Investigation On Heterogeneity And Its Characteristic Of Several Alloy Melts

A great number of investigations indicate that the structure, state and nature of liquid metal vary with the condition and correlate tightly to its as-cast structure. So, the studies on liquid metal are becoming popular for foundrymen and metallurgists which have ever been topics only in field of condensed matter physics. A popular result is that many alloy melts are in heterogeneous state in various manners. Such as, the metastable heterogeneity in heating process, the liquid phase separation of immiscible alloy, stable atom clusters formed in melt which make the melt in microheterogenous state. It should be stressed that the influence of heterogeneity in melts on the fabrication of alloy can not be neglected. So it is of great significance to further disclose the characteristic of heterogeneity in alloy melt. The works in present dissertation are carried out as follows:1. Metastable heterogeneity in alloy melts during heatingThe electrical resistivity of Bi-Ga melt during heating and cooling process was measured by resistivity measurement apparatus. The results show that the resistivity of miscible Bi-Ga alloy melt behaves anomalously with temperatue in heating process but linearly in cooling process, which indicates the existence of metastable microheterogeneity in melt during heating. Micro-domains enriched in Bi or Ga in solid state are retained to liquid state due to the relaxation of dissolution. This phenomenon is attributed to the more intensive bond between Bi-Bi and Ga-Ga than Bi-Ga, as well as the large density difference between Bi and Ga. The composition of micro-domains is found to depend on melt's composition. Further heating leads to the dissolution of micro-domains and the irreversible transition of melt from microheterogenous state to real homogeneous state.It is found that immiscible Bi33.3Ga66.7 melt is still in serious heterogeneous state even is heated up to far above phase separation temperature. Isothermal measurements of resistivity were carried out to study the homogenization process of immiscible alloy melt. Based on the characteristic of resistivity in isothermal process a semi-empirical formulation is established for the description of isothermal homogenization process of melt, i.e. the volume of second phase droplets in melt is written as a fuction of isothermal time:Discussions based on the formulation indicate that the second phase Ga droplets have large size in heterogeneous melt below 573 K, and the buoyancy greatly impedes the diffusion process, so the droplets present high stability. As the temperature goes up to 673 K, the size of droplets decrease sharply, then the influence of buoyancy on diffusion is feeble, so, with the increase of isothermal temperature the time need for homogenization decreaes nearly linearly.Through isothermal experiment, the influence of tin addition on the metastable heterogeneity in Bi33.3Ga66.7 melt was studied. It is found that tin addition decreases the size of second phase droplets, as well as the interface tension between droplets and matrix melt, as a result the stability of heterogeneity and the time need for homogenization is decreased. The effect of tin is more powerful at lower temperature 573 K.1. Study on phase separation of immiscible alloy meltThe phase separation process of immiscible Bi33.3Ga66.7 alloy was studied through resistivity. Especially, the influence of tin and indium on phase separation of Bi33.3Ga66.7 alloy was investigated. The results indicate that tin addition weakens the phase separation tendency of melt through decreaing the phase separation temperature, monotectic reaction temperature and the width of immiscible gap. Contrarily, indium enhances the phase separation tendency by increasing the phase separation temperature and width of immiscible gap, as well as decreasing the monotectic reaction temperature.Based on thermodynamic calculation, the mixing behaviours of addition element with the components of immiscible monotectic alloy were discussed. Comparative analysis between the mixing behaviours of addition element with the components of immiscible alloy and its effect on phase separation shows that:the addition element which presents hetero-coordination tendency with one of the constituent of immiscible alloy but self-coordination tendency with the other could enhance the phase separation of immiscible alloy; The addition element which shows self-coordination tendency with both of the constituents of immiscible alloy could weaken the phase separation.The electrical resistivity of liquid In-Bi system was measured. The measured resistivity is well coincident in heating and cooling process, indicating that this liquid system does not contain metastable heterogeneity like that in liquid Bi-Ga system during heating. But, the analysis of resistivity of In-Bi melt based on Nordheim rule manifests the stable existence of InBi type atom clusters which make the melt in thermodynamically stable microheterogeneous state. The calculated mole fraction of InBi atom clusters at different temperature follows a good empirical relationship:As the temperature decreases to the vicinity of melting point, the content of InBi atom clusters in In50Bi50 melt attains to a large value leading to an apparent concentration fluctuation and discrepancy of resistivity of melt.The structural characteristics of liquid In-Bi system are deduced based on the characteristic of resistivity: in the composition interval 0-50 at.% In, the melt can be seen as InBi type atom clusters distributed in Bi-like melt; in the composition interval 50 at.%-100 at.% In, the melt can be seen as InBi type atom clusters distributed in In-like melt.Preliminary study on the correlation between resistivity and thermodynamic parameters was carried out. For liquid In-Bi system, a good relationship between△ρand |△Hm△Sm| is calculated as:The viscosity of A1-12 wt.% Sn-4 wt.% Si melt was measured by high-temperature viscometer. It is found that the viscosity increases anomalously at 1103 K and 968 K accompanied by the abrupt changes of size of fluid units and viscous activation energy. It is speculated that Si-Si and Sn-Sn atom clusters grow abruptly in the temperature interval 1103 K-1138 K, and Si-Si atom clusters may increase greatly between 968 K and 998 K. Melt chilling was carried out according to the above-mentioned characteristic of melt structure, and it is found that smaller atom clusters at high temperature can be retained to pouring temperature by chilling, which leads to deep undercooling of melt and hence refinement of solidification structure.