Heterogeneous Nucleation Behavior Of Al On Al₂O₃ And MgAl₂O₄ Substrates

The solidification nucleation of molten metal in the traditional production is heterogeneous nucleation.Effectively promoting heterogeneous nucleation is the most convenient,economical and effective approach to obtain materials of great properties.In this paper,heterogeneous nucleation ability,influence factors and heterogeneous nucleation mechanism of Al melt on different kinds and different orientations oxide substrates were studied by considering the interface energy,undercooling and the interface between heterogeneous core and molten metal.Firstly,the mismatch degree and the lattice electron density difference between the nucleation phase of Al and the different orientation oxide single crystal substrates?Al₂O₃,SiO₂,ZrO₂,MgO,MgAl₂O₄?were calculated according to the two-dimensional mismatch degree theory and the electrostatic interaction theory,and it can predict the possibility of the different orientation oxide as metallic Al heterogeneous nucleation core from the perspective of interface energy.According to the results of the Al heterogeneous nucleation theory,six single crystals of the two materials(?MgAl₂O₄?100?,?110?,?111?and Al₂O₃?1-102?,?0001?,?11-20??were selected as the experimental substrates for the study of the Al heterogeneous nucleation.The roughnesses of the selected substrates were detected by atomic force microscopy?AFM?.Then,the undercooling of Al heterogeneous nucleation on the different orientation oxide single crystal substrates were measured by the jointly developed DSC&contact angle measurement instrument from the German Linseis company.The experimental results show that the undercooling of Al heterogeneous nucleation is different not only for different substrates,but also for the same substrates with different orientations.By further analyzing the relationship between mismatch degree,lattice electron density difference and undercooling degree,it is found that the influence of mismatch degree on interface energy is more significant in the two factors of mismatch degree and lattice electron density difference.Finally,the optical microscopy?OM?and high resolution transmission electron microscopy?HRTEM?were used to characterize the interface between the substrate?MgAl₂O₄?100?and Al₂O₃?0001??and the metal Al from the macroscopic morphology to the atomic scale.The interfacial mesoscopic structure between Al and two substrates has no significant difference by using metallographic microscope.However,under the high resolution transmission electron microscope,the microstructure of the interface between Al and two substrates is significantly different.It was observed that many intermittent and small protrusions exist at the interface between Al and MgAl₂O₄?100?substrate,while the interface of Al and Al₂O₃?0001?substrate are clear and flat.By exploring the enrichment and diffusion of the relevant elements at the front of the undercooling melt interface,it is pointed out that the epitaxial layer may be caused by the diffusion of elements at the interface.These protrusions,as substrate epitaxial layers,may be the reason of the low nucleation undercooling of Al on MgAl₂O₄?100?substrate,it is also verified that epitaxial growth is presented in the actual nucleation.Combined with the epitaxial growth model,the thickness of the epitaxial layer on the effective nucleation substrate was predicted,and the heterogeneous nucleation mechanism of Al on MgAl₂O₄?100?plane was discussed.Results of this paper have certain reference value to enrich the heterogeneous nucleation theory.