The effect of oxygen on void stability in nickel and austenitic steel

Surface energy values lower than those determined experimentally are often utilized in theories of void nucleation and growth in metals. Utilization of established surface energy values generally predicts no swelling in the absence of helium. However, swelling occurs in many metals even in the absence of helium. Surface active impurities, such as oxygen, can readily account for this discrepancy by reducing the surface energy of metals. This investigation shows that very low concentrations of oxygen in nickel and austenitic stainless steel can achieve the necessary decrease in surface energy. A model has been developed to calculate the requisite quantity of oxygen in solution to stabilize voids. The criterion for void stability is that the void be the most energetically stable vacancy cluster in the metal. Knowing the fraction of oxygen which chemisorbs and the surface coverage required permits the determination of initial oxygen concentration needed to promote void stability. Calculations have been performed for austenitic stainless steel and nickel. The model has been tested by irradiating nickel with 14-MeV Ni ions at 500 °C. Oxygen was preinjected into one sample to a concentration of 75 appm. The irradiation reached a fluence of 3 × 10 20 Ni-ion/m 2 (28 dpa at the damage peak). The irradiated foils were examined in cross section in the electron microscope.