Time evolution of Kirkendall porosity in single-phase Ni-based diffusion couples

The development of the Kirkendall porosity was studied in fcc Ni–30Cr/Ni–10Si diffusion couples at 1176 °C using X-ray tomography, optical and scanning electron microscopy, and multicomponent diffusion simulation. Diffusion experiments were interrupted multiple times to monitor the porosity ex situ by tomography. This allowed tracking the position and size of thousands of pores over tens of hours. Pores detected by tomography were also observed by microscopy to determine the surrounding grain structure. The porosity depth profiles (number density, equivalent diameter, area/volume fraction) derived from 2D cross-sectional observations and 3D tomography were compared and the benefits of both methods discussed. Alloys of different grain sizes were used as starting materials to study the influence of the grain boundary density on the spatial pore distribution. Local analysis showed that pore nucleation was not significantly accelerated on grain boundaries compared to the grain interior, but that pore growth was faster along grain boundaries. The time-resolved pore distribution data indicated that the porosity evolved through both pore movement and growth-shrinkage. These mechanisms were discussed in view of the simulated vacancy flux profile.