Yongbing Dai

First-principles calculations on the stability of Al∕TiB2 interface

The atomic structure, adhesion, and interfacial energy of Al∕TiB2 interfaces were studied using first-principles calculations. The preferable interfacial atomic structure is that wherein the Al atoms continue the natural stacking sequence of the bulk TiB2. The interface adhesion for Ti-terminated interface is stronger than that for B-terminated interface. The structural stabilities of two interface terminations depend greatly on the Ti interfacial chemical potential. Calculation of the interfacial energies provides theoretical evidence for the hypernucleation and duplex nucleation grain refinement mechanisms.

First-principles study of TiB2(0001) surfaces

The TiB(2)(0001) surfaces are calculated using the first-principles total-energy plane-wave pseudopotential method based on density functional theory. It is found that there are large relaxations within the top three layers for both termination surfaces, and the outermost and second interlayer relaxations for B-terminated surfaces are much larger than those for Ti-terminated surfaces. The charge depletion in the vacuum and the charge accumulations in the interlayer region between the first and second layers reinforces the interlayer Ti-B chemical bonds and reduces the outermost interlayer distance. Simultaneously, the charge accumulation for B-terminated surface is more than that for Ti-terminated surface, and the interlayer Ti-B bonds between the second and third layers are weakened more for the B-terminated surface. The Ti-terminated surface is thermodynamically more favourable in most of the range of μ(B)(slab).

Atomic study on the ordered structure in Al melts induced by liquid/substrate interface with Ti solute

Atomic ordering in Al melts induced by liquid/substrate interface with Ti solute was investigated by ab initio molecular dynamics simulations and in-situ synchrotron X-ray diffraction. It is predicted that deformed nanoscale ordering Al layers with a rhombohedral-centered hexagonal structure ( R 3 ¯ m space group) instead of the intrinsic fcc structure ( F m 3 ¯ m space group) form on substrate at temperature above Al liquids. With Al atoms stacking away from the interface, the ordering structure reaches a critical thickness, which inhibits the consecutive stacking of Al atoms on substrates. The locally stacking reconstruction induced by Ti atom relieves the accumulated elastic strain energy in ordered Al layers, facilitating fully heterogeneous nucleation on substrate beyond the deformed ordering Al layer around the melting point. The roles of liquid/substrate interface with Ti solute in the physical behavior of heterogeneous nucleation on substrate were discussed.

In situ study on columnar-equiaxed transition and anaxial columnar dendrite growth of Al-15%Cu alloy by synchrotron radiography

Directional solidification of Al-15% (mass fraction) Cu alloy was investigated by in situ and real time radiography which was performed by Shanghai synchrotron radiation facility (SSRF). The imaging results reveal that columnar to equiaxed transition (CET) is provoked by external thermal disturbance. The detaching and floating of fragments of dendrite arms are the prelude of the transition when the solute boundary layer in front of the solid-liquid interface is thin. And the dendrite triangular tip is the fracture sensitive zone. When the conditions are suitable, new dendrites can sprout and grow up. This kind of dendrite has no obvious stem and is named anaxial columnar dendrites.

Hydrogen Diffusion in Aluminum Melts: An ab initio Molecular Dynamics Study

The diffusion process of hydrogen in aluminum melts was investigated by molecular dynamics simulation. The pair correlation function, first peak position, and coordination number was calculated and differences in the structural properties among Al-H, Cl-H, and Al-Cl pair were examined. The mechanism of chlorine on improving hydrogen diffusion was discussed. From an ab initio molecular dynamics calculations, the diffusivity of hydrogen in liquid aluminum as D(T)=(0.118×10−4m2/s)exp(−0.316 eV/kT) is obtained, which is in good agreement with the experimental data. Correspondingly the diffusivity with presence of chlorine is promoted as D(T)=(0.09×10−4m2/s)exp(−0.251 eV/kT). It can be concluded that the diffusion of hydrogen in aluminum melts can be enhanced in the presence of chlorine.

Atomic-scale study of boron implantation into amorphous carbon

Boron implantation into amorphous carbon substrate has been investigated by molecular-dynamics simulation based on Tersoff empirical potential. The results show that the implanted boron atom is mainly fourfold coordinated. The average size of the implantation-affected region increases linearly with the kinetic energy of the incident boron atom from 150to300eV. Boron implantation leads to a great increase of the total number of the rings in amorphous carbon network and the larger the kinetic energy, the bigger the increasing number. A time-resolved analysis shows that the implantation process can be naturally divided into four stages, among which the second one featuring a decreased coordination number could be analogous to an endothermic reaction, while the third one featuring an increased coordination number could be analogous to an exothermic reaction, which explains why lower substrate temperature and higher thermal conductivity are favorable for achieving higher average coordination number.

Microstructure Evolution in High Purity Aluminum Single Crystal Processed by Equal Channel Angular Pressing (ECAP)

Aluminum single crystal with 99.999% purity was deformed at room temperature by equal channel angular pressing (ECAP) up to 16 passes. Grain size and misorientation of processed samples were quantitatively characterized by TEM and EBSD. The results show that the refinement efficiency of high purity aluminum single crystal was poor in the initial stage. Extrusion by fewer ECAP passes (n ≤ 8) resulted in only elongated grains containing a large number of subgrains and small misorientations between grains. Stable microstructures of nearly equiaxed grains with high misorientations were obtained by 15 passages, indicating that the initial extremely coarse grains and highly uniform grain orientation are not conducive to the accumulation of strain energy. The initial state of high purity aluminum has a significant effect on the refining efficiency of the ECAP process.

Study of columnar-equiaxed transition and anaxial columnar dendrites growth of hypoeutectic alloy with synchrotron radiation

Among solidification processes, the columnar to equiaxed transition (CET) is critical component for formation either of microstructure or macrostructure. Directional solidification of Al-Cu alloy was performed at the BL13W beamline of Shanghai Synchrotron Radiation Facility (SSRF). The imaging results reveal that CET was provoked by external thermal disturbance. The fragment of dendrite tips were come from solute-rich-zone. And they floated to the mushy zone to form the equiaxed dendrites. As the conditions are suitable, a new dendrite morphology sprout and grow up, in which dendrites grow along directions in a binary Al-Cu alloy. These dendrites have no obvious primary arms and were named anaxial columnar dendrites.

Solidification structure refining of 409L ferritic stainless steel using Fe-Ti-N master alloy

A novel in-situ synthesized Fe-Ti-N master alloy was employed as a grain refiner for refining of the 409L ferritic stainless steel solidification structure. Two groups of experiments have been carried out to test the grain refinement performance of the Fe-Ti-N master alloy. In one group, 2 wt% of the Fe-Ti-N master alloy has been added into 409L ferritic stainless steel melts with various temperatures just before casting. In the other group, 409L ferritic stainless steel melts with 2 wt% Fe-Ti-N master alloy added at 1803K were held for various times before casting. It was found that high melt temperature resulted in a lower proportion of equiaxed grains and coarser structures. As the holding time increased, the proportion of the equiaxed grain zone decreased quickly. However, the proportion of the equiaxed grain zone re-increased when the holding time was extended longer than extreme points. The mechanisms of these experimental phenomena have been analyzed in terms of thermodynamics.