Measurements of stress evolution during low-energy argon ion bombardment of Si have been made using a real-time wafer curvature technique. During irradiation, the stress reaches a steady-state compressive value that depends on the flux and energy. Once irradiation is terminated, the measured stress relaxes slightly in a short period of time to a final value. To understand the ion-induced stress evolution and relaxation mechanisms, we account for the measured behavior with a model for viscous relaxation that includes the ion-induced generation and annihilation of flow defects in an amorphous Si surface layer. The analysis indicates that bimolecular annihilation (i.e., defect recombination) is the dominant mechanism controlling the defect concentration both during irradiation and after the cessation of irradiation. From the analysis, we determine a value for the fluidity per flow defect.