Materials Modeling, Simulation, and Characterization

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Authors: Xi Zhong An, Guo Quan Liu, Chao Li
Abstract: The growth of nano CVD diamond films on low index faces such as (111) face and (100) face under different proposed surface chemical reaction model was simulated by using Kinetic Monte Carlo (KMC) method from atomic scale. The results, for example the influence of deposition time t, substrate temperature Ts, and atomic hydrogen concentration [H] on the film deposition rate, surface roughness, and H embedded in the film under different processing conditions, were systematically analyzed and compared. And the adsorption of various species on {111}-oriented diamond cluster was preliminarily computed from electronic scale by Local Density Approximation (LDA) method to assist understanding the surface adsorption mechanism. It is indicated that the film morphology and quality obtained from atomic scale KMC simulation varies according to the chemical reaction models. And our initial electronic scale computation on {111}-oriented diamond cluster showed that single-carbon species can be adsorbed on the activated site more easily than double-carbon species and the former will result in a more stable state than the latter. In order to reveal the nano CVD diamond film growth mechanism, more work about various species adsorption on many different morphological CVD diamond surfaces is needed.
Authors: Hui Li Wei, Feng Mao, Xin Yu Tan, Xiang Ping Huang, Zhao Wang, Hui Jin Xu, Chang Yuan Zhang, Jia Yi
Abstract: The numerical simulation of high-energy femtosecond laser ablation on metal target is studied in this paper. Based on the two-temperature model (TTM), a new model considering the effects of the electron density of states (DOS) on electronic heat capacity, electron-phonon coupling coefficient and electronic thermal conductivity is established. As an example of gold target, the relationship between the melting threshold and the thickness of gold films is numerically calculated. Our result is more consistent with the experimental datum in contrast to the results without considering the DOS effects. This shows that the revised TTM of high-energy femtosecond laser ablation (i.e. DOS-TTM) is more reasonable compared with general used TTM.
Authors: Meng Ou Tang, Jun Xu, Zhi Feng Zhang, Yue Long Bai
Abstract: Annular electromagnetic stirring (A-EMS) process is an advanced semi-solid metal processing technology. It could avoid the effect of skin effect, increase the shear rate, reduce the temperature gradient and refine microstructures. A three-dimensional computational model sequentially coupling electromagnetic stirring with a macroscopic heat and fluid flow in A357 alloy semisolid slurry preparation by A-EMS has been developed. Comparison between microstructures of A357 alloy produced by EMS and A-EMS has been made. The results show that the annulus gap avoids the part of low magnetic induction intensity and A-EMS avoids the effect of skin effect. The annulus gap prohibits the circular flow and increases the shear flow. The circular flow is more effective than the shear flow on heat dissipation. A lower temperature difference in the stirred melt and subsequent uniformly fine microstructures were obtained as compared with the normal electromagnetic stirring.
Authors: Hong Tao Yu, Wen Bo Zhang, Jing Song Liu, Lin Hong Cao, Han Xing Liu
Abstract: In this work, we have proposed a simple model to predicte the dielectric constant of CaCu3Ti4O12-SrTiO3 composite ceramics. The model has been established based on the analysis of composition and microstructure. Two different compositions in the composite ceramics correspond to two different grain sizes, large for CaCu3Ti4O12, and small for SrTiO3, which has been identified by the x-ray diffraction, the scanning electron microscope and the energy spectrum analysis. All specimens have been assumed to be the barrier layer dielectrics in the model, according to the complex impedance spectra. The dielectric constant of serial, parallel, and logarithmic mixture models has been discussed. Compared with the experimental results, the model shows the similar variation tendency as SrTiO3 content increases.
Authors: Yan Xu, Kai Zhang, Hong Liang Zheng, Yu Cheng Sun, Xue Lei Tian
Abstract: It is very important to predict the hot spots of castings properly, which is known as a criterion for riser design. In this paper, an improved geometric model for hot spot prediction is proposed, and subsequently, its application to hot spot analysis is presented. As we know, the heat dissipation potential of a location in a casting depends on its distance to the heat transfer surfaces. In a meshed casting, the reciprocal of distance from a certain cell to surfaces is calculated at all the six orthogonal directions, by which the heat dissipation potentials of every cell will be evaluated considering the influences of the neighboring grids. With the improved geometric model, there is no iteration during calculation, and only twice of cell traverse is required. The first traverse gets the distance reciprocal and the second focuses on the heat dissipation potential. The result of this model, which turns out similar to that of procedures based on heat transfer equations, reflects solidification sequence in a casting, hence the hot spots will be known instantaneously. Obviously this geometric model ignores many conditions during solidification process. However, messages like locations of hot spots are shown much faster and more conveniently than that of procedures based on heat transfer equations. Therefore, it is believed that it will shorten much time for casting technology design.
Authors: Ze Feng Liu, Qing Sen Meng, Shao Ping Chen, L.J. Liang, P.F. Xue
Abstract: TiB2-TiC+Ni/TiAl/Ti graded materials were prepared by field-activated pressure-assisted synthesis process (FAPAS) and the mechanical properties and residual stress were investigated. Shear fracture that occurs at the interface between the cermets and TiAl with the maximum shear strength of 85.88 MPa. The residual stress and deformation induced by the thermal effect of chemical reaction and Joule heat during the synthesis process were analyzed by nonlinear finite element simulation. It is demonstrated that the maximum equivalent residual stress locate in the transition layer between the ceramics and TiAl, consistent with the shear test result.
Authors: K.M. Fan, Li Yang, Shu Ming Peng, Xing Gui Long, Z.C. Wu, Xiao Tao Zu
Abstract: A reactive interatomic potential based on an analytical bond-order scheme is developed for hexagonal close-packed (hcp) scandium, and the model is fitted to the lattice parameters, elastic constants, cohesive energy and vacancy formation energy of scandium. The potential was used to calculate the structural energy differences of bcc-hcp, fcc-hcp, sc-hcp and diamond-hcp, as well as self-interstitial atom (SIA) formation energy, vacancy migration energy, divacancy binding energy, surface energy and stacking fault energy. The developed potential is shown to be able to reproduce energetics and structural properties of hcp-scandium.
Authors: Min Jie Liang, Hai Hong Liao, Wen Jiang Ding, Zheng Chen
Abstract: The influence of temperature on atomic site occupation probability (SOP) in L12--Ni3(Al1-xFex) has been studied using microscopic phase-field simulation for Ni3Al-Fe alloy with the lower Fe content. Microscopic phase-field kinetics model was further extended to study the temporal evolution of atomic site occupation and atomic anti-sites behavior, which was revealed in detail at the different temperatures. The results show that the size, number and the precipitation mechanism of L12 phases also present obvious changes with the increase of temperature. The SOP of each atom on its right-sites decreases slowly; while the SOP of Fe atoms are far lower than those of Al atoms. On the other hand, the SOP of each atom on its anti-sites increases gradually. The formation of NiAl (Ni atoms form anti-sites on Al sites) is more easily and the formation of NiAl is more sensitive to temperature than that of FeNi (Fe atoms form anti-sites on Ni sites). Atomic site occupation will not evolve during the early stage of precipitation, which becomes more and more obvious.
Authors: Fu Ling Tang, F.C. Wan, X.Q. Dai, W.J. Lu
Abstract: We studied in detail the lattice transition and local lattice structure (including Jahn-Teller distortion) in LaMnO3/SrMnO3 surperlattices by classical atomistic simulation. For a certain doping density, it is found that the superlattices with short modulation period have small lattice energies and larger differences among lattice parameters a, b/√2 and c. The average La-Mn (Mn3+-O) distance is larger than the average Sr-Mn (Mn4+-O) distance for all doping densities and superlattice configurations at certain doping density. The standard deviation of Mn-O bond lengths and Jahn-Teller distortion of MnO6 octahedra have been calculated. Both the standard deviation and Jahn-Teller distortion of Mn3+O6 octahedra in the superlattices are much smaller than those of Mn3+O6 octahedra in LaMnO3, while Mn4+O6 octahedra in the superlattices have the smallest lattice distortion, but larger than those in SrMnO3.

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