Elucidating the Mechanisms of Irradiation Induced Softening in Nanocrystalline BCC Metals

NON-TECHNICAL DESCRIPTION: Coupled extremes of high stresses and irradiation necessitate new materials design methodologies for enhancing strength and radiation tolerance through precise control over material structure. Over the past two decades, nanocrystalline metals that contain a large fraction of internal interfaces have been pursued to address the performance limitations of traditional engineering alloys. These unique materials exhibit significant improvements in mechanical properties such as strength and wear resistance, but under certain irradiation conditions, develop internal defects that degrade these properties and limit their technological utility for extreme environment applications. Using an integrated computational and experimental framework, this research will build a fundamental understanding of the mechanisms responsible for such property degradations, specifically characterizing the intricate defect networks that produce softening and hardening in nanocrystalline metals under irradiation. Technologically, new insights into the impact of irradiation on the mechanical performance of nanocrystalline metals will foster innovations in materials design for extreme environments to advance next-generation nuclear technologies as safe, sustainable energy sources with drastically reduced environmental impacts. The integration of research activities into educational initiatives will advance the engagement of underrepresented students in materials science and provide curriculum enrichment for engineering majors through the establishment of a new materials science and engineering major at Stony Brook University. TECHNICAL DESCRIPTION: This research will elucidate the mechanisms responsible for the transition from softening to hardening in nanocrystalline body centered cubic (BCC) metals containing helium irradiation defects. The guiding hypothesis is nanoscale helium defects aggregated in grain boundaries act as stress concentrations that reduce the energetic barrier for grain boundary mediated dislocation nucleation, which in turn manifests as a softening effect that ultimately competes with classical irradiation hardening. The research team will combine atomistic simulations with in situ mechanical testing experiments and nanoindentation to explore the influence of helium irradiation defects on the fundamental deformation processes in the nanocrystalline BCC metals tungsten and iron. The competition between classical irradiation hardening found in coarse-grained polycrystalline metals and softening prevalent in nanocrystalline materials will also be explored by considering the coupling of intragranular defect loop damage with grain boundary defects in the deformation behavior. The manifestation of these mechanisms in the mechanical response will be quantified and used to build mechanism-property maps on the basis of collective interactions of deformation mechanisms with irradiation defects. From this research, the team will gain a new understanding of radiation effects and their implications for the mechanical performance of nanocrystalline BCC metals. A fundamental understanding of competing mechanisms due to coupled defect states will foster new innovations for combating mechanical property degradation from irradiation damage, thereby providing opportunities for interface engineering through synergistic alloying-by-design methodologies. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.