Kirkendall porosity in diffusion couples: Formation and morphological unstable growth investigated by synchrotron X-ray tomography and phase-field simulation

• Applicants: Professorin Dr.-Ing. Bettina Camin; Dr. Julia Kundin
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials ; Experimental Condensed Matter Physics ; Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 496083821

Interdiffusion in diffusion couples is accompanied by porosity growth via absorption of migrating vacancies, the so-called Kirkendall porosity. This phenomenon is of a great technological and scientific interest since this porosity causes detrimental damage, in particular close to joints of different materials. The investigations of Kirkendall porosity aim on the kinetics of interdiffusion, the pore morphology and the modeling of the porosity growth. A further aspect is the inhomogeneous growth of the porosity and the pore morphology in a strong, directed vacancy flux, as observed in our preliminary experiments. These aspects have attained only little attention in the scientific literature so far. Using sub-micrometer synchrotron 3D tomography in diffusion couples of nickel-base alloys we found that Kirkendall pores can develop in different ways taking plenty of different morphologies (cubic, dendritic, octahedral, drop-like, cup-like etc.), and they interact with each other during growth. Clarifying mechanisms and a deep physical understanding of the unstable development of Kirkendall pores is the goal of the present proposal. The investigations will focus on the investigation of diffusion couples of nickel-base alloys using high resolution synchrotron X-ray tomography, including in-situ experiments, which provide dynamic observation of the nucleation and evolution of Kirkendall pores. In order to distinguish compositional and temperature effects, diffusion couples will be assembled and annealed at different temperatures. In-situ Synchrotron tomography experiments will be accompanied by electron probe microanalysis of interdiffusion. In parallel, to achieve a better theoretical understanding of Kirkendall porosity, nucleation and growth will be simulated using the phase-field method, coupled to a recently developed pair-wise multicomponent diffusion model generalized to the consideration of the vacancy diffusion. A special focus will be given to the inhomogeneous growth of pore morphologies and porosity in a strong, directed vacancy flux.It is expected that the combined experimental and theoretical investigations will contribute to a fundamental understanding of nucleation and growth of Kirkendall porosity in joints of technically relevant materials of different compositions, and to a better control of these detrimental defects.

• DFG Programme: Research Grants