The Origins of Tritium and Helium Effects on the Tensile Properties of Metals

The effects of internal tritium and helium on the tensile properties of two austenitic stainless steels and an iron-based superalloy have been studied. The materials tested were, forged 21Cr-6Ni-9Mn and 304L (tested in the annealed condition and two forged conditions), and a modified A-286 alloy. The accumulation of ³He from the radioactive decay of tritium caused an increase in the yield strength and a continuous decrease in the ductility in almost all materials tested. Increased ³He concentrations also caused a change in fracture mode from ductile rupture to predominantly intergranular fracture. The property changes resulted from ³He bubble-induced strengthening, which produced a change in deformation mode from long-range dislocation activity to deformation twinning. In the deformation-twinning mode, the ³He-accelerated fracture initiated at the intersections of deformation twins with grain boundaries. High-strength forged 304L was most resistant to ³He effects, owing to the redistribution of ³He on dislocations.