Morphology-mechanism correlation of irradiation-induced perfect loops: Insights from a model FCC Pd-25Ag alloy

Clarifying the formation pathways of a₀/2⟨110⟩ perfect loops (PLs) is pivotal for understanding radiation damage evolution in face-centered cubic (FCC) alloys. While traditional models emphasize the transformation of Frank loops (FLs), direct nucleation of PLs under irradiation remains underexplored. Here, using Pd-25Ag as a model alloy, we employ in-situ ion irradiation to trace dislocation loop evolution. We identify and characterize three distinct formation mechanisms for PLs: direct nucleation, FL self-unfaulting, and loop reaction. A morphology-mechanism correlation of dislocation loops is established based on in-situ TEM observation and detailed crystallographic analysis, providing a diagnostic framework for microstructural analysis. Crucially, the proportion of directly nucleated PLs is significantly higher in Pd-25Ag than in pure Pd. This enhancement suggests the dual role of the alloying element Ag: It thermodynamically promotes nucleation by lowering the stacking fault energy, and kinetically aids loop stabilization by suppressing point-defect migration. This work decouples the solute effects on defect kinetics and offers a morphology-based paradigm for interpreting irradiation-induced defects in FCC alloys.