Understanding Kirkendall effect in Ni(W) diffusion-induced recrystallization region

This study elucidates the formation of distinct phases, such as intermetallic compounds (IMCs), and the evolution of Kirkendall voids. We not only observed the emergence of polycrystalline IMC layers and distinct diffusion-induced recrystallization (DIR) regions using various experimental methods but also quantified the elusive Kirkendall vacancy fraction by an extended numerical model. Experiments revealed the formation of polycrystalline layers with Ni4W and a distinct DIR region. Irregularly shaped Kirkendall voids were found in the DIR region in the Ni matrix. The observed DIR region thickness exceeding W penetration depth indicated rapid alloying. Experimental data and insights from the Ni-W phase diagram showed exclusive Ni4W formation. EBSD analysis revealed smaller Ni4W grains and larger, irregularly shaped DIR region grains. Quantification of Kirkendall voids through Blob analysis linked numerical results to experiments. Noteworthy findings include the fraction of Kirkendall vacancy and the evolution of the possible vacancy region calculations, highlighting their dependence on the W diffusivity and grain boundary diffusion. Temporal evolution of the vacancy region showed the absence of vacancies near Ni4W/DIR, with the leading edge at DIR/Ni, providing insights into annealing-induced vacancy distribution. This integrated approach enhances understanding of thermodynamics and kinetics in the Ni-W diffusion couple, advancing knowledge crucial for high-temperature materials science applications.