Mechanistic Study And Qualitative Physical Modeling Of The Viscosity Jump Phenomenon In Metal Melts

The dynamic viscosity coefficient of molten alloys is directly related to their flow characteristics,thus profoundly impacting the quality and efficiency of casting.Through indepth research on the characteristics and control mechanisms of the viscosity of molten alloy,a better understanding of the formation mechanisms of casting defects can be achieved.This understanding,in turn,allows for the optimization of casting processes,the development of innovative materials and techniques,and the enhancement of the competitiveness and sustainable development capability of the casting industry.This paper,combining the Wulff cluster model with density functional perturbation calculations,semi-quantitatively describes the variation of molten structure with temperature and its impact on viscosity.The nonArrhenius viscosity jump phenomenon is elucidated,providing insights into the mechanisms behind it.By establishing a micro-scale model for viscosity,the non-Arrhenius viscosity jumps in pure Al and pure Pb melts are explained.Additionally,the temperature range and type of viscosity jump in pure Sn melts are predicted and verified.Preliminary predictions of potential non-Arrhenius viscosity jumps in the InBi melt system are also provided.Simultaneously,utilizing the Wulff cluster model and high-temperature liquid X-ray diffraction experiments,the molten structure of single-component multiphase pure metal systems and intermetallic compound systems are studied,expanding the application scope of the Wulff cluster model.Firstly,employing density functional perturbation theory combined with phonon free energy calculations,the temperature-dependent variations in the Wulff shape in pure Al and pure Pb melts are calculated.The highly consistent disappearance temperatures of specific facets on the Wulff shape with the occurrence temperatures of non-Arrhenius viscosity jumps are observed.For Al(100),the disappearance occurs beyond 1075 K,aligning well with the experimental non-Arrhenius viscosity jump temperature range of 1050 K to 1100 K.Similarly,for Pb(321),the disappearance occurs beyond 975 K,with the experimental non-Arrhenius viscosity jump temperature range falling between 950 K and 1000 K.This consistency supports the notion that non-Arrhenius viscosity jumps are highly correlated with changes in specific melt structure facets.In other words,the non-Arrhenius viscosity jumps are caused by the disappearance of characteristic melt structures.Furthermore,the microscale interaction strength between different facets and free atoms is studied to accurately determine the type of nonArrhenius viscosity jump.When non-Arrhenius viscosity jumps occur in pure Al and pure Pb melts,Al(100)is the lowest adsorption energy facet on the Wulff shape,and Pb(321)is the highest adsorption energy facet on the Wulff shape.The disappearance of Al(100)leads to a non-Arrhenius viscosity jump in Al,while the disappearance of Pb(321)corresponds to a nonArrhenius viscosity drop in Pb.This difference results in different viscosity jump patterns,consistent with experimental results.Thus,viscosity can be qualitatively defined as a macroscopic manifestation of microscale interaction strength.Subsequently,the viscosity model is used to predict the non-Arrhenius viscosity jump temperature in Sn,indicating a jump rise.The predicted α-β phase transition temperature points are highly consistent with experimental values,confirming the theoretically predicted β-γ solidstate phase transition and its transition temperature.Using the Wulff cluster model combined with high-temperature X-ray diffraction experiments,the molten structure of pure Sn is investigated,confirming the existence of cluster structures corresponding to the three solid phases of Sn in the actual melt.The viscosity changes in pure Sn melts are then predicted.By calculating the evolution patterns of Wulff shapes for all three clusters,it is found that α-Sn(111)appears on the Wulff shape beyond 1075 K.Calculations of interaction strength reveal that due to the reconstruction of α-Sn(112)and α-Sn(122),the adsorption energy decreases,making αSn(111)the second-highest adsorption energy facet on the Wulff shape at this time.Meanwhile,the proportion of low adsorption energy facets increases significantly after the appearance ofα-Sn(111)due to geometric relationships,based on these results,it can be predicted that a nonArrhenius viscosity jump will occur in pure Sn melts near 1075 K.Through rotational oscillation testing of high-purity Sn melts,a viscosity non-Arrhenius jump phenomenon is observed experimentally between 950 K and 1100 K,consistent with the predicted results.Next,the molten structure of the InBi intermetallic compound system is studied using the Wulff cluster model,and the viscosity changes in InBi melts are predicted.Simulated XRD matches well with experimental XRD,showing the presence of InBi intermetallic compound clusters and Bi clusters in In50Bi50 melts,while In exists primarily in the form of free atoms.Near the melting point,a shift between simulated XRD and experimental values is observed.By linearly combining the diffraction intensity of bulk Bi into the simulated XRD spectrum,the shift is largely eliminated.This implies that,near the melting point,Bi clusters in the melt significantly increase in size,and the nucleation tendency of the melt may have already formed even before it drops below the melting point.Attempts to predict potential non-Arrhenius viscosity jumps in In50Bi50 melts are made,considering the weights of two adsorbing atoms.After considering the weights,it is predicted that InBi melts may exhibit one or two viscosity non-Arrhenius jumps between 500 K and 800 K.