Heterogeneous Nucleation Mechanism Of Aluminum On Substrates

In the solidification process of the metals and alloys,almost all the nucleation occurs heterogeneously.Investigating the heterogeneous nucleation process and the effect of heterogeneous nucleus will help to find a more efficient grain refiner in industry,which will improve the mechanical properties such as the plasticity,toughness and strength of the material.Since heterogeneous nucleation essentially triggers a crystallization on surface of substrates,the interface structure and characteristics play a crucial role.Deeply understanding the heterogeneous nucleation mechanism not only gives more insights into the nucleation theory,but also substantially promotes the development of grain refinement techniques in industry scale.The present thesis focus on the heterogeneous nucleation mechanism of pure Al on the substrates of MgO,MgAl₂O₄ and Al₂O₃ with different orientations.Based on Bramfitt's two dimensional lattice matching and electrostatic interaction theory,the lattice misfit and electron mobility was calculated.In addition,the preferred orientations of interfaces between Al and the substrates were also predicted according to edge to edge matching models.The nucleation undercooling of Al achieved on different substrates was measured using an improved differential scanning calorimeter.The corresponding interface reaction,structure and oritation relationship were systemactically charactered by TEM thechnique.Finally,two heterogeneous nucleation models were proposed for Al/MgAl₂O₄,Al/MgO system and Al/Al₂O₃ system,respectively.The corresponding results are as follows:?1?The lattice misfit and electron mobility between Al crystal and SiO₂,ZrO₂,MgO,Al₂O₃,MgAl₂O₄ substrates with different plane orientations were calculated,respectively.The results of lattice misfit show that the MgO and MgAl₂O₄ is more effective heterogeneous nucleation substrates for Al than Al₂O₃,SiO₂ and ZrO₂substrates.However,according to electrostatic interaction theory,the Al₂O₃ and SiO₂ is the substrates with higher efficiency for Al.?2?The measured undercooling results show that the undercooling of Al/MgO and Al/MgAl₂O₄ is significantly lower than Al/Al₂O₃.As for Al/MgAl₂O₄ with different orientations of?100?,?110?and?111?,the lowest undercooling is achieved in Al/MgAl₂O₄?100?.The undercooling in the Al/MgO system with different orientations of?100?,?110?and?111?is almost equal,but is a little lower than that of Al/MgAl₂O₄with the same orientations.Nevertheless,the achieved undercooling proves that the MgAl₂O₄ and MgO substrates is more effective than Al₂O₃ for the nucleation of Al,which is consistent with the calculated results according to the lattic misfit theory.?3?The original substrate MgO is completely replaced by the reaction product MgAl₂O₄ at the interface owing to the chemical reaction between liquid Al and the MgO substrate.In addition,the same crystal structure with the original MgO substrate is achieved in the buffer MgAl₂O₄ layer with the orientation relationships of MgO?100?//MgAl2O4?100?//Al?100?,MgO?110?//MgAl2O4?110?//Al?110?and MgO_?111?//MgAl₂O_4?111?//Al_?111?,which is the same with Al/MgAl₂O₄ system.No matched orientation relationship is observed at the interface of Al/Al₂O₃.The achieved orientation relationships between heterogeneous and nucleation phase in the present study is consistent with the theoretical prediction according to Edge-to-Edge model.?4?The results with respect to interface structure present that the quasi-substrate transition layer appears in all of Al/MgO,Al/MgAl₂O₄ and Al/Al₂O₃.The ordering and thickness of transition layer is strongly dependent on the substrate structure and orientation.As for the Al/MgO and Al/MgAl₂O₄,the transition layer is in-plane ordering and laying at the interface.The corresponding interfacial energy between the nucleation phase and substrate is released by the lattice elastic distortion.However,only layering transition layer is observed at the interface.The interfaciale nergy is released by edge dislocations.?5?A diffused-ordering model was developed for the heterogeneous nucleation of Al/MgAl₂O₄ and Al/MgO system.Since the surface energy between nucleation phase and heterogerenous substrate is relatively low,liquid Al layers would form in-plane ordering and laying at the MgAl₂O₄ interface.As the adsorption energy of Al-O presents a larger adsorption strength than that of Al-Al and Al-Mg,oxygen element diffused from substrate to Al melt would promote in-plane atomic ordering with the hump profile.As the temperature decreasing,the interface of the quasi-substrate layer and liquid Al will form edge dislocation to release the energy.Meanwhile,partial energy would be released by the lattice elastic distortion in the transition layer.With the increasing of undercooling,the Al atoms continuously grows on the hump ordering layer to trigger nucleation.?6?A partial ordering model was developed for the heterogeneous nucleation of Al/Al₂O₃ system.As the interface of Al₂O₃,the liquid Al atoms can transform to intrinsic FCC-Al structure to apprear a quasi-Al₂O₃ structure.Since the inter-atomic potential of Al is incompatible with the hexagonal symmetry,the ordering of transition layer with FCC-Al structure is only layering rather than in-plane ordering.During the initial solidification,a mass of dislocations and defects occur to relase energy at the interface.Eventually,only a few layering is survived.With the increase of undercooling,the size of embryo would grow more than critical nucleation radius to trigger nucleation.