When the number of silicon atoms was small, the silicon atomic chains were linear, when the silicon atomic chains had seven or more silicon atoms zigzag structures appeared. With the increase of the distance between atoms, atomic chains were gathering. When the number of silicon atoms was between 10 and 20, the charges on the silicon atoms appeared as a symmetrical distribution. With the increase of the number of atoms, the energy of silicon atomic chains decreased gradually. As the distance between atoms and atomic number changed, HOMO (highest occupied molecular orbital electrons) -LUMO (lowest unoccupied molecular orbital electrons) energy gap changed as well.

Abstract: We use the cellular automaton (CA) modeling to investigate the ferrite nucleation on the austenite grains. On the basis of the thermodynamics and kinetics of phase transformation from austenite to ferrite, the CA modeling demonstrates that the size of nucleated ferrite grains is increased with increasing of cooling rates, and nucleation process is finished instantly at a given cooling rate. The initial austenite grain size plays an important role in the obtained ferrite nucleation number, and the potential nucleation cells are increased.

Abstract: The lowest-energy geometrical structures of a cluster containing 55 atoms were searched by using the Density Functional Tight Binding (DFTB) combined with unbiased global optimization genetic algorithms (GAs) method. Two lowest-energy structures were obtained for the Si₅₅ cluster with the appearance of “Y shape” and “like-spherical shape” configurations. The configuration dependence average energy, highest occupied and lowest unoccupied molecular (HOMO-LUMO) gap, electron transfer and molecular dipole moment were also discussed in details for this cluster.

Abstract: Mechanical spectra of an Al-12.7Si-0.7Mg alloy under extrusion and the solid solution treatment followed artificial aging (T6) states were measured at different frequencies. It has been found that their spectra present differences. There was an obvious internal friction peak near to 200°C with increasing the temperature. The value of internal friction presented increase started from 300°Cand height of the peak was low, there existed a peak near to 270°C during the cooling processes in the spectrum for the extrusion state. For the T6 state alloy, no peaks could be observed during the heating processes. With decreasing the temperature, the internal friction peaks were again observed near 200°C and 270°C.

Abstract: TiAl alloys have great potential because of its low density and the outstanding performance at high temperature. However, the brittleness influences its industrialization process. It is known that the macroscopic nature is greatly influenced by its microscopic structure, and the fault development plays a vital role during the material working process. The paper performed the molecular dynamics (MD) study of the thermodynamic shear deformation in TiAl/Ti₃Al system to promote the understanding in this aspect. Above all, we adopt a special shear deformation model based on the experimental consideration, and conduct the optimal calculation of the related parameters. Then, a series of thermodynamic deformation simulation were carried out using the previous optimized model. The analysis of the potential variation and the structural snapshots showed that the shear deformation is related with the “stick-slip” behavior. The Ti₃Al (TiAl) shows obvious (little) covariant deformation stage before the initiation of the fault transition. For Ti₃Al region near the interface, the final structure is the continued FCC stacking. For TiAl, twin and SISF are observed and the block of twin is the main remnant. The atomic diffusion is locally observed in Ti3Al phase. The interface transits the energy and counterpoises the deformation between the hetero-phases.

Abstract: Relaxation and local structure changes of a molten TiAl alloy film during quenching have been investigated by molecular dynamics simulations within the framework of embedded atom method (EAM). The details of atom motions are analyzed using mean square displacement (MSD). Accompanying with massive atom rearrangement at a certain quenched temperature and time, local structural patterns are identified by decomposing peaks of pair distribution functions (PDFs) according to the pair analysis(PA) technique. The relaxation factor clearly reveals two relaxation processes involving in slow relaxation and fast relaxation of the quenched liquid TiAl film. Concerning the studied film, the obtained results reveal how quenched temperatures affect local structure changes.