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Thermophysical Properties And Rapid Solidification Of Undercooled Metallic Melts

Posted on:2003-07-07Degree:DoctorType:Dissertation
Country:ChinaCandidate:X J HanFull Text:PDF
GTID:1101360092466127Subject:Materials Physics and Chemistry
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Both the thermophysical properties and the rapid solidification of undercooled metallic melts are the important research subjects in the physics of condensed matter and materials science in space. The present dissertation carried out detailed investigations on the measurement of surface tensions and specific heats of undercooled metallic melts of Ni, Co and Co-Mo alloys. Meanwhile, rapid solidification of Co-Mo and Ni-Sb binary alloy systems were accomplished by electromagnetic levitation, drop tube, and glass fluxing techniques.An oscillating drop method combined with electromagnetic levitation was applied to determine the surface tensions of metallic melts of Ni, Co, Co-10%Mo, Co-26.3%Mo and Co-37.6%Mo as functions of the temperature. During experiments, the above five metallic melts were undercooled by up to 185K (0.11 Tm ) , 231K (0.13 rm) , 223K(0.13 TL), 213K(0.13 TL\ and 110K (0.07 TE) , respectively. In the experimentally covered undercooling range, crNi=1767-0.31(r-rm) mN/m, crCo=1875-0.332(r-rj mN/m, aCo.w/M=l894-0.31(7-7^, crCo.26,%Mo=l932-0.33(7^) and <7Co. 37 6%Mo=l 985-0.34(r-rE) mN/m. On basis of the Butler equation, the surface tensions of Co-Mo alloys were calculated, which agree well with the experimental results. Meanwhile, from the measured surface tension of liquid cobalt, the viscosity, the self-diffusion coefficient, the density, and the thermal diffusivity were derived.Using the electromagnetic levitation in combination with drop calorimeter method, the specific heats of undercooled liquid Co-10%Mo, Co-26.3%Mo, and Co-37.6%Mo alloys were determined to be 41.75, 43.73, and 44.82 Jmor'K'1 within their respective undercooling range. It was found that the specific heats of Co-Mo alloys increase linearly with the enhancement of molybdenum content.On basis of the measured surface tensions and specific heats of Co-Mo alloys, the Gibbs free energy difference AGLS, the crystal nucleation, and the fluid dynamics of liquid Co-Mo alloys were investigated. Calculations indicate that Turnbull's linear model for AGLS deviates greatly from its rigorous expression with the experimental data when undercooling is large. A minor deviation of Gibbs free energy difference will lead to a major deviation of the calculated nucleation rate. The fluid dynamics calculations indicate that the Marangoni number and Rayleigy number increase gently with the decrease of the temperature. Furthermore, the increase ofmolybdenum content in a Co-Mo alloy suppresses the gravity-driven convection and strengthens the Marangoni convection.The nucleation and growth characteristics of Co-Mo alloys were intensively investigated with electromagnetic levitation and drop tube containerless processing techniques. For Co-26.3%Mo and Co-37.6%Mo alloys, the containerless states produce substantial undercoolings up to 213K(0.13rL) and 39IK (0.24TK) before crystallization. Heterogeneous nucleation is the main nucleation mechanism for these two Co-Mo alloys. The microstructure of Co-26.3%Mo alloy is characterized by single phase of a-Co solid solution. a-Co dendrite transformed from solute diffusion controlled growth to thermal diffusion controlled growth when undercooling exceeds about 100K. Meanwhile, calculation shows that the difference between the measured thermophysical parameters and those approximate estimations influence the calculations greatly at the stage of thermal diffusion controlled growth. For Co-37.6%Mo eutectic alloy, the growth morphology transforms from lamellar eutectic to anomalous eutectic if the droplet undercooling exceeds a critical value. Meanwhile, a kind of spherical anomalous eutectic was found, which was ascribed to the spatial concentration field and thermal diffusion field caused by reduced gravity condition. The independent nucleation and cooperative dendrite growth of two eutectic phases are responsible for this growth mechanism transition. The three-dimensional structural model of anomalous eutectic grain is a two-phase composite dendrite.Microstructural characteristics of Ni-Sb eutecti...
Keywords/Search Tags:undercooling, liquid metal, surface tension, specific heat, rapid solidification, dendrite growth, eutectic growth
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