Investigation Of Transition State Phenomenon Of Metals During Solidification

Solidification is the phase transition process from liquid state to solid state, which is important in the material preparation and formation. Investigations on phenomenon and law of solidification process have sustained for thousands of years. Till now, considerably less is known about their earliest stages compared with the behavior of bulk crystals, when they nucleate and grow from a liquid. This present work investigates the temperature dependence of viscosity and the liquid structure near the liquidus, and the energy changes during the solidification process. This work discovers phenomenon of transition state near the liquidus, which provides the theory and experimental data for the solidification.The viscosity of the eutectic Pb_38.iSn_6i.9 alloy has been measured by a torsional oscillation viscometer using three different crucibles which are made of the materials of highly sintered alumina (Al₂O₃), quartz (SiO₂), and graphite (C), respectively. The roughness of crucibles has effect on the viscosity. The viscosity data obtained for SiO₂ and C crucibles are concentrated in the narrow range of about 0.5% and show almost the same activation energy. However, the viscosity obtained using Al₂O₃ crucible with the maximal roughness is higher than that using the other two crucibles. The discrepancy of viscosity obtained using those crucibles increases with the viscosity. A deviation behavior of the temperature dependence of viscosity from the Arrhenius law in certain ranges of temperature (about 60K above the melting point) has been found. The deviation behavior near the melting point is defined as the transition state phenomenon. The starting deviation temperature (T') is considered as the structural transition temperature. The transition state exists between the solid state and the liquid state.The melt transfers to solidify through the liquid-solid phase transition. There are relationship between the melt and the crystals. Exploring the characteristics of melts near the melting point will be especially helpful to understand the nature of liquids and explain melting and freezing mechanisms. The investigations on the temperature dependence of the physical property of melt in the transition state are important for understanding the liquid-solid phase transition. The temperature dependence of viscosities of liquid Sn, Cu, eutectic alloy Pb_44.8Bi_55.2,Cu_0.7Sn_99.3,Ag_3.5Sn_96.5, Al₂₈Ag₇₂, Sn₄₃Bi₅₇, peritectic alloy Ag₄₈Sn₅₂, ultimate mutual solubility alloy Sb₄₀Bi₆₀ has been studied by a torsional oscillation viscometer. There is deviation phenomenon when the superheated temperature is 300K above the liquidus. The region of transition state is abour 60K, except for Sb₄₀Bi₆₀ alloy. The range of transition state is about 10K. There is not deviating behavior of temperature dependence of viscosity from Arrhenius equation near the melting point, when the superheated temperature of Sn₄₃Bi₅₇ melt is below 100K. Those results indicate that there is only periodic structure destroyed during the metal crystal melting. The other valence bonds still exist in the melt and there is locally order structure. With increase of temperature, the valence bonds will be destroyed gradually. In the transition state, the homogeneous atoms congregate and form solid-like ordered structure and lead to the deviation of temperature dependence of viscosity from Arrhenius equation. If the superheated temperature is not high enough to destroy the bond, the activation energy and the adjacent distance among atoms of the melt are invariability near the melting point and the transition state is not obvious (Sn₄₃Bi₅₇ melt).Viscosity as an important physical property is sensitive to the melt structure and friction among atoms. The structure of In-Sn,Cu-Ag and Cu-Sn alloys has been measured by X-ray diffraction in our previous work. The structure of liquid metals changes unusually in the transition state and the correlation length has a linear relationship with the viscosity. The experimental results indicate that the deviating behavior of temperature dependence of viscosity from Arrhenius equation is related to the change of melt structure: the solid-like cluster or ordered structure formed. It is well known that the energy changes with the structure. For observing the change of energy above the melting point, the melting and the solidification processes of pure Sn and eutectic alloys Pb_38.1Sn_61.9, Pb_44.8Bi_55.2 and Cu_0.7Sn_99.3 are studied by DSC experiments. The experimental results indicate that the holding time at a certain temperature (in the transition state and lower than the structural transition temperature) decreases the degree of supercooling and advances the exothermic reaction. These results indicate that the structure of liquid metal changes above the melting point after maintaining enough time in the transition state. The changes of structure prepare for the growth of the crystal nucleus and make it is easy to meet the condition of kinetics.There are not mutations on the DSC curves. The phenomenon of transition state does not arouse the change of energy, which is similar to the second-order phase transition, i.e. the earliest stage of liquid-solid phase transition possesses of the property of second-order phase transition. The temperature dependence of viscosity of immiscible alloys Al%Bi4, Al₉₃Bi₇ and Al%Bi₁₀ has been measured by torsional oscillation viscometer. There is deviating phenomenon of temperature dependence of viscosity from Arrhenius equation near the critical point. Although the viscosity in the transition state and the critical temperature range both deviate from Arrhenius equation, there are discrepancies: The increase amplitude of viscosity with temperature in transition state is smaller than that in the critical temperature range.There are differences in essence betweem the phenomenon in the transition state and the critical temperature range: the transition state exists between the liquid state and the solid state and prepares for the solidification on the kinetics and the structure; the deviating of Al-Bi alloy near the critical temperature results in the congregation of Al and Bi atoms separately and cause the phase separation.Considering that the melt has the solid-like property and the correlation length increases near the liquidus, the transition state is similar to the critical phenomenon. Based on the critical dynamics theory, Arrhenius equation is amended and fits the temperature dependence of viscosity. The fitted results are well in the transition state and the critical temperature range for the melts measured in this work. In the high temperature, the temperature dependence of viscosity deviates from the amended Arrhenius equation.The melts has gas-like property in the range of high temperature and the viscosity increases with increasing temperature. According to the collision theory, the cavity theory and the critical dynamics theory, we obtain semi-empirical viscosity equation, which can describe the temperature dependence of viscosity in a wide temperature range.The effects of rare earths (RE) on viscosity, the structural transition temperature and the transition state are investigated in this present work. A mount of RE can increase the viscosity and the structural transition temperature, and accelerate the structural transformation. Above T, the temperature dependence of viscosity fits well with the Arrhenius equation. The effect of Ce on Ag_3.5Sn_96.5 alloy is greater than that on Pb_38.1Sn_61.9 alloy due to the existence of Ag₃Sn cluster and the increasing of Sn content. During the holding time, the viscosity increases rapidly and reaches a steady value finally at the measured temperature below T'. The time dependence of viscosity reflects the process of the structural transition in the melt, which can be considered as the thermodynamic equilibrium relaxation process. The time required for the viscosity increasing and reaching a steady value is defined as the thermodynamic equilibrium relaxation time. During the relaxation process, the melt transfers from metastable state to equilibrium state.The size, correlation radius, and correlation length of cluster increase with increasing viscosity. In the transition state, the viscosity increases and reaches a steady value with the holding time. Then, the size of clusters of the melt increases and reaches a steady value with the holding time. Based on the reported solidified texture of Pb-Sn-RE and the results in the present work, we deduce that the clusters mainly consist of RE and Sn atoms. Considering T' is below the liquidus of RE-Sn compounds, the cluster including RE and Sn is considered as solid-like phase, which represents a stable structure in the melt. The structure of melt is regarded as two kinds of clusters: liquid-like cluster and solid-like cluster. The content and the size of the two kinds of clusters reach equilibrium under the thermodynamic condition.The melt presents thixotropy property due to the solid-like cluster formed in the transition state. The thixotropy has been found in the transition state of (Pb_38.1Sn_61.9)_100-xCe_x(x=0, 0.05, 0.1wt.%) and (Ag_3.5Sn_96.5)_100-xCe_x(x=0, 0.05, 0.1wt.%) and the temperature range near the critical temperature of Al-Bi alloys. Near the structural transition temperature, it becomes easy for atoms to jump the energy barrier and form large cluster under the shear force. The formation and growth of clusters with the shear time arouse the increasing of viscosity and the melt represents anti-thixotropy. That anti-thixotropy is related to the formation of solid-like cluster and is different with the classical thixotropy theory. At the temperature range near the liquidus (or the critical temperature), the clusters congregate and form reticular structure or aggregation structure. That ordered structure is destroyed by the shear force, and the viscosity decreases with the shear time. The melt exhibits thixotropy.