Microtructure Evolution Of Al-In And Cu-Pb Monotectic Alloys Directionally Solidified In High Temperature Gradient

In this paper, Al-In and Cu-Pb monotectic alloy, are chosen as the investigated subject. Experimental study of microstructure evolution and theoretical analysis are carried out during the directional solidification, and the droplets coarsening are simulated using a phase-field method.In the condition of high temperature gradient, the effect of the growth rate, chemical composition and the third element on the fibres are examined. The results show that the regular fibre of L2 phase of the directionally solidified Al-In alloy change from fibre, spindle fibre, periodical and regular array of In droplets to random dispersion of In droplets in the aluminum matrix with increasing growth rate. And the critical velocity from fibre to droplet becomes larger with increasing the temperature gradient.Microstructure of the directionally solidified Cu-Pb hypomonotectic alloy changes from columnar dendrite to the irregular rod composite structure with increasing lead content and growth rate, and the transition of the Cu-Pb hypermonotectic alloys from the band structure and elongated droplets to the irregular rod composite structure is observed with increasing growth rate. The range of compositions of forming the rod composite structure around the monotectic points increases with increasing the growth rate.The modified Jackson-Hunt model of eutectic solidification is employed to analyze the coupled growth of monotectic directional solidification due to the similarity between the eutectic alloys and monotectic alloys. The effect of interfacial energy, volume fraction and liquidus slope on the morphologies are included in the criterion. The criterion can determine that whether regular or irregular morphology occurs. The results show that the prediction calculated by the modified model agrees well with the morphologies observed in experiments. Sinusoidal perturbation is imposed on the regular fibre of L2 phase. The fibre stays liquid state in the temperature domain lying between the monotectic and eutectic temperature. If the time required for the specimen to cross the domain is larger than the time needed for the fibres to pinch-off, the spheroidization of fibre can take place. A criterion for the spheroidization of fibre is developed. Cahn-Hilliard equation coupling with thermodynamic data is solved by Fourier transformation spectrum method due to the high efficiency and accuracy to simulate droplet growth and Ostwald ripening for Cu-Pb hypermonotectic alloy. The effect of flow on the microstructure evolution during the liquid separation process is included by computing the Navier-Stokes equation. The simulation results show that growth rate of droplet radius is in good agreement with the well accepted Zener's law. Size distribution of reduced droplet radius matches well with classical LSW theory. In addition, the marangoni motion drived by interfacial force gradient can take place besides the diffusion, Ostwald ripening during the droplet growth. In the subsequent solidification, the liquid droplets can collide due to the different velocity drived by the various marangoni forces. Therefore, the coagulation is dramatically accelerated by flow. The size of droplets increases along the temperature gradient.Under the dry sliding condition, the wear coefficient of directionally solidified Cu-Pb alloy becomes small and the anti-attrition is improved with increasing the lead content. Under the oil lubrication, the mixed film is formed between the bearing and journal surface. Mass transfer becomes weak between the sample and grinding disc. The wear coefficient becomes small due to the existence of the film. In the mean time, the wear coefficient of the sample prepared by the directional solidification is smaller than the one of sample prepared by the conventional casting, because the distribution of second liquid droplets in the sample prepared by the directional solidification is more homogenous than that of sample prepared by the conventional casting.