Materials Science Forum Vol. 850

Abstract: The low-index (001), (111) and (110) surfaces of austenitic NiTi were investigated by the use of first principles calculations. The calculated results showed that the non-polar NiTi (110) surface was the most stable under most of the Ti chemical potential. The polar Ni terminated NiTi (001) surface was the most stable under Ni-rich conditions. The Ni-terminated surfaces were more stable than their corresponding Ti-terminated surfaces in the entire range of Ti chemical potential. The surface interlayer relaxations of the Ni-terminated surfaces were much larger than those of the corresponding Ti-terminated surfaces. The Ti atoms in the surface layer of the non-polar NiTi (110) surface were more outward than Ni atoms.

Abstract: The research on the decarbonizing behavior of the austenite region of SCM435 steel was carried out. And the experimental results shewed that the relationship between the diffusion coefficient and temperature totally agreed with the Arrhenius equation and that the diffusion constant and the diffusion activation energy were uniform within the temperature range of 900-1100°C. However, when the austenite reached certain temperature, the carbon diffusion coefficient decreased significantly as temperature increased and its relationship with temperature no longer agreed with the Arrhenius equation.

Abstract: Aluminum melt structures play significant roles in process of impurity element eliminations. In order to investigate the aluminum melt structures containing small amount of silicon element, several compositions of aluminum melts were simulated. Molecular dynamics simulation method was employed to investigate the melt structures, concentration profiles and diffusion properties. For purpose of studying structure stability, related calculations were performed by first principles molecular dynamics method. The calculated results suggest that silicon concentration has obvious influence on silicon-silicon radial distribution function; silicon element segregates in local zone in each concentration of aluminum; there is no obvious relationship between silicon concentration and silicon diffusion coefficient; the stable Al-Si phase is formed by substitution lattice point or interstitial space; and position of Fermi energy of the stable structure corresponds to the valley of density of states.

Abstract: In this study, the deformation process of AZ31 magnesium alloy during equal channel angular pressing (ECAP) was simulated using the commercial software Deform-3D under different extrusion condition (passes and temperatures). To investigate the effect of temperature and deformation rate on grain refinement, the rules of flow and deformation homogeneity and also the extrusion load during ECAP was discussed. The simulation results indicate that the AZ31 magnesium alloy obtain homogenous and larger strain magnitude after 4 passes ECAP at 250°C~275°C. To verify the 3D finite element simulation results, the microstructure in the cross-section was observed. It shows that the grain of AZ31 magnesium alloy is homogenous refined by finite element method (FEM) results, thus the mechanical property is improved.

Abstract: The deposition of a single Cu cluster on the Si (001) and Si (111) surfaces was studied by molecular dynamics simulations. EAM, SW and Morse potentials were used to describe the interaction of Cu-Cu, Si-Si, and the Cu-Si atoms, respectively. The results show that the crystallographic surface of the substrate had a large effect on film growth. On the Si (111) surface, the cluster structure demonstrate good uniformity and epitaxial relation to the substrate. There is no appreciable intermixing or silicide formation at the film-substrate interface, nor silicon segregation on the surface with incident energy between 0.1 and 1.0eV/atom. However, as the incident energy increases to 3.0eV/atom, intermixing began to appear for Cu₁₃ and Cu₁₉ clusters. For the Cu₁₄₇ cluster intermixing starts to appear at a lower energy of 1.6eV/atom. In contrast, intermixing appears on the Si (001) surface for a range of incident energy from 0.1 to 3.0eV/atom for all the three clusters studied. Furthermore, the epitaxial relation with the Si substrate result in (001)- or (111)-oriented Cu clusters on Si (001) or Si (111), respectively. This behavior is consistent with experimental observations. We found that the effect of cluster deposition on film property depends not only on the substrate-film material combination and cluster size but also on the substrate crystallographic orientation and incident energy.

Abstract: Large steel ingots are the important material for the equipment manufacturing industry. It is still difficult to predict and control the macrosegregation in ingot. In this paper, the cooling curves at the surface of ingot and temperature variation of the mold were measured. The carbon distribution was measured through the local region dissection of ingot. Then, based on the definite the heat transfer coefficient at the interface of mold/ingot, a two-phase model with consideration of the motion of equiaxed grains is applied for the prediction of macrosegregation in 160-t steel ingot formed during the solidification. The results indicate that the heat transfer coefficient at the interface of mold/ingot decreases sharply after starting solidification and then varies slowly. Negative segregation at the bottom of ingot forms due to the interaction of solidification interface and equiaxed grains deposition during solidification. The positive segregation appears in the riser with thanks to the solidification shrinkage and the floating enriched solute. Finally, the results of the predicted and the measured are in good agreement.

Abstract: A phase field model has been established to simulate the grain growth of AZ31 magnesium alloy containing spherical particles with different sizes and contents under realistic spatial-temporal scales. The expression term of second phase particles are added into the local free energy density equation, and the simulated results show that the pinning effect of particles on the grain growth is increased when the contents of particles is increasing, which is consistent with the law of Zener pinning. There is a critical particle size to affect the grain growth in the microstructure. If the size of particles is higher than the critical value, the pinning effect of particles for grain growth will be increased with further decreasing the particle size; however the effect goes opposite if the particle size is lower than the critical value.

Abstract: In this study, size-dependent bond length of metallic clusters is established by introducing bond number. This model, free of any adjustable parameters, can be utilized to predict the change rule of bond length with size. If the atomic structure of a cluster is known, the size and shape-dependent bond number are obtained. The cubooctahedral structure is taken for simplicity to describe the shape and geometric characteristics of metallic clusters. It is found that the bond length decreases with the decreased size of metallic clusters, which is due to the structure relaxation and enhanced single bond energy. The theoretical predictions are consistent with the evidences of the simulations for Au and Ag clusters. This confirms the validity of taking cubooctahedron structure, even if the simulated Au and Ag clusters are not cuboctahedron ones. This can be expected to other metallic clusters even with other atomic structures.

Abstract: In this study, Mn, Zn and Si elements which are the most common alloying elements in Al, were chosen as solute atoms to be analyzed. The structure of molten Al, local structure around solute atoms and diffusion of the solute atoms are studied using ab initio molecular dynamics simulations. The results show that minimum addition of a solute (1 atom) does not significantly influence the structure of liquid Al as a whole. However, the local structure around foreign atoms varies dramatically for the different solute species. The local structure around Mn is the most compact and stable among the three types of solute atoms, leading to its lower diffusion coefficient. Conversely, Si possess the highest diffusion coefficient among those three kinds of elements derive from the local structure around Si is the most relaxed structure. In summary, the close packing and stable spherical shell around the solute atoms affect their diffusion behaviors in the melt. In addition, it is suggested that more alloying elements should be investigated to corroborate the results of this study.

Abstract: Plasticity zone at crack tip of aluminum alloy with fcc structure is investigated in order to analyze the effect of crystal orientation to the plasticity distribution on crack tip, as well as the effect to CTOD and J-integral, which is implemented using finite element code in Abaqus with a rate dependent crystal plasticity theory. The results show that the crack tip plasticity, stresses and CTOD are significantly affected by grain orientations. When the grains have single textures, Cube and S orientations have a strong ability to against crack propagation. However, when the grains combine textures, the increasing of misorientation enhances the resistance for crack growth. And when the tilt angle is higher, the crack deflection is promoted to reduce the crack propagation rate.