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

在建议的项目中，在固态相变过程中的界面的热力学状态的影响将被调查的大规模转变的例子。大规模转换是至少部分界面控制的转换。有争议的讨论问题是：i）在多大程度上母相和产物相之间的界面处于热力学平衡; ii）在迁移界面前的母相中是否形成浓度尖峰。该提案的目标是表征多组分体系中界面和扩散控制相变之间的转变，为这种转变提供一个通用标准，如例如介观建模所需。结合实验结果和模拟计算，采用考虑界面热力学的热力学和动力学模型，（即在局部平衡或其偏差中的界面处的过程）将允许量化在界面处的能量耗散，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转变的表征，通过工艺条件（加热或冷却速率）和添加三元合金化元素来改变转变速率。将使用基于二元Ag-Zn和Cu-Zn系统的扩散偶。在制备了明显的浓度梯度后，可以在单个样品中观察到宽范围的局部浓度。加热通过用电流脉冲加热导线来实现。对于快速和可变的温度控制，允许加热，保持，冷却和循环周围选定的温度，一个国家的最先进的光子探测器相机将被采用。相机将以空间和时间上最高的可能分辨率控制沿着整个样品的温度。FIB薄片制备后，将使用高分辨率透射电子显微镜结合能量色散X射线光谱法研究转化前沿周围的浓度分布。

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.