【作者】 朱晶；

【导师】 王同敏；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2013， 博士

【摘要】 合金的宏观力学性能与其微观组织紧密相关,而微观组织演变主要受控于凝固过程。电场通过作用于合金熔体的力与热来改变合金凝固过程的热力学与动力学,是一种先进而有效的凝固过程调控手段。探究电场调控下合金凝固微观行为,深入系统地研究凝固过程的动力学机制,从而为科学合理地制定凝固参数提供理论依据。传统的实验研究方法是借助金相显微镜、扫描电镜等对淬火后的凝固组织进行观察和分析,但仅可以得到某一特定时刻某一截面的微观组织特征,这样势必会错失一些重要的凝固过程动态信息。高能同步辐射硬X射线原位成像技术的出现为研究合金凝固过程动态行为提出了新的思路和技术途径。利用具有高亮度、高能量、高分辨率的同步辐射X射线光源以及具有高速读写的CCD成像系统,能够对合金凝固过程中微观组织演变的动态过程进行实时成像。其成像结果可为合金凝固过程微观行为及内在机制的研究提供直观的实验数据。本文应用同步辐射实时成像技术,原位研究了电场调控下Sn-Pb、 Sn-Bi合金的枝晶生长行为以及A1-Bi难混溶合金的液液相变／分离过程,深入理解了电场调控下枝晶生长与两相分离的动力学机制。研究了直流电场对Sn-Pb合金的枝晶生长形貌和生长速率的影响。实时成像结果表明,由电场产生的洛伦兹力驱动的熔体流动能够冲刷枝晶的底部和侧枝,减少其周围的溶质富集,使得枝晶臂在各个方向均衡发展,大大减弱了枝晶的“自我中毒”现象(即枝晶臂生长不均衡),最终呈现出近似于规则的等轴晶形貌。同时发现电流的作用会改变枝晶生长前沿的溶质分布,使得邻近的枝晶彼此之间通过溶质扩散层相互干扰,从而降低了生长速率,并提前终止了枝晶的自由生长。研究了直流电场对Sn-Bi合金凝固过程的影响。实验结果发现,由于固液相之间电导率的差异,电流聚集于枝晶尖端,枝晶尖端以分散聚集电流(降低焦耳热)的自我调节方式来维持继续生长,尖端形状呈圆润或平直形貌。在施加直流电场的熔体中并且存在温度梯度情况下,熔体中会产生具有一定扰动性的电磁体积力,加之自然对流和溶质偏析导致的密度差,驱使上浮枝晶发生明显的大角度旋转。在凝固过程中的不同阶段施加脉冲电流对Sn-Bi合金的凝固组织有很大的影响。成像结果表明,在Sn-Bi合金的整个凝固过程持续施加脉冲电流时,经历了长时间的孕育形核期后,枝晶在很短的时间内迅速形核与生长,一次枝晶细化现象非常明显。仅在形核过后的枝晶生长阶段施加脉冲电流时,已经长大的枝晶先被脉冲电流加热重熔,而后重新凝固生长的枝晶仍然在原来的枝晶臂位置处生长,呈现一定的金属遗传性特征,并且未观察到一次枝晶细化现象,实时成像印证了脉冲电流施加时机的重要性。对Al-Bi难混溶合金凝固过程中第二相液滴的微观行为进行了原位观察。证实了两个半径尺寸相近的液滴之间是以碰撞的形式发生聚合进而粗化成一个接近于椭圆形的液滴；半径尺寸相差悬殊的液滴之间是通过尺寸较大的液滴慢慢吞并周围无数的小液滴而发生Oswald熟化,形成一个近似于圆形的液滴。原位观察了脉冲电流对Al-Bi难混溶合金第二相液滴微观行为的影响。结果表明,施加脉冲电流后,在液液相难混溶区中分离出的富Bi液滴不再是圆球形貌,而是凝聚成许多云状的大团簇。第二相液滴因为受到Marangoni力和电磁挤压力的共同影响而沿着两力的合力方向运动,最终在基体中形成了一个明显的倒三角形状。基于同步辐射X射线三维CT成像技术研究了Al-Bi难混溶合金第二相颗粒的三维形貌及分布特征,同时考察了三种不同晶粒细化剂Al-3B、Al-5Ti-B和Al-3Ti对第二相颗粒的影响。三维重构结果可清晰地显示Al-Bi难混溶合金的内部结构信息,结果表明A1-3B和Al-5Ti-B细化剂对Al-Bi难混溶合金凝固组织细化的效果不明显,球形的第二相颗粒尺寸较大且在基体中分布不均匀。经Al-3Ti细化的第二相颗粒尺寸较小,颗粒三维形貌呈球形,而且均匀弥散地分布在基体中,在所选的几种细化剂中对Al-Bi难混溶合金凝固组织的细化效果最优。更多还原

【Abstract】 Macroscopical mechanical properties of alloys mainly depend on its microstructure. The solidification process plays a dominate role in the evolution of microstructure. Electric field is an advanced and effective methold to vary thermodynamics and kinetics during solidification of alloys by affecting the force and heat in alloy melt. It is helpful to study the kinetics mechanism of microstructure under electric field based on probing microscopic behavior of alloys with electric field treatment during solidification. Accordingly, it can provide theoretical proofs to scientifically and reasonably work out electric field parameters.The conventional experimental methods are based on scanning electron microscope (SEM) and transmission electron microscope (TEM) to observe and analyze the final solidification structure after complete solidification or interrupting solidification by quenching. It can be only obtained the characters of microstructure in the particular moment and the particular section. It will lead to the missing of some crucial dynamic information during solidification process. Synchrotron radiation real-time in situ imaging technology raises a new research idea and opens a novel window for the study on dynamic solidification process of alloys. This method, which utilizes the high brightness, energy and resolution synchrotron radiation X-ray source combined with a high fast-readout charge coupled device (CCD) could capture many dynamic process of microstructure evolution during solidification process of alloys and real-time record the whole solidification process of alloys. The real-time imaging results can offer the direct proofs to investigate microscopic behavior and intrinsic mechanism during solidification of alloys. In present work, the study of the microscopic behavior in Sn-Pb, Sn-Bi alloys and the liquid-liquid phase transformation and separation in Al-Bi alloys during solidification process with electric field treatment has been achieved by synchrotron radiation X-ray real-time in situ imaging technology. The kinetics mechanism of dendrites growth and two-phase separation under eceltric field has been understood in depth.The modification of dendrite growth morphology and growth rate in Sn-Pb alloy with direct current (DC) treatment has been studied. The results show that the Lorentz force generated by DC washes the sides and underpart of the dendrites, which decreases the solute concentration nearby. The dendrites in every direction grow with the same rate and the dendrite "self-poisoning" is thus weakened. Finally, a nearly regular equiaxed dendrite appears. Meantime, it is clear that the electric current can vary solute distribution ahead of dendrite growth front. The neighbouring dendrites growth will be affected by solute diffusion layer. Consequently, the growth rate decteases and dendrite growth stops ahead of time.The influence of DC on the solidification process of Sn-Bi alloy is observed. The results demonstrate that the electric current will gather on the dendrite tip due to the difference of conductivity between solid and liquid phase. The sharp tip tends to be round or flat with an ability of self-adjustment to scatter the electric current for reducing Joule heat in order to keep growing. When the melt with temperature gradient subjected to the case of direct current, an electrical body force appears which can disturb the melt intensively, together with natural convection and density difference caused by solute segregation, leading to the floating dendrite rotate obviously.The effect of electric current pulse (ECP) on microstructure of Sn-Bi alloy during different solidification stages has been investigated. When ECP was imposed on the whole solidification process, dendrites break out to grow with a large growth rate in such a short time after the long nucleation incubation period. It can be seen that the refinement of Sn-Bi alloy is clear. When ECP was imposed only on the stage of dendrite growth, the grown dendrites are melted due to the heat effect of ECP. As the solidification proceeding, the melted dendrites reappear based on the previously growing trajectories and present hereditary character. However, no refined dendrites can be found in this experiment. The imaging experiments confirme the importance of time to impose ECP.The microscopic behavior of second-phase droplets in Al-Bi immiscible alloy during solidification process is in situ observed. It is demonstrated that the droplets with little difference in radius size collide with each other to form a new ellipse-like droplet. While the Swald coarsening occurred among droplets with extreme difference in radius size. The bigger droplet gradually engulfs a great number of droplets around it and form a nearly rounded droplet with tremendous radius.The influence of ECP on microscopic behavior of second-phase droplets in Al-Bi immiscible alloy has been investigated. It is clear that the separated Bi-rich droplets form lots of big clusters which are no longer the sphere in shape but the cloud morphology. Due to the effect of Marangoni force and electromagnetic pinch force, the second-phase droplets move along the direction of their resultant force and form an inverted triangle shape in the Al matrix finally.Based on synchrotron tomography imaging technology, the three-dimensional morphology and distribution of second-phase particles in Al-Bi immiscible alloy have been studied. Meanwhile, the effect of A1-3B, Al-5Ti-B and Al-3Ti grain refiner on Al-Bi immiscible alloy has been probed. After restructuring the solidification microstructure of the alloys, it is found that A1-3B and Al-5Ti-B can refine Al-Bi alloy, but the second-phase particles distribute inhomogeneously in matrix and the larger particles still exist. After Al-3Ti grain refiner added, not only second-phase particles appear as spherical morphology with tiny radius size, but also distribute homogeneously in matrix. The refining effect of Al-3Ti on Al-Bi immiscible alloy is excellent.更多还原