| The application of external physical field is an important means to control the metal solidification structure. A lot of studies have shown that ultrasonic field and electromagnetic field can significantly refine the metal solidification structure, thus improve the deformation properties and functional performance of metallic materials. One of the mechanisms of external physical field affecting the solidification structure is by changing the microstructure of the metallic melt, thereby influencing the nucleation process. However, the mechanism of how external physical field affects melt structure still remains unclear, so an intensive study of the effects of external physical field on melt structure is of vital importance to understand the essence of melt solidification under external field. But, it is difficult to study the melt and test its microstructure directly, since the melt is always in high temperature state. Electric resistance and thermoelectric power, being sensitive physical quantities to melt structure, are important parameters to characterize structural variation of metallic melt. Therefore, this paper explores the effect of external physical field on melt microstructure by investigating the change law of electric resistance and thermoelectric power caused by ultrasonic field or electromagnetic field and solidification microstructure observation. It also explains the mechanism of this effect with metal electronic transport theory and cluster theory, which lays both a theoretical foundation and important experiment data for the realization of precise control of solidification structure with the application of external physical field.The main study contents are as following:the apparatus which can be used to continuously, real-time and synchronously detect the electric resistance and thermoelectric power of metallic melt under the treatment of external physical field is designed; taking Pb-Sn alloy as subject, the effects of ultrasonic field and AC magnetic field on electric resistance and solidification structure of melt are investigated, and a correlation among ultrasonic, electric resistance, melt structure and solidification structure is built; based on this, taking pure Al and Al-X (X= Fe, Cu, Ni, Si) alloys as subjects, the effect of DC magnetic field on melt electric resistance and thermoelectric power are studied, and the influence of Fe composition and alloy component on the change of electric resistance and thermoelectric power caused by magnetic field is analyzed.In this study, an apparatus, derived from improved four electrode method, is designed and constructed, which can measure the variations of melt's electric resistance and thermoelectric power induced by external field. It is for the first time that a continuous, real-time, synchronous, precise detect of melt's electric resistance and thermoelectric power under the treatment of ultrasonic field or electromagnetic fields is realized. The method of utilizing electric resistance and thermoelectric power to characterize the variation of melt structure induced by external field is proposed, with persistent variation of resistance (?r) and persistent variation of potential difference (?u3) characterizing the persistent variety degree of melt structure induced by external field, and with the effective time (At) being a vital parameter to reflect the stability of melt structure's changes caused by external field. With this method, the effect of ultrasonic field and electromagnetic field on metallic melt structure is systematically researched and analyzed. By applying DC magnetic field on metallic melt, it shows that thermoelectric power is a more sensitive parameter to characterize metallic melt structure than electric resistance is.With Pb-Sn alloy being the research subject, the effect of ultrasonic parameters on metallic melt electric resistance is studied. The results show that transient variation of resistance (?R) increases with the enhancement of ultrasonic power and treating temperature, but it has no correlation with ultrasonic treating time; ?r also increases with the enhancement of ultrasonic power and treating temperature, and with the ultrasonic treating time prolonging, Ar increases to a saturation value, then remains constant; At increases with the enhancement of ultrasonic power, and with the treating temperature and treating time increasing, At first increases then decreases, and there is an optimum treating temperature and an optimum treating time. The effect of ultrasonic field on the electric resistance of Pb-Sn alloys, which is related to the content of element Sn, enhances with the increase of Sn content.Through the measurement and observation of metallic melt electric resistance and its solidification structure under the treatment of ultrasonic field or AC magnetic field, it is found that there is a corresponding relationship between Ar and solidification structure. The refinement and homogenization degrees of melt solidification structure induced by ultrasonic field or AC magnetic field both increase with the increase of|?r|. The correlation among external field, electric resistance property, melt structure and solidification structure is revealed here. Ultrasonic field diminishes melt electric resistance, while AC magnetic field enhances melt electric resistance, but both fields lead to the refinement of Pb-80% Sn alloy's solidification structure. The reason for this phenomenon is that ultrasonic field changes the homogenization of size and distribution of melt atomic clusters, decreases the disorder degree of system, and thus decreases the electric resistance; while AC magnetic field not only alters the size and distribution of melt atomic clusters, but also changes their bonding type, increases the content of Pb-Sn atomic clusters, enhances element disorder degree of system, thus increases the electric resistance.With pure Al and Al-X alloys being experiment subjects, the results demonstrate that during the applicatioin of DC magnetic field, electric resistance increases at once and then stays constant; after the magnetic field is shut down, electric resistance/ecovers to its initial value rapidly. This magneto-resistance effect has no relationship with magnetic field's treating time, in stead it is closely related to the disorder degree of system. The higher the temperature is, the higher disorder degree of system is, and the more significant this magneto-resistance effect will be.DC magnetic field can change the thermoelectric power of pure Al and Al alloys melt. During DC magnetic field being applied, potential difference experiences a fast changing process and a slow changing process. After the field is stopped, potential difference goes through a dramatic recovery, a slow recovery and a steady state process. Applying DC magnetic field to pure Al, hypoeutectic Al-Fe, near eutectic Al-Fe, Al-2.44%Cu and Al-15.5%Si alloys under a low temperature near liquidus line can decrease potential difference (U) and increase thermoelectric power (S); applying DC magnetic field under a high temperature far from liquidus line can increase potential difference. Applying DC magnetic field on hypereutectic Al-2.89%Fe and Al-7.37%Ni alloys within the temperature range of this experiment can decrease their potential differences. This is due to a higher transition-element composition in those two hypereutectic alloys.The experiment shows that the changing trend of |?U|,|?u3| and At with the variation of temperature is unrelated with solute species, i.e.??U?increases with the increase of temperature, ??u3| and At first increase and then decrease with the increase of temperature, the temperatures that correspond with two extreme points are similar, but the position of extreme points is different according to different solute species. The extreme points of Al-Fe alloy are close to the liquidus line, the extreme points are about 30? above the liquidus line for Al-Cu and Al-Si alloys, and for Al-Ni alloy it is approximately 50? above the liquidus line. For Al-2.89% Fe alloy,|?U| increases with the increase of treating time of DC magnetic field, until it reaches its saturation value, while|?u3| and At both increase gradually. With the increase of the magnetic flux density,|?U|,|?u3| and ?t all increase gradually.Applying DC magnetic field on metallic melt can result in two kinds of magnetic effects: quantum magnetic effect and classic magnetic effect. Quantum magnetic effect, which is normally unrelated to temperature, changes the number density and distribution of extended state electrons; classic magnetic effect only alters extended state electron distribution, not changing the number density, and its effect is enhanced with increase of temperature. It is the combination of these two magnetic effects that results in the above mentioned changes of metallic melt electric properties. |
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