定向凝固晶粒竞争生长过程、机制及控制的相场法研究

How to control the competitive grain growth during directional solidification precisely is the first and very common issue encountered during the preparation of advanced single-crystal materials. However, the traditional mechanism of competitive grain growth based on Walton-Chalmers model not only cannot supply enough theoretical supports to solve this issue perfectly, but also has faced serious challenges. The main reasons for this situation are the traditional investigation ignored the solute interactions which plays a more important role in the microstructual evolution during solidification and could only supply simple segment information for the complex competitive growth process due to the limitation of research tools. In the present project, we will investigate the process and mechanisms of competitive grain growth during directional solidification using phase field simulation and in-situ observation of transparent alloys. Through the analysis of the microstructual evolution and solute distribution during bi-crystal directional solidification, effects of growth mode, growth dimensions, controlling parameters and material parameters on the dendritic tip characteristics, primary dendritic spacing, the development of secondary and thirdly dendritic arms, the formation and development of grain boundary and grain elimination will be studied in a systematic way. We will try to reveal the mechanisms of the competitive grain growth during directional solidification, and clarity the influence of materials' parameters and controlling parameters on the competitive growth. With the directional solidification experimental verification of superalloys, a grain elimination criterion for two-grain system will be built. The accomplishment of this project can supply some useful information and theoretical supports for the preparation of high performance single crystal materials for precisely controlling the integrity during directional solidification.

如何精确控制定向凝固过程中的晶粒竞争生长是先进单晶(定向)材料制备的共性和关键问题。但当前基于Walton-Chalmers思想的传统晶粒竞争生长理论不仅不能为完满解决这一问题提供足够的理论支撑，而且受到了严重的挑战。其主要原因在于传统研究手段单一，仅能获得晶粒竞争生长这一复杂过程时空上的片段信息，并且忽略了对凝固微观组织演化更具影响力的溶质场的作用。本项目拟结合相场法数值模拟及透明模型合金实时观测，通过对定向凝固双晶晶粒竞争生长过程中组织演化及溶质输运的数值模拟和实时观测，系统研究生长方式、生长维度、工艺参数及材料特性等对尖端状态、枝晶间距、侧向分枝发展、晶界形成与演化及晶粒淘汰与否等的影响。阐明定向凝固晶粒竞争生长机制及其作用条件，明确材料特性及工艺参数对晶粒竞争生长的影响规律，并最终通过实际高温合金实验验证，建立双晶晶粒淘汰临界条件判据，为实现单晶完整性的主动控制提供理论和技术支持。

如何精确控制定向凝固过程中的晶粒竞争生长是先进单晶(定向)材料制备的共性和关键问题。本项目结合相场法数值模拟及透明合金实时观测，通过考察不同条件下定向凝固双晶晶粒竞争生长过程中组织演化及溶质输运过程，阐明了定向凝固晶粒竞争生长过程、机制及控制取得的主要成果如下：.1..建立了适用于不同取向多晶竞争生长的定量相场模型，并基于GPU加速算法实现了相场模型的高效大规模并行计算求解；搭建了透明合金双晶系统定向凝固组织形貌演化的实时观测平台。.2..阐明了定向凝固枝晶侧向分枝起源及波包演化规律，揭示了侧向分枝调控规律与侧向分枝产生机制的内在关联；建立了枝晶倾斜生长角度与抽拉速度及枝晶一次间距间的定量关系。.3..再现了定向凝固双晶汇聚生长中非择优枝晶淘汰择优枝晶的反常淘汰现象，基于溶质场的相互作用提出了反常淘汰的内在机制；确定了抽拉速度对汇聚晶粒竞争淘汰的影响规律，揭示了抽拉速度对双晶汇聚生长晶粒淘汰规律影响的内在机制。.4..发现了定向凝固发散双晶竞争生长过程中晶界处一次枝晶臂的形成具有局部随机性，阐明了侧向分枝的形成与生长对新生一次枝晶臂局部随机性的影响机制；揭示了定向凝固发散双晶竞争生长过程中的晶界演化规律，建立了定向凝固发散双晶竞争生长的晶界角度选择模型。.5..系统研究了三维条件下双晶共面和异面竞争生长过程，阐明了试样盒厚度对反常淘汰的影响规律及机制；获得了发散生长过程中一次枝晶臂和二次枝晶臂取向对枝晶生长方向和晶粒竞争淘汰结果的影响规律；再现了三维空间中异面双晶晶界组织形貌的演化过程，阐明了晶粒淘汰速率随着非择优晶粒偏角变化的演化规律；获得了双晶一次臂的位置关系及侧向分枝的生长位向对汇聚生长过程中晶粒淘汰速率的影响规律。. 以上研究结果可为实现单晶完整性的主动控制提供了理论和技术支持。