Liquid-solid Interface Between Al Melts And Substrate During Heterogeneous Nucleation On Atomic Scale

The grain refinement of metals and alloys during solidification can enhance their strength and plasticity, prohibit porosity and hot cracking and improve the uniformity of as-cast structure. In aluminium industries, the effective method to refine the grain size of as-cast structure is the inoculation of heterogeneous substrates. It is of high value to study the nucleation process of overgrowth phase on different solid substrates in order to select efficient substrates to induce heterogeneous nucleation. The current heterogeneous nucleation mechanism under the classic nucleation theory or the epitaxial growth model is mainly based on solid-solid interface energy by assessing the misfit between the substrate and overgrowth phase from the view of solid state. However, a small misfit is not equivalent to the effective heterogeneous nucleation under some circumstances. Therefore the current misfit criterion is not adequate to assess the heterogeneous nucleation of overgrowth phase on substrate.The heterogeneous nucleation of overgrowth phase on substrate belongs to the liquid-solid phase transformation, and the structure of corresponding liquid-solid interfaces controls the process of heterogeneous nucleation. Atomic-scale studies on the structural evolution at the liquid-solid interface during heterogneous nucleation can provide more precise description of the physical process of heterogeneous nucleation than that in solid view.The dissertation investigated the liquid-solid interfacial structures between Al melts with solutes and TiB2 substrates on atomic scale through ab initio molecular dynamics simulation, synchrotron X-ray diffraction and highresolution transmission electron microscopy. To elucidate the physical process of heterogeneous nucleation, the ordered structure of Al melts close to the interface was analyzed at the inoculation stage of heterogeneous nucleation(at 70 K above liquidus temperature) and during heterogeneous nucleation(around the melting point of aluminum). The major conclusions are as follows.The simulation and experimental results reveal the atom termination of TiB2(0001) surface decides the nucleation potency of overgrowth phase on substrate. The B-terminated(0001) surface can absorb the neighbouring Al atoms in the melts forming an ordered monolayer structure, which is similar to AlB2. The AlB2-like monolayer structure of Al atoms cannot further extent into Al melts. Thus, the B-terminated TiB2 substrate is not effective to induce the heterogeneous nucleation of Al. The Ti-terminated(0001) surface can induce the neighbouring Al atoms forming nano-scale quasi-solid structure during the heterogeneous nucleation, which is the basis of Ti-terminated TiB2 as a potent substrate for the heterogeneous nucleation of Al.The reason that the ordered monolayer structure of Al atoms at the Bterminated surface cannot further extend lies in two factors. On one hand, the absorption effect in the B-terminated interface comes from the weak 2s(B)-3s(Al) and 2p(B)-3p(Al) hybridization, which cannot guarantee enough electronic hybridization between Al atoms to support the extension of ordered structure. On the other hand, the monolayer structure similar to AlB2 at interface results in a sparse region of stacking Al atoms close to the interface due to the high interfacial energy between AlB2(0001) surface and α-Al, which prohibits the extension of ordered structure of Al atoms.The simulation and synchrotron X-ray diffraction analysis show two effects of Ti-terminated TiB2 substrates on heterogeneous nucleation. On one hand, the formation of nano-scale Al quasi-solid structure results from the strong 3d(Ti)-3p(Al) hybridization at interface, which is prerequisite for the heterogeneous nucleation of overgrowth phase from disordered liquids to ordered solids. On the other hand, the electronic hybridizations induce the formation of a rhombohedral-centered Al structrure in R3 m space group by accommodating the in-plane structure of TiB2 substrate. The R3m-Al structure deviates from its intrinsic structure of fcc-Al, which results in the emergence of distortion energy in quasi-solid structure. With the extension of quasi-solid structure into Al melts, the electronic hybridization between new-formed Al layers decays accompanying with the accumulation of distortion energy, which leads to the thickness limitation of the Al quasi-solid structure.Zr solutes at interfacial area decrease the structure order of Al atoms close to the liquid-solid interface and prohibit the extension of the ordered structure. Although Zr solutes locating in the quasi-solid structure can enhance the electronic hybridization among the neighboring Al atoms, the misfit between the quasi-solid structure and its intrinsic fcc-Al structure is also enlarged. As a result, the distortion energy is enhanced, which restricts the extension of the quasi-solid structure.That Ti solutes close to the liquid-solid interface favoring the extension of Al quasi-solid structure lies in three factors. The Ti atoms locating in the liquid bulk region can restrict the thermal movements of neighboring Al atoms improving the structural order of Al quasi-solid structure. The Ti atoms locating in the Al quasi-solid structure can enhance the electronic hybridization among neighboring Al atoms reinforcing the cohesion of atoms in the quasi-solid structure. More important, the Ti atoms in the quasi-solid structure can induce the neighboring Al atoms transforming from the R3m-Al to intrinsic fcc-Al structure during heterogeneous nucleation with the help of two-dimension stacking fault. As a result, the distorted quasi-solid R3m-Al is selfaccommodated into its intrinsic fcc-Al structure by relaxing the distortion energy.Based on the above simulation and experimental results, a “duplex accommodation” heterogeneous nucleation mechanism is elucidated. The effective nucleation of overgrowth phase on heterogeneous substrate consists of two steps: the first is the formation of the quasi-solid structure induced by the substrate surface, and the second is the structural accommodation from distorted quasi-solid structure to its intrinsic structure induced by solutes. The quasi-solid structure results from the strong electronic hybridization between the substrate surface and neighboring liquid atoms by accommodating to the in-plane structure of the substrate. The accommodation generates the difference between the quasi-solid structure and its intrinsic structure accompanying with the distortion energy, which results in the limited extension of the quasi-solid structure into Al melts. Induced by the solutes in the interfacial area, the quasisolid structure can transform from the R3m-Al to intrinsic fcc-Al structure by two-dimension stacking fault. The second structural accommodation eliminates the misfit between the quasi-solid structure and its intrinsic structure by relaxing distortion energy.