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

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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.