Investigating vacancy cluster diffusion mechanisms in FeNiCr concentrated solid solution using the kinetic activation-relaxation technique

A better understanding of the mechanisms of vacancy clustering and diffusion in concentrated solid solutions is essential to improve the design of alloys with enhanced radiation tolerance and thermal stability. In this study, the kinetic activation-relaxation technique (kART) was employed to investigate vacancy cluster behavior in FeNiCr CSAs at the atomistic level. By analyzing di-, tri-, and tetravacancy clusters, we reveal that cluster stability increases with size, driven by reduced formation energies, while local chemical composition---particularly Ni's stabilizing effect and Cr's role in enhancing mobility---influences defect evolution. By computing the diffusion barriers and entropic prefactors for more than 700 000 diffusion events, we find that divacancies combine notable stability with substantial mobility, while trivacancies exhibit significant mobility despite reduced stability, enabling efficient defect recombination through short-range migration mechanisms. In contrast, tetravacancies demonstrate greater stability but significantly restricted mobility, influencing localized defect interactions and microstructural evolution. These findings provide fundamental insights into the complex interplay between cluster size and local composition that affect both the enthalpic and entropic contributions to diffusion kinetics, offering a predictive framework for rationally designing alloy compositions to optimize radiation resistance and structural integrity in advanced nuclear and aerospace applications.