【作者】 刘礼；

【导师】 李金富；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2011， 博士

【摘要】 利用熔融玻璃净化和循环过热相结合的方法对Co-24at%Sn共晶合金,Co-20at%Sn亚共晶合金和Co-28at%Sn过共晶合金以及添加少量第三组元Mn、Sb后的Co-24at%Sn共晶合金进行了过冷凝固实验。利用高速红外测温仪对熔化和凝固过程中的温度变化进行了监测,并采用双探头方法测定了不同过冷度下凝固时的晶体生长速度。利用光学显微镜和扫描电镜对凝固后试样的宏观与微观组织进行了观察,采用电子背散射衍射技术分析了异常共晶组织中共晶两相α-Co和β-Co₃Sn₂晶粒的位向情况。在假设过冷熔体中共晶凝固界面以旋转抛物面枝晶生长,且第三组元加入不产生新相的前提下,建立了含少量第三组元过冷共晶合金凝固的理论模型。对过冷共晶合金凝固界面形态、凝固组织形成机制等进行了深入分析。随第三组元加入量的增大,共晶枝晶的尖端半径减小。第三组元的加入量小于某一临界值时,较小过冷度下共晶生长速度因第三组元的加入而增大,大过冷度下则因第三组元的加入而减小。第三组元的加入量超过临界值时,共晶生长速度恒变小。共晶层片（棒）间距在第三组元加入后的变化与生长速度正好相反。第三组元在共晶两相中的溶质再分配系数相差大时,第三组元加入对共晶生长的影响增大。Co-24at%Sn共晶合金的凝固组织,在过冷度小于20 K时全部由层片共晶组成。在更高的过冷度下由异常共晶和层片共晶组成,且随着过冷度的增大,异常共晶逐渐增多。所有过冷度下异常共晶中的两相晶粒均是随机取向分布,表明快速凝固阶段共晶两相以耦合方式生长,只是由于温度再辉初生层片共晶发生了重熔分解与熟化。在所研究的过冷度范围内,Co-24at%Sn共晶凝固界面以海藻状方式生长,其原因是耦合生长时共晶两相的固液界面能各向异性均较低,加之共晶界面分裂后的尖端半径远大于共晶层片间距,使得共晶凝固界面的有效界面能各向异性进一步降低。过冷度较低时,Co-24at%Sn共晶凝固界面为分形海藻状,而当过冷度达到175 K后,界面形貌转变为密集海藻状,并伴随有生长速度的快速上升。少量Mn和Sb的添加显著改变过冷Co-24at%Sn共晶合金的凝固行为。Mn的添加增大共晶凝固界面的界面能及其各向异性,导致小过冷度下共晶凝固界面由海藻状转变为树枝状,并使中等过冷度下分形海藻向高过冷度下密集海藻转变的临界过冷度增加至182 K,但Mn对共晶生长速度的影响较小,尤其在过冷度较低时。与Mn不同,Sb的添加主要影响共晶生长速度,使低过冷度下的生长速度上升,高过冷度下的生长速度降低,但Sb对共晶凝固界面形态影响较小。利用相关理论模型对过冷Co-24at%Sn共晶合金凝固时的晶体生长速度进行计算后发现,当稳定性参数采用临界界面稳定性理论所确定的值0.025时,计算值与实验结果相差很大。当采用“微观可解性理论”,令稳定性参数为0.001时,计算值与实验结果在共晶生长界面为分形海藻状时符合较好。更多还原

【Abstract】 Bulk Co-24at%Sn eutectic alloy, Co-20at%Sn hypo-eutectic alloy, Co-28at%Sn hyper-eutectic alloy and the Co-24at%Sn alloy with a small addition of Mn and Sb were undercooled to different degrees below the equilibrium liquidus temperature by the glass fluxing technique in combination with cyclical superheating, and a series of samples with different undercoolings for each alloy were solidified. The temperature change in melting and solidification was monitored by a high-speed infrared pyrometer. The crystal growth velocity was measured by two infrared pyrometers set along the sample axis （the dual-probe method）. The solidification structure was observed by an optical microscope （OM） and a scanning electron microscope （SEM）. The grain orientations of theα-Co andβ-Co₃Sn₂ phases in the anomalous eutectics were identified by the electron back scattered diffraction （EBSD） technology. Assmuing that the eutectic solidification interface advances in a dendritic form with a tip of paraboloid of revolution in the undercooled melt, and the small addition of a third element does not induce a new phase, a theoretical model was estabilished for the solidification of undercooled eutectic alloys containing a third element. The solidification interface morphology and the solidification structure formation of the undercooled alloys were comprehensively analyzed.As the content of the third element increases, the eutectic dendrite tip radius reduces. When the content of the third element is less than a critical value, the eutectic growth velocity becomes larger at low undercoolings but smaller at high undercoolings. Once the content of the third element exceeds the critical value, the eutectic growth velocity is always reduced. After the addition of the third element, the eutectic lamellar （rod） spacing changes in an opposite way of the growth velocity. When the partitioning coefficients of the third element in two eutectic phases differ greatly from each other, the effects of the third element on the eutectic growth become remarkable.The solidification structure of Co-24at%Sn alloy is composed of lamellar eutectics at undercoolings below 20 K, and lamellar eutectics plus anomalous eutectics at larger undercoolings. The fraction of anomalous etuectics in the solidification structure increases with undercooling. The grains of two eutectic phases in the anomalous eutectics orient randomly whether the undercooling is how large, indicating that coupled eutectic growth takes place in the rapid solidification stage. However, the primarily formed lamellar eutectics are subjected to superheating and partially remelting during the temperature recalescence and ripening in the subsequent solidification, and therefore evolved into anomalous eutectics.The eutectic solidification interface in the Co-24at%Sn alloy melt grows in seaweed modes at all experimental undercooling. The reason is that the solid/liquid interface energy anisotropies of the two eutectic phases are relatively low during coupled growth, and the eutectic interface tip radius after tip-splitting is much larger than the eutectic lamellar spacing, which leads to an extremely low interface energy anisotropy of the eutectic solidification interface. When undercooling is not too large, the eutectic solidification interface of Co-24at%Sn is of fractal seaweed pattern. Once undercooling exceeds 172 K, the interface morphology transforms to a compact seaweed pattern, accompanyied with a fast increase of growth velocity.Small additions of Mn and Sb remarkablely change the solidification behavior of undercooled Co-24at%Sn eutectic alloy. The addition of Mn increases the interface energy and its anisotropy, making the eutectic solidification interface change from a seaweed into dendrite form at low undercooling, and the critical undercooing for the eutectic interface to transit from a fractal seaweed at moderate undercooling to a compact seaweed at large undercooling increases to 182 K. Different from Mn, the addition of Sb mainly influences the eutectic growth velocity, i.e. the addition of Sb increases the growth velocity at lower undercoolings, and decreases the growth velocity at larger undercoolings.The eutectic growth velocity in the undercooled Co-24at%Sn eutectic melt was calculated using related theoretical models. It is found that when the critical stability parameter is let to be 0.025 according to the interface critical stability theory, the calculated growth velocities are obviously different from the experimental values. If the“microscopic solvabiltycriterion”is adopted and the critical stability parameter is let to be 0.001, the calculation results fit the experimental growth velocities well even if the eutectic growth interface is of fractal seaweed pattern.更多还原