Influence of oversized atoms X = Ti, Ta, W, Mo on the dynamics of native defects under annealing in Fe35Cr35V20Cu5-(X) HEA studied by positron annihilation spectroscopy (PAS)

The dynamics of native defects in the high entropy alloy family produced by arc melting Cu₅Cr₃₅Fe₃₅V₂₀X₅ (at. %), where X = Ti, Ta, W or Mo corresponds to oversized atoms, were studied using positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening (CDB) spectroscopy. Isochronal annealing from room temperature (RT) to 900 °C revealed three stages common to all HEAs. The first stage (RT- 300 °C) corresponds to the stabilization of the native defects. This is followed by a second recovery stage (400–600 °C, or 700 °C for W-containing HEAs), involving the onset of diffusion and recombination of the defects. The final stage, up to the upper annealing temperature, is characterized by the interaction of the thermal vacancies produced during isochronal annealing with the precipitates. Cu precipitation, driven by its high mixing enthalpy with Fe and Cr, dominates the chemical environment of positron annihilation sites initially and, later, by the coarsening of the Cu precipitates at high temperatures. Oversized atoms like Ti and Ta suppress void growth, as revealed by combining PAS results with microstructural analysis. These results provide insights into defect behavior and stability in HEAs, emphasizing the role of oversized atoms in enhancing resistance to void growth.