非晶形成能力的拓扑密堆团簇表征研究

The difficulty in describing the microstructure of disordered system leads to the result that the physics of vitrification and the glass-forming ability (GFA) of materials are still to be uncovered; this seriously limits its application and hampers the progress of related studies. The proposed empirical criteria mostly based on related properties (such as Tg and elastic constants) lack and the ability of prediction. Metallic glasses are the frozen liquids, thus it is the study on metallic liquids that is possible to fundamentally solve the prediction problem of GFA, where an effective structural characteristic is crucial. The topological close-packed cluster (TCPC) can consistently describe the prevalent topological order in disordered systems, including icosahedral, local five-fold symmetry order, and so on. A topological closed-packed cluster (TCPC) is a local compact structure where 4 atoms contacting with each other always comprise a perfect or slight-deformed tetrahedron, is the common and essential structures of metallic liquids and glasses. With TCPC as breakthrough, the relationship between structure indexes (the proportion, distribution, combination, transformation, heredity of TCPC) and system parameters (the components, temperature, pressure, and cooling/heating rate of alloy), will be systematically examined for typical metallic liquids (such as Cu, Ta, NiAl, CuZr, CaMgCu, and CaMgZn), this project aims to quantify the critical role of TCPC in glass formation, clarify the dynamics and thermodynamics mechanisms, establish the prediction model for GFA based on the evolution characteristic of TCPC in liquid metals, provide theoretical guide for developing novel high-GFA amorphous alloys.

无序体系结构表征上的困难，导致非晶化的物理机制，材料的非晶形成能力（GFA）至今不明，严重限制其应用和相关研究；现有的GFA判据多基于已有非晶材料的相关特性(如Tg和弹性常数)，预测能力不够。金属玻璃是冻结的液体，因此研究液态金属才能从根本上解决GFA的预测问题，而有效的结构表征是关键；拓扑密堆团簇（TCPC）能统一描述包括二十面体、局域五重对称等普遍存在的拓扑序。TCPC是原子间隙均为理想或略有变形的四面体的致密局域结构，是金属液体与玻璃共有的基本结构特征。本项目以TCPC为突破口，系统研究典型液态金属（Cu、Ta、NiAl、CuZr、CaMgCu和CaMgZn）内TCPC的数量、分布、结合、转化、遗传等与合金成分、温度、冷却（加热）速率等的关系，揭示其在玻璃形成中的关键作用，阐明热力学与动力学机制，建立基于液态金属TCPC演化特征的GFA预测模型，为开发GFA强的非晶合金提供理论基础。

金属玻璃的结构描述是建立非晶理论的基础，但其结构的无序性使得基于对称性的结构描述方法无能为力，本课题根据原子间相互作用力随距离快速减小的物理事实，基于欧几里得距离，采用拓扑判据严格定义了局域结构—最大标准团簇（SC，the largest standard cluster）—的识别方法，开发了基于LSC的微观结构分析程序，并论证了Voronoi方法应用于材料领域的局限性。根据金属键仍然是决定金属玻璃属性的关键因素和金属键没有方向性和饱和性，因而局域结构倾向于紧密堆积的特点，提出拓扑密堆是金属玻璃的结构特征；并基于LSC严格定义了拓扑密堆（TCP，topologically clos-packed）团簇（原子），用TCP团簇表征复杂晶体的结构，成功描述sigma相和A15相的结晶度。将非晶合金体系的原子按是否为TCP原子将体系看成二元体系，能够完美解释PDF曲线第一峰和第二峰分裂出来的次峰的位置的结构起源。与液态金属结晶过程的四个阶段：晶胚、形核、长大和粗化相对应，液态金属非晶化过程也经过四个阶段：初胚、集聚、长大和粗化四个阶段；液态金属非晶化就是TCP结构聚集、生长和粗化的过程。对于同系列的合金，如不同成分的Ni64Zr36-xMox合金和不同压强下的Mg70Zn30合金，固态非晶体系内的TCP含量与其非晶形成能力正相关。TCP结构具有高的稳定性和低的能量，具有强大的抗压缩能力，因此提高体系内的TCP的含量就能提高材料的强度。本项目的研究表明TCP结构与金属液体、过冷液体、非晶固体的基本属性密切相关，且具有能量低、结构对称性好、相对密度高等特点，如果能发展出实验方法来检测液态金属内TCP结构的信息，则可以通过熔融状态的金属来预测其GFA，提高对非晶属性预测的有效性。