Heterogeneous Nucleation Behavior Of Aluminum And Its Alloys

Grain refinement of metals and alloys during solidification will improve the uniformity of as-cast structure and enhance their strength and plasticity.As the initial stage of solidification process,nucleation controls the microstructure of the primary phase,grain size and phase distribution to a great extent.However,heterogeneous phases with different scales,shapes and uneven distribution are inevitable in the actual metal melt,such as oxides,intermetallic compounds and other solid particles.These potential nucleation sites usually have a huge impact on the metal melt nucleation and growth processes given an appropriate condition.In industrial practice,the refiner or modifier are often introduced for a finer and homogeneous mirostructure.Therefore,it is of great theoretical and practical importance to investigate the nucleation behavior of the liquid,the controlling factors and the behavioral variation during solidification.The current study focuses on the heterogeneous nucleation behavior of pure A1 and Al-Cu alloys,which is triggered by different Al₂O₃ substrates with various crystal orientations.The undercooling of pure A1 and Al-Cu alloys triggered by different substrates was obtained using undercontrolled nucleation method.The interfacial structure of A1 and Al-Cu alloys was also investigated,together with the preferred orientations of nucleating phase and the effect of alloying element Cu on the heterogeneous nucleation of Al.Based on Bramfitt's two-dimensional lattice matching and various interfacial energy-mismatch models,the relationship between the lattice misfit and nucleation undercooling was discussed.Finally,the liquid structure of pure Al and Al-Cu alloys during the nucleation was also investigated with or limited with the heterogeneous substrate.The main results are as follows:1.The lattice misfits of A1 crystals and Al₂O₃ substrate with nine plane orientations from 7.77%to 25.03%are obtained.The heterogeneous interfacial structure between them can be divided into two types:When f<13%,the elastic deformation and mismatch dislocations appear to match the interface.When f>13%,there are micro-twin regions with composition of Al at the interface.2.The existing interfacial energy-mismatch model was modified and developed as:When f<3.1%,the interfacial energy between the nucleation phase and the substrate is released by the lattice elastic distortion of the new phase;When 3.1%<f<7.8%,the mixed dislocations are introduced to release the energy accumulation under large mismatch conditions,which extends the original edge dislocation model to a mixed dislocation model;When 7.8%<f<13%,the periodic coincidence dislocations are introduced to release the interfacial energy;When f>13%,the stacking faults such as micro-twins are assumed to be introduced into the interface to release the interface energy and a new energy model is established based on the stacking fault defects.The modified model and new developed stacking fault model can accurately predict the nucleation undercooling of Al/Al₂O₃ system together.3.The undercooling,preferred growth orientation and lattice parameters of Al-1.4 wt.%Cu and Al-4.0 wt.%Cu alloys on?10???0?,?10???1?,?10???4?,?0001?A1₂0₃ were obtained from experiments.A Cu-rich layer with 1?2nm thick adsorbs at the interface which is confirmed by high angle annular dark field scanning transmission electron microscope and electron energy loss spectroscopy.The presence of this Cu-rich layer reduces the lattice misfit between Al and Al₂O₃ and promotes the heterogeneous nucleation of Al according to the above model analysis.The adsorption layer is not affected by the content of Cu element and the plane orientation of Al₂O₃ substrate.The undercooling of Al-1.4 wt.%Cu alloy on Al₂O₃ substrate with different orientation is lower than that of pure Al.There is no significant effect on the nucleation undercooling of A1 with the further increase of Cu content.The preferred growth orientation of Al can be affected by the addition of Cu and the plane orientations of Al₂O₃.The undercooling of Al-Cu alloy can also be well predicted by the above energy-mismatch model.4.In order to further understand the influence of different substrates on the formation of the primary phase,the melt structure of Al and Al-Cu alloy on the heterogeneous substrate during the solidification process was studied by in-situ synchrotron X-ray diffraction.The evolutionary process shows that the first neighbour distance of Al and Al-1.4 wt.%Cu alloy melt is affected by the lattice structure of the substrate surface during the solidification process.Above the melting point temperature,near the nucleation temperature,the first neighbour distance of aluminum and its alloy melt near the substrate is approximate.With the decrease of temperature,the crystal phase of the aluminum melt on the?10???0?Al₂O₃ substrate gradually increases,but the increase amplitude of the first neighbour distance decreases.On?0001?Al₂O₃ substrate,more nano-clusters are found in the melt.The first neighbour distance increases with the growth of the crystal after recalescence.5.The majority of Al retained in the melt state and some of Al changed into nanocrystals with or limited with the trigger of heterogeneous substrate at recalescence state.When it nucleated on polycrystalline ceramics,Al melt nucleated and formed polycrystalline solids after solidification.On?10???0?and?0001?Al₂O₃ substrates,the formed crystal shows typical preferred orientation and the undercooling is decreased.The crystal with preferred orientation is consistent with the crystals at room temperature.The addition of alloying element Cu promotes the formation of Al nanocrystals,which further reduces the nucleation undercooling.