Atomic insights into the oxidation-induced softening of Fe-Ni nanowires

Understanding how surface oxidation impacts the mechanical reliability of Fe-Ni nanowires (NWs) is crucial for next-generation devices, yet the underlying mechanisms remain elucidated. Here, we employ reactive force field molecular dynamics (ReaxFF MD) simulations to investigate the oxidation of Fe-Ni NWs and its effect on their tensile deformation response. Our results show that a pronounced size dependence is observed in the oxidation of Fe-Ni NWs. Smaller-diameter NWs (3 nm and 4 nm) experience rapid oxygen diffusion and pronounced selective oxidation on Fe sites, forming thicker amorphous oxide shells with complex local coordination environments on the surface. In contrast, larger counterparts (5 nm and 6 nm) keep thinner and more ordered oxide shells. The mechanical properties of Fe-Ni NWs also depend on size. Fe-Ni NWs show a ‘smaller is stronger’ trend before oxidation, but the presence of an amorphous oxide shell greatly reduces their mechanical properties and reverses the size effect to ‘smaller is softer,’ due to an abnormal decrease in potential energy from tension-induced reconstruction of the metal-oxide interface. Furthermore, surface oxidation can alter the plastic deformation mechanism, replacing the phase transformation (BCC→FCC) dominant in unoxidized NWs with an amorphization-dominated deformation mechanism.