The Relationship Between Undercooling And Heterogeneous Nucleation Interface Of Aluminum And Its Alloys

Nucleation plays an important role in solidification process, and the interface behavior of liquid-substrate dominates nucleation behavior of liquid. This study majorly focuses on the interface behavior of liquid Aluminum and substrates and its factors. The intrinsic undercoolings of pure aluminum and its alloys are obtained through aerodynamic levitation technology and heterogeneous nucleation behavior of liquid is studied on different single crystal substrates with various crystal planes（Al₂O₃ and MgO）. Undercooling and wetting angle were recorded and measured under high vacumm environment. SEM（Scanning Electronic Microscopy）, EDS（Energy Dispersive Spectrometer）, XRD（X-Ray Diffraction）, TEM（Transmission Electron Microscopy）,HRTEM（High Resolution Transmission Electron Microscopy） are employed to investigate the microstructure adjacent to the interface and the formed intermetallics. Bramfitt’s two dimensional lattice match model and heterogeneous phase epitaxial nucleation model are introduced to discuss the relationship among the lattice misfit between nucleated crystal and substrate, the undercooling and the wettability between melt and heterogeneous phases. And the influence of alloy elements on the nucleation process also be discussed. The main results are as follows:1. When the lattice misfit is smaller than 14%, undercooling have an linear relationship with the square of lattice misfit. The better lattice matching, the smaller undercooling for nucleation. The undercooling will increase with the increase of lattice misfit. While when the lattice misfit is larger than 14%, the undercooling triggered by aforementioned substrates will decrease with the increase lattice misfit.2. TEM results show the interface is plat and there isn’t the middle strained layer when the lattice misfit is smaller than 14% and there are middle layers with thickness from dozens to hundreds and continuous and regular shape occuring at the interface between nucleated crystal and subsrates when the lattice misfit is larger than 14%. The middle layer is full of micro twin defects3. The total interfacial energy between the nucleated phase and the substrates can be released by the lattice distortion or dislocation or twins introducing. When the lattice misfit is smaller than 3.1%, the total interfacial energy is sum of the elastic strain energy and misfit energy, Etotal=Eε +Em. When the lattice misfit is between 3.1% and 5.8%, the total interfacial energy is sum of the elastic strain energy and edge dislocation energy,Etotal=Es +Eed. When the lattice misfit is between 3.1% and 5.8%, the total interfacial energy is sum of the elastic strain energy and screw dislocation energy,Etotal=Es +Esd. When the lattice misfit is larger than 14%, the total interfacial energy is sum of the elastic strain energy and twinning energy, Etotal=Es +ET.4. The undercoolings of the Al/MgO heterogeneous nucleation systems are not affected by the crystal planes of MgO substrates indicating different lattice misfits. The reason is that a new layer of Mg Al2O4 occurs at the interface. The new layer of Mg Al2O4 fully covers the whole interface of Al/MgO, serving as the new substrate. Therefore similar undercoolings are obtained although adopting different MgO substrates.5. Alloying elements added into the pure Al can affect the nucleated process due to the new formed intermetallics, and change undercooling of melt and the grain orientation.6. The wetting angle shows that Al melt has different wettability on different substrates. The continuous middle Al layer has greatly improved the wettability. Wettability decreases with the increase of lattice misfit without the middle layer.