同步辐射X射线原位观测研究固相受力对固/液平界面稳定性的影响

Solid/liquid interface is the starting point of the various patterns during the solidification of metals, and its stability determines metallic materials’ initial morphology, the formation of defects and the homogeneity of the compositions. Understanding the stability of solid/liquid interface is the preconditions to study the formation mechanism of the solidification structure, and therefore to realize the active control of solidification process. From the 'constitutional supercooling' theory to the dynamics perturbation analysis of MS (Mullins-Sekerka) theory, and then the non-linear analysis theory with time correlations consideration, the stability of interface has been developed and gradually perfected. Nevertheless, the general cases that the solidification shrinkage causing stresses in the solid has not been taken into account when analyzed the solid/liquid interface stability. In fact, except stresses caused by solidification shrinkage, during some other casting processes like high-pressure die casting and the centrifugal casting, the forces acting on the solid are stronger and inevitable. To understand how the structure during those casting processes and then to optimize them, how the forces in solid affect the solid/liquid interface stability must be known in advance. Aiming at this problem, the proposed project will use the synchrotron X-rays radiography to observe the solidification process in-situ and real time, and the effect that applying magnetic field can make the solid phases subjected to the controllable and quantifiable forces to experimentally study how the forces (magnetizing force、thermoelectric magnetic force)in solid affect the stability of solid/liquid planar interface. The criterion for the instability of solid/liquid interface with the stressed solid will be founded, and the formation mechanism of solidification structure with the stressed solid phases will be illuminated. This would enrich the theories of solidification and interface stability, and therefore provide a new theoretical foundation to optimize the solidification control method.

固/液界面是金属凝固过程中各类复杂花样的起点，其稳定性决定了金属材料的初始组织形态、缺陷的产生及其成分均匀性。对固/液界面稳定性的理解，是认识凝固组织形成机制，进而实现凝固过程主动控制的前提。自“成分过冷”理论，到MS理论的动力学扰动分析，再至考虑时间相关性的非线性分析理论，人们对界面稳定性的认识不断的发展和完善。然而，凝固收缩所致固相中的应力，以及有些工艺过程中（如压铸、离心铸造）固相受到的外力，对界面稳定性的影响至今尚未得到充分的认识和研究。本项目正是针对这一问题，拟采用同步辐射X射线原位观测的方法，利用外加磁场可使已凝固固相受到可控、可定量外力（磁化力、热电磁力）的效应，实验研究固相受力对固/液平界面稳定性的影响规律。建立固相受力情况下固/液平界面失稳的判据，阐明固相受力情况下凝固组织形成机制。这将进一步丰富金属凝固以及固/液界面稳定性理论，为优化金属凝固过程控制提供新的理论依据。

固/液界面稳定性是金属凝固形成各类复杂组织花样的起源，对于这一基础问题的深刻认识是实现凝固过程主动控制的前提。研究者们通常采用两种方法来研究固/液界面稳定性的问题，一是理论推导和凝固模型的建立，界面稳定性的理论经历了“成分过冷”理论→MS理论→非线性界面稳定性分析的发展历程；二是从精确控制凝固条件的定向凝固实验中研究固/液相界面的形态从非稳定到稳定态的演变过程，以此来揭示晶体生长过程中凝固界面形态的演变规律。本文以Al-Zn、Al-Cu二元合金为研究对象，结合定向凝固过程X射线原位观测和淬火法保留界面两种实验手段，对固/液界面演化过程和固相受力对界面稳定性的影响等问题进行了研究。获得主要结果如下：.通过对Al-Zn二元固溶体合金的定向凝固过程进行X射线二维与三维原位观测，发现在降温速率R恒定，温度梯度G随时间线性变化的动态控温过程中，固/液界面形貌经历由树枝晶向胞状晶，胞状晶向平面晶生长的转变。实验与经典界面稳定性理论一致，定向凝固温度梯度越高，界面稳定性越强。.定向凝固过程中，由于界面前沿固相溶质排出而引发的液相自然对流会对界面形态产生影响，造成所谓宏观尺度下的长波界面失稳现象。这种长波失稳表现为宏观水平界面变形为局部凸起的界面。对界面形态的三维空间结构表征显示，在高温度梯度、低生长速率的条件下长波失稳程度较小，随着生长速率的提高，长波失稳呈现出先增大后减小的趋势。这种变化规律是自然对流与生长速率共同影响下产生的结果。.在纵向稳恒磁场作用下，液相对流被洛伦兹力抑制，因此液相流动对界面形态的影响消失，而固相则由于热电磁效应受到热电磁力的作用。该力在固相组织中会引起扭矩和应力，受此影响，固/液界面稳定性降低。宏观组织的生长方向也因此受到影响，糊状区的固相会向受力方向发生偏移，从沿温度梯度方向竖直生长转变为环绕圆柱试样螺旋式的生长。