The mechanism of stress influence on swelling of 20% cold-worked 16Cr15Ni2MoTiMnSi steel

Argon-pressurized tubes of 20% cold-worked 16Cr15Ni2MoTiMnSi steel were irradiated at hoop stresses of 0, 100 and 200 MPa at ∼740 K in the BN-600 fast reactor to 108 dpa. Following nondestructive measurements of strain, density measurements and microscopy were conducted. Voids were categorized into three types depending on their association with other microstructural features. Stress enhanced the nucleation of all void types, but nucleation of voids associated with dislocations was increased the most. Swelling increased as a consequence, even though the average size of each type void decreases. Swelling measured by TEM and density change gave identical results. A stress-enhanced void nucleation model is presented to explain these results. It invokes collection and diffusion of helium–vacancy complexes in dislocation cores and intersections to produce void nuclei, followed by stress-induced breakaway of the pinned dislocation to reach new obstacles and initiate the next sequence of helium collection and void nucleation.