Study of massive transformation using diffusion couples of multicomponent alloys for CALPHAD consistent modeling of interface controlled phase transformations

在拟议的项目中，将以块状相变为例，研究固态相变过程中界面热力学状态的影响。大规模转变至少是一种部分界面控制的转变。有争议的问题是：i)母相和产物相之间的界面在多大程度上处于热力学平衡状态，以及ii)迁移界面前面的母相中是否形成浓度尖峰。本提案的目标是描述多组分体系中界面和扩散控制的相变之间的转变，为这种转变提供一个通用的标准，例如介观模拟所需的。结合实验结果和考虑界面热力学的热力学和动力学模型(即处于局部平衡的界面过程或其偏差)的模拟计算，可以量化界面处的能量耗散，从而增强对界面控制相变的定量理解。对于从块状相变到扩散控制相变的表征，相变速率将随工艺条件(加热或冷却速度)和添加的三元合金元素而变化。将使用以银锌和铜锌二元系为基础的扩散偶。在制备了明显的浓度梯度后，在单个样品中可以观察到大范围的局部浓度。加热是通过电流脉冲加热导线来实现的。为了快速和可变的温度控制，允许在选定的温度周围加热、保持、冷却和循环，将使用最先进的光子探测器相机。相机将以尽可能高的空间和时间分辨率控制整个样品的温度。在FIB片层制备后，将使用高分辨率电子显微镜结合能量色散X射线光谱来研究相变前沿附近的浓度分布。

In the proposed project the influence of the thermodynamic state of the interface during solid state phase transformations will be investigated on the example of massive transformation. Massive transformation is an at least partially interface controlled transformation. Controversially discussed questions are i) to what degree the interface between parent and product phase is in thermodynamic equilibrium and ii) if a concentration spike forms in the parent phase in front of the migration interface.The goal of the proposal is the characterization of the transition between interface and diffusion controlled phase transformation in multicomponent systems for providing a general criterion for this transition, as e.g. needed for mesoscopic modeling. The combination of experimental results and simulation calculations using a thermodynamic and kinetic model considering interface thermodynamics (i.e. processes at the interface in local equilibrium or deviations thereof) will allow the quantification of the energy dissipation at the interface and thus enhance the quantitative understanding of interface controlled phase transformations.For a characterization of the transition from massive to diffusion controlled phase transformations, the transformation rate will be varied by the process conditions (heating or cooling rate) and adding ternary alloying elements. Diffusion couples on the basis of the binary Ag-Zn and Cu-Zn systems will be used. After the preparation of a distinct concentration gradient, a broad range of local concentrations can be observed within a single sample.Heating is realized by pulse heating a wire with electric current. For fast and variable temperature control that allows the heating, holding, cooling and cycling around a chosen temperature a state-of-the-art photon detector camera will be employed. The camera will control the temperature along the entire sample in spatially and temporally highest possible resolution. After FIB lamella preparation the concentration distribution around the transformation front will be investigated using high resolution transmission electron microscopy in combination with energy dispersive X-ray spectroscopy.