We present a first-principles investigation of defect energetics and vacancy-mediated diffusion in pristine and Zr-doped CoCrFeNi high-entropy alloys using density functional theory and climbing-image nudged elastic band calculations. The Zr substitution stabilizes the fcc solid solution, introducing pronounced local chemical disorder and electronic inhomogeneity. Analysis of the electronic density of states reveals significant broadening and redistribution of $d$ states near the Fermi level upon Zr addition, indicating modified hybridization with the transition-metal matrix. These electronic changes lead to a substantial increase in intrinsic stacking-fault energy from $-67.74\pm 5.38\mathrm{mJ}{\mathrm{m}}^{-2}$ to $93.13\pm 12.72\mathrm{mJ}{\mathrm{m}}^{-2}$, signaling a shift in deformation mechanisms. Bader charge analysis reveals that Zr acts as the dominant electron donor in the system, transferring approximately $+0.90e$ to the surrounding transition-metal matrix, substantially greater than the maximum charge donation of Cr ($\sim +0.60e$) in the undoped alloy. Vacancy formation energies and migration barriers exhibit strong configurational variability, reflecting the heterogeneous local bonding environment. In the presence of Zr, vacancy formation is locally facilitated, accompanied by strong solute-vacancy binding that promotes vacancy trapping. Migration pathways evolve from relatively symmetric, single-stage profiles in the pristine alloy to broadened, asymmetric, and multistage energy landscapes in the Zr-containing system. Zr itself exhibits the highest mean migration barrier ($1.315\pm 0.507\mathrm{eV}$) and an anomalous double-peaked migration profile attributed to its weak $4d-3d$ hybridization. Overall, these results establish a direct connection between solute-induced modifications in electronic structure and the resulting changes in the energetics of the defect and diffusion behavior, providing a unified framework for tailoring defect properties in high-entropy alloys through electronic and chemical design.

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• Charge
• Density of states
• Electronic structure
• First-principles calculations