Investigation On Heterogeneous Nucleation Potency Of MgO And MgAl₂O₄ Oxide Substrates For Pure Al

MgO and MgAl₂O₄ are common oxide inclusions in smelting process of Al-Mg alloys.Both MgO and MgAl₂O₄ are believed to be effective heterogeneous nuclei for Al based alloys due to their small lattice misfits with Al.However,some research results show that there are complex chemical reactions between liquid Al and MgO or MgAl₂O₄ at high temperatures,but the heterogeneous nucleation behavior of such phases is rarely discussed.The truly effective nucleating agents in Al-Mg alloys and their nucleation behavior are still not very clear.In order to explore the heterogeneous nucleation potency of MgO and MgAl₂O₄ oxide substrates for pure Al,and to clarify the effect of Al/MgO and Al/MgAl₂O₄ interfacial reactions at high temperatures on the nucleation behavior of Al,in this study,the undercooling of pure Al nucleated on MgO and MgAl₂O₄ single crystal substrates was measured by infrared pyrometer in high vacuum condition.The preferred orientaions of newly nucleated Al crystals on MgO and MgAl₂O₄ substrates with disparate terminated crystal planes were obtained by XRD?X-ray diffraction?.Then the element distribution and phase compositions of Al/MgO and Al/MgAl₂O₄ nucleation interfaces were analyzed using SEM?Scanning Electronic Microscopy?,EDS?Energy Dispersive Spectrometer?and XRD.Furthermore,TEM?Transmission Electron Microscopy?and HRTEM?High Resolution Transmission Electron Microscopy?were conducted to analyze the OR?orientation relationship?and interface lattice misfits?f?of Al/Mgo,Al/MgAl₂O₄ and Al/Al₂O₃ three detected nucleation interfaces,and the nucleation undercooling of Al on MgO and MgAl₂O₄ substrates were predicted based on classic misfit-undercooling theory,which were consistent with the experimental undercooling results.The main results of this project are illustrated below:1.Al melt would react with MgO and MgAl₂O₄ at high temperatures forming Al₂O₃ at contacting interface accompanying with the dissipation of Mg gas;the interfacial reaction of Al/MgO is a multi-step reaction process and MgAl₂O₄ is an intermediate reaction product.2.The interfacial reaction layer Al₂O₃ hindered liquid Al from directly contacting with MgO or MgAl₂O₄ substrates,inhibiting their nucleation potency to some extent.The Al/MgO and Al/MgAl₂O₄ interfacial reactions corroded the exposed planes of original substrates,leading to scattered growth orientations of the new Al₂O₃ grains.3.TEM analysis on three detected nucleation interfaces Al/Al₂O₃,Al/MgO and Al/MgAl₂O₄ indicated that either new formed Al₂O₃ or original MgO and MgAl₂O₄ could all act as heterogeneous substrates to trigger nucleation process of Al.4.The nucleation processes of Al solidified on MgO or M substrates were triggered by multi heterogeneous substrates simultaneously.Disparate lattice misfits at the nucleation interfaces led to scattered undercooling results.