•The correlation between local structures and diffusion of the fourth period transition metals were studied by AIMD.•The diffusion coefficients of solute atoms are influenced by their local structures, the 1551 bonded pair plays a fatal role.•The PCFs and packing densities illustrate a parabolic behavior which can be well explained by the DOS of the solute atoms.The fourth period transition metals (FPTM) atoms were chosen as solute elements of Al binary alloy to be analyzed. The local structures around solute atoms, diffusion coefficients and the electronic densities of states of solute atoms in dilute Al-FPTM molten alloys were studied by ab initio molecular dynamics simulation. We find that the local structures around solute atoms from Sc to Zn varies dramatically which are corresponding to the various electronic densities of states of solute atoms. The different local structures lead to the various migration behaviors of solute atoms. The diffusivity of solute atoms are related to the packing density of local structure, while the local environment (1551 bonded pair) plays a more fatal role in determining the diffusion coefficient.Download high-res image (181KB)Download full-size image

•The local structure around solute atoms in molten Al was studied by AIMD.•The local structure around Fe shows a higher packing density than the ones around V, Ti and Si.•The diffusion coefficient of Fe is lower than V, Ti and Si in the melt.•The characteristics of Si in molten Al are greatly different with Fe, V and Ti.In this study, the elements Fe, Si, V and Ti, which exhibit typical segregation behavior in Al, were chosen as solute atoms to be analyzed. The structure of molten Al, the local structure around solute atoms and the diffusion of the solute atoms were studied using ab initio molecular dynamics simulations. The results showed that the 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 Fe was the most compact and stable among the four types of solute atoms, leading to its lower diffusion coefficient. Conversely, the local structure around Si was the most relaxed structure. For the transition metal elements (Fe, V and Ti), there was a correlation between the equilibrium partition coefficients and local structures around these solutes in the molten Al. This simple relationship between the solute atmosphere, chemistry and temperature, requires further study. In summary, the close packing and stable local structure around the solute atoms can affect both their diffusion and segregation behavior in the melt. In addition, we suggest that more transition metal elements should be investigated to corroborate the results of this study.

•Cluster aggregates in the Al–Cu melt present as incompact 3D chain network structure.•Added TiB2 particles can break the incompact 3D chain network structure.•This process was more effective than increasing the temperature.•Appearance of microstructure in Al–Cu alloy melt was prohibited by TiB2 particles.Small angle X-ray scattering (SAXS) studies were carried out in Shanghai Synchrotron Radiation Facility (SSRF) to study the effect of addition of TiB2 particles on the melt structures of Al-15 wt% Cu. It was found that doping with TiB2 particles could dramatically reduce the sizes of the melt aggregates at temperatures ranging from the melting point to 800 °C. The results show that the aggregates in the Al–Cu melt present mass fractal characteristics and distribution of incompact 3D flocs.

A new dendrite morphology, anaxial columnar dendrites, was found in directional solidification using the real-time X-ray imaging technique. The dendrites are composed of a pair of stems, which are divided by a narrow liquid zone located in their center. The formation process of a free dendrite in the melt indicates that it grows along < 110> directions. According to the basic morphology of the dendrites, a tip model was developed to explain the growth preference of the secondary arms. The formation of anaxial columnar dendrites indicates that the dendrite morphology is closely related to the selection of the growth direction of the dendrites.Highlights► A new dendrite morphology in directional solidification of an Al–15 wt.% Cu alloy. ► Growing along < 110>, this newly observed dendrites are composed of a pair of stems. ► There is a solute enrichment in the melt around the top of these dendrites. ► Microscopic heat flow in the dendrites is analyzed. ► Growth preference of the secondary arms.

•The morphology of dendrite structure at the region of variable cross-section presents the fan-shaped distribution due to the arc-shaped temperature field.•The solute accumulate to the bottom region of the variable cross-section to cause massive eutectic and induced solute convection.•The addition of refiners can greatly refine microstructure and eliminate the variable cross-section effect.The formation of microstructures during solidification is strongly affected by the interaction that occurs between factors, such as heat field, melt flow, solute distribution, and number of effective nucleation cores. In this study, the microstructure evolution of a high-purity hypoeutectic Al–Cu alloy at its region of variable cross-section was studied using synchrotron radiation imaging. The characteristics of the thermal field, solute field, and flow field were analyzed according to these radiographs. The results showed that the region of variable cross-section is the site that is more prone to dendrite arms fracture. These dendrites spread radially from the center of the thin wall and the isotherms distribute as an arc-shape. These are the main reasons for the uneven distribution of the microstructure, which lead to undesirable phase formation and aggregation at the region of variable cross-section. The microstructure can be significantly refined and solute enrichment can be eliminated by adding refiners to alloys. Comparing these two conditions, it can be concluded that the solute and flow field distribution at the variable cross-section region is significantly affected, uniformity of the solidification microstructure is improved, and undesirable phases are eliminated by increase in effective heterogeneous nucleation cores.