易形成非晶Zr-基合金熔体深过冷与晶体长大速度

The special structures and properties of metallic glasses have made them a most active research area in metallic meterials in recent years. Although great attention has been given to the alloy systems, the glass formation ability, structure and properties as well, a lot of foundamental problems related to the glass formation of metallic materials are still unclear. In this project, it will be mainly focussed on the study of the compitation process between the crystal growth and the glass formation in deep undercooled Zr-based alloy melts, in order to reveal the physical mechanisms of glass formation, which is undoubtedly an avoidance of crystal formation. A series of Zr based glass-forming alloys will be selected to be undercooled by using magnetic levitation and electrostatic levitation, by which the contamination from the crucible wall and the heterogeneous nucleation will be eliminated, thus, a wide undercooling regime becomes accessible. Combined with the high-speed camera system the velocities of crystal growth under different undercoolings will be measured. Further more, by using differential scanning calorimeter, some thermophysical parameters for glass forming systems like crystallization- and melting-temperature, the heat of fusion and the specific heat in the liquid regime will be measured. And the viscosity as a function of temperature will be measured by a rotating cylinder analyser. The morpholoy of the dendrite will be simulated by phase field method and processing of nuleation will be calculated by Molecular Dynamics method. The relationship among undercooling, crystal growth velocity, viscosity of the melts and the glass-forming ability for different Zr-basd alloys will be discussed. The velocity as a function of underccoling will be explained based on planar or dendrite growth theories. The underlying physical processes in crystal growth including the nature of the crystal-liquid interface, the thermodynamic driving force, and growth crystal-growth kinetics will also be expatiated. Through these studies, the nature of glass formation is hopefully clearer.

非晶合金结构与性能上的特殊性，使得它成为近年来金属材料最活跃的研究领域之一。虽然关于合金体系、非晶形成能力、微观结构等方面已有大量研究工作，但迄今为止仍有一系列非晶形成中的基础科学问题尚未得到解决。本课题主要针对易形成非晶合金熔体凝固过程中的晶态与非晶态竞争形成的相关影响因素开展对比研究，以期查明晶态与非晶态相形成的内在规律。以易形成非晶的Zr基合金为研究对象，用电磁悬浮和静电悬浮手段并结合高速摄像技术，一方面开展熔体中最大过冷度、晶体形成与生长速度及其主要影响因素、抑制晶体形成的主要条件等实验与理论研究；另一方面开展合金体系成分、过冷度、样品尺寸等与非晶形成的相互关系研究，建立过冷熔体CCT曲线，找出非晶形成临界冷却速度。同时结合熔化潜热、过冷熔体比热、过冷熔体粘度等热力学与动力学参数的测量，结合计算机模拟与理论研究，最终查明非晶形成时过冷熔体所处的临界物理状态，建立非晶形成的新判据

本课题主要针对易形成非晶的合金体系，将其熔体凝固过程中的晶态与非晶态竞争形成的相关影响因素，进行对比研究，以期查明晶态与非晶态相形成的内在规律。以易形成非晶的 Zr 基合金为研究对象，采用旋转柱体法测定熔体粘度及热物性能，并结合分子动力学模拟研究进行对比分析，系统研究了纯 Zr、Zr50Cu50、Zr36Cu64、Zr55Ni5Al10Cu30 等成分合金熔体粘度特性，揭示了不同的冷却条件对粘度变化产生影响的本质，即冷却速率增加或压力减少，导致粘度下降；粘度对温度最敏感，压力和冷速对粘度的影响较小。提出了粘滞激活能作为非晶形成能力的重要判据。用电磁悬浮和静电悬浮手段并结合高速摄像技术，一方面开展熔体中最大过冷度、晶体形成与生长速度及其主要影响因素、抑制晶体形成的主要条件等实验与理论研究；另一方面开展合金体系成分、过冷度、样品尺寸等与非晶形成的相互关系研究，系统研究了Zr50Cu50、Zr60Cu40、Zr64Ni36合金深过冷凝固及生长过程。采用分子动力学模拟研究了合金熔体凝固过程中的动力学性能，通过对原子均方位移和非高斯参数的分析发现扩散行为在空间上是不均匀的。扩散激活能的减小是导致自扩散系数增加的重要原因，从而揭示了原子的扩散运动对于微观结构演化以及金属玻璃的形成、晶化等过程发挥了重要的作用。