RADIATION-ANNEAL HARDENING IN NIOBIUM: AN EFFECT OF POST-IRRADIATION ANNEALING ON THE YIELD STRESS.

The effect of low-temperature annealing on the yield stress of niobium following neutron irradiation was studied. The initial increase in yield stress upon irradiation was sensitive to the interstitial carbon content. A further increase in the yield stress (“radiation-anneal hardening”) was observed after two-hour anneals near 150 and 300 °C. When the annealing temperature was raised above 400 °C, the yield stress gradually recovered toward the preirradiation value. Changes in the density and size of the radiation-produced defect clusters were determined by transmission electron microscopy following post-irradiation anneals. An analysis of the observed hardening based on a dispersed barrier model and the density and size of defect clusters indicated that post-irradiation annealing strengthened the clusters as barriers to dislocation motion by as much as a factor of two. Previous resistivity and internal friction measuremements have shown that interstitial oxygen and carbon in irradiated niobium migrate to the defect clusters at the temperatures near 150 and 300 °C, respectively. Thus, the radiation-anneal-hardening peaks at these temperatures are attributed to the trapping of oxygen and carbon, respectively, at defect clusters. There is a reduced tendency for dislocation channeling (i.e., defect cluster removal by slip dislocations) in radiation-anneal-hardened niobium, suggesting that indeed the clusters have been strengthened.