<p>Understanding how irradiation affects phase stability in nanocrystalline metals is critical for developing radiation-tolerant materials for nuclear environments. In particular, iron (Fe) exhibits size- and defect-dependent polymorphic transitions that remain poorly understood under low-temperature irradiation. This study investigates how irradiation at temperatures below 550&#xa0;K influences the phase behavior of nanocrystalline Fe particles, focusing on the stabilization of body-centered cubic (bcc) and face-centered cubic (fcc) phases.</p><p>We employ a Gibbs thermodynamic framework incorporating defect energetics and a chemical kinetics approach to model defect formation and diffusion during irradiation. Using this combined model, we construct size–temperature and size–energy diagrams to capture phase stability domains and transitions.</p><p>Our results confirm that nanoscale Fe particles exhibit a pronounced size effect in stabilizing the fcc phase without irradiation. We show that defect accumulation at low temperatures significantly shifts phase boundaries, promoting bcc phase formation in small particles. These findings provide new insight into irradiation-driven phase behavior and offer design principles for radiation-resistant nanostructured materials.</p> Graphical Abstract <p></p>