【作者】 赵素；

【导师】 周尧和； 李金富；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2009， 博士

【摘要】 深过冷是大体积液态金属实现快速凝固的唯一途径,广泛用于金属非平衡凝固理论研究和块体非平衡材料的制备。在考察共晶合金凝固组织和熔体过冷度之间关系的过程中研究者发现,原本在小过冷度下以耦合生长方式凝固的共晶合金在大过冷度下以离异生长方式进行凝固,并存在规则层片共晶向反常共晶组织的转变。对于这样一些凝固现象产生的原因,至今仍存在较大的争议。Ag-28.1wt.% Cu共晶是极少数由端部固溶体构成的共晶合金体系,共晶两相成分差别大,凝固过程中溶质分凝显著,本实验选择该合金为研究对象,可望在揭示新凝固现象的同时,完善对已有问题的认识。鉴于此,本文用熔融玻璃净化法和循环过热法将Ag-28.1wt.% Cu(x=0)共晶合金过冷至不同过冷度,研究了其凝固行为及组织形成机制。并通过向Ag-28.1wt.% Cu共晶合金中分别添加0.5和1wt.% Sb(x=0.5,1),研究了共晶凝固界面在第三组元扩散和热扩散联合作用下在过冷熔体中的界面形态演化规律及Sb对Ag-28.1wt.% Cu共晶合金过冷凝固的组织和生长速度等的影响。获得了以下主要研究成果:在过冷共晶LZ(Li-Zhou)理论模型的基础上,讨论了溶质截留对过冷共晶生长的影响。与LZ模型相比,溶质截留使共晶生长速度增大,层片间距和共晶枝晶尖端半径减小,而且随着过冷度的增大,生长速度增加越显著。通过实验测试Ag-Cu共晶生长速度,验证了考虑溶质截留后理论模型的正确性,说明当平衡溶质分配系数小时,过冷共晶生长理论模型中应该考虑溶质截留效应。在实验所得到的过冷度范围内(≤100 K),Ag-28.1wt.%Cu共晶合金始终以层片耦合生长方式凝固。其中当过冷度小于临界值76 K时,层片共晶从试样表面形核点处向外以胞状形态生长,凝固后的试样中存在三种微观组织区域:形核点附近的反常共晶区,围绕反常共晶组织的胞状层片共晶区,试样末端的等轴层片共晶区。Ag-28.1wt.%Cu共晶合金中长的共晶线(两个共晶相成分之差)和合金熔体大的热扩散系数是导致共晶胞状生长发生的原因。分析表明,形核点附近非平衡凝固条件下形成的层片共晶组织处于较高的溶质过饱和状态,再辉过程中部分被重熔,随后熟化为反常共晶组织。柱状晶区中最细层片间距沿生长方向随离开形核点距离的增加而增加,表明快速凝固过程中生长速度逐渐减小,晶体进行非稳态生长,共晶合金试样中具有区域特征的微观组织特点也支持了非稳态生长这一观点。当过冷度等于或者大于76 K时,Ag-28.1wt.%Cu层片共晶转为以树枝状方式生长。试样凝固后的组织也不再具有分区特征。在试样内部快速凝固过程中形成的共晶枝晶因过热熔断,并于随后的慢速凝固阶段熟化,从而形成了反常共晶组织。但试样表面较好的散热条件则使该处的共晶枝晶组织能够幸存下来。在临界过冷度76 K处,伴随非稳态胞状生长向稳态共晶枝晶生长的转变,生长速度突然增大。第三组元Sb的添加导致了固-液界面前沿形成“成分过冷”区,但并不改变原Ag-28.1Cu共晶合金低过冷度下(<76 K)非稳态生长及高过冷度下(76 K～100 K)稳态生长的特性。然而随着Sb加入量的增大,低过冷度下界面形态发生了从胞状到胞枝状,再到不发达树枝状的转变。添加第三组元Sb后,Ag-28.1wt.% Cu共晶胞(枝)晶的尖端半径减小,生长速度增大,而且随着Sb添加量的增大,这样一些效应更为显著。加入第三组元Sb后,由于再辉度和生长速度的增大,反常共晶组织的体积分数增加,反常共晶组织中粒状晶的晶粒尺寸增大。更多还原

【Abstract】 Deep undercooling is the only way of achieving rapid solidification of bulk liquid metals, and has been widely used in investigating the non-equilibrium solidification theory and preparing of bulk non-equilibrium materials. While studying the relationship between the solidification structure of eutectic alloys and the melt undercooling, it has been discovered that eutectic phases grow in a coupled mode at small undercoolings but in a decoupled mode at large undercoolings. The solidification far from equilibrium results in a transition from regular lamellar eutectics to anomalous eutectics. So far there are furious controversies among researchers on the causes of such solidification behaviors.Ag-28.1wt.% Cu eutectic alloy is one of few eutectic alloy systems composed of terminal solid solutions. The large difference in composition between the two eutectic phases leads to a considerable lateral diffusion of solute ahead of the solidification interface. Novel solidification behaviors are expected to be revealed as well as the mentioned problems can be understood deeply, if the Ag-28.1wt.% Cu eutectic is considered as a research object. Therefore, in this dissertation,the Ag-28.1wt.% Cu (x=0) eutectic alloy was undercooled by the glass fluxing technique in combination with cyclical superheating and the solidification behaviors and formation mechanism of structures were investigated. Furthermore, 0.5 and 1 wt.% Sb (x=0.5, 1) is added to the Ag-28.1wt.% Cu eutectic alloy respectively. The evolution of eutectic interface morphology with the Sb addition, and the other solidification behaviors were studied. The main research achievements are as follows:The effect of solute trapping on the eutectic growth of undercooled alloy melts has been investigated on the base of the LZ (Li-Zhou) eutectic growth model. It is revealed that solute trapping makes growth rate increase, and lamellar spacing and dendrite tip radius decrease. And the increase of growth rate becomes more remarkable with the increasing undercooling. By measuring the eutectic growth rate at different undercoolings, it is verified that the LZ theoretical models including solute trapping can predict the eutectic grow very well, and the solute trapping should be taken into account while analyzing eutectic growth if the equilibrium solute partition coefficients are small.The Ag-28.1wt.% Cu eutectic alloy melt solidifys in coupled eutectic lamellae during rapid solidification up to the largest experimental undercooling 100 K. However, when undercooling is less than a critical value of 76 K, cellular growth of lamellar eutectics from the nucleation site takes place because of the large difference in composition between two eutectic phases and the very large thermal diffusion coefficient of the liquid. Three regions of microstructures are observed in the sample. They are the anomalous eutectic region near the nucleation site that generally located at the sample surface, cellular eutectic region in the middle, and equiaxed lamellar eutectic region at the end. The primary lamellar eutectics near the nucleation site solidify under conditions far from equilibrium, and therefore are supersaturated with more solute, and then partially remelted and ripened into anomalous eutectics. With the distance along the growth direction increasing, the finest lamellar spacing across the cellular eutectics rises, indicating a gradually decreasing growth velocity of the primary eutectics. This means that the eutectic growth during rapid solidification is unsteady. Such an argument is also supported by the appearance of the three regions of microstructures.When undercooling is equal to or higher than 76 K, lamellar eutectics grow in a dendritic form during rapid solidification. There are no longer regional characteristics in the solidification microstructures. The significant remelting and ripening of the primary eutectic dendrites result in appearance of anomalous eutectics inside the sample. But the eutectic dendrites on the sample surfaces survive because of the better heat dissipation conditions. The transition from the unsteady cellular growth to steady eutectic dendrite growth leads to a sudden increase in recalescence rate at the critical undercooling 76 K.The addition of a third element Sb into the Ag-28.1wt.% Cu eutectic alloy creates an additional constitutional undercooling ahead of the solid/liquid interface, but does not change the unsteady eutectic growth at low undercoolings (<76 K), and the steady eutectic dendrite growth at high undercoolings (76 K ~ 100 K). The solidification morphology changes from a cellular into a cellular dendritic and then an undeveloped dendritic form with the increasing addition of Sb at low undercoolings. After the addition of Sb, the cell or dendrite tip of the Ag-28.1wt.% Cu eutectic is sharpened. As a result, the eutectic growth is accelerated and recalescence rate is increased. Meanwhile, the volume fraction of anomalous eutectics in the microstructure and the size of the particles in the anomalous eutectics increase because of the increasing recalescence degree and recalescence rate.更多还原