【作者】 崔红保；

【导师】 郭景杰； 苏彦庆；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2007， 博士

【摘要】 本文选择典型的Al-In和Cu-Pb偏晶合金作为研究对象,研究了在定向凝固工艺条件下微观组织的转变过程并对其进行了理论分析,同时采用相场法模拟了偏晶合金微观组织演化过程。在高温度梯度下对Al-In偏晶合金进行定向凝固,考察了不同凝固速度、化学成分以及第三组元对偏晶合金凝固组织的影响,获得了定向凝固条件下微观组织的演化规律,即在固定的温度梯度下,随着抽拉速度的增大,第二相形态演化的过程依次为:规则的纤维状,发生颈缩的纺锤状,粗化的球状,均匀弥散分布于基体中。纤维间距随着凝固速度的增大而减小,随着温度梯度的增大,纤维状向球状转变的临界速度相应增大。在Cu-Pb亚偏晶成分范围内进行定向凝固时,组织呈现枝晶的形貌,当铅含量增加时,组织由枝晶向不规则的棒状组织转变。随着铅含量的增大,组织由枝晶向不规则的棒状组织转变所需要的凝固速度减小。在Cu-Pb过偏晶成分范围内进行定向凝固时,抽拉速度和铅含量较低时,容易形成拉长的粒子状第二相,而后随着抽拉速度的进一步增加向不规则的棒状第二相转变。而抽拉速度较低和铅含量较高的时候形成带状组织,随着抽拉速度的进一步提高组织会由带状向拉长的粒子状组织转变。在偏晶点附近形成不规则的棒状复合组织的铅含量随着凝固速度的增加而增大。基于共晶理论模型考虑生成相密度的不同对凝固界面前沿溶质场进行求解,在此基础上对过冷度计算,进而进行偏晶合金定向凝固中第二相纤维球化以及规则第二相纤维与非规则第二相纤维之间转变的理论预测,综合考虑了界面能、第二相体积分数以及液相线斜率对偏晶合金定向凝固组织的影响因素的情况下,对偏晶合金定向凝固规则与非规则第二相纤维转变提出一个新的判据。并利用此判据对五种典型的偏晶合金系进行计算,所获得预测结果与实验观察的第二相纤维规则情况吻合较好。通过对沿着规则L2相纤维表面成正弦分布的扰动进行分析,以在纤维表面扰动幅度达到R0时所需要的时间tp小于试样在凝固界面以下TM到共晶反应温度TE之间运动时间t为临界条件,推导出纤维状的第二相可以发生颈缩,从而导致球化的判据。采用计算效率和计算精度较高的Fourier变换谱分析方法求解Cahn-Hilliard相场方程,对Cu-Pb过偏晶合金液相分离过程中液滴生长和Ostwald熟化进行模拟,研究了第二相液滴扩散长大过程,同时耦合Navier-Stokes方程计算了流动对Cu-Pb过偏晶合金的液相分离过程中液滴生长过程的影响。研究表明在过饱和基体中,单个第二相液滴生长的扩散过程主要是系统自由能作用的结果,液滴半径的长大速率与经典的Zener理论吻合良好。计算区域中多个第二相液滴粗化后液滴半径的分布情况,与LSW解析理论吻合较好。第二相液滴在生长过程中,除了扩散生长、Ostwald熟化以外,还受到定向凝固温度场中温度梯度的作用,产生Marangoni运动,在Marangoni力的驱动下,液滴向温度高的区域发生移动,在随后的凝固过程中,由于不同半径的液滴之间受到Marangoni力的大小不同,移动速度也不同,导致各种半径大小的液滴之间发生碰撞,从而加速了液滴半径粗化的进程,使液滴半径尺寸沿着温度梯度方向逐渐的增加。干摩擦条件下定向凝固Cu-Pb金属基自润滑复合材料的摩擦系数随固体润滑剂体积分数的增加而下降,减摩自润滑效果逐渐提高。铜铅自润滑材料在含油的状态下,形成了一种复合润滑体制,具有很强的吸油能力,能形成高效能的胶体油铅混合液态薄膜。由于摩擦接触表面间油膜的存在,减轻了微凸凹体的接触及表面间元素传递过程,降低了摩擦系数。由于定向凝固工艺方法使得第二相更加的均匀,因此定向凝固制备的铜铅自润滑材料普遍比熔铸方法所制备的铜铅自润滑材料在油润滑状态下的摩擦系数要低。更多还原

【Abstract】 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.更多还原