Helium Porosity Development During Annealing of Helium-Implanted 18Cr10NiTi Steel

Abstract

The kinetics of helium porosity development during annealing of 18Cr10NiTi stainless steel irradiated with 20 keV helium ions at room temperature for simultaneous creation of displacement damage at a level of 0.5–5 dpa and a helium concentration of 1–12 at.%, have been investigated by electron microscopy and thermal desorption spectrometry. The temperature ranges of helium release from steel and their dependence on the irradiation dose are determined. The evolution of 18Cr10NiTi steel microstructure was investigated during post-implantation annealing in the temperature range from T_room to 1420 K. At a dose of 1·10²⁰ m^-2, helium bubbles were detected only after annealing to a temperature of 890 K, while at a dose of 1·10²¹ m^-2, bubbles were observed immediately after radiation at T_room. During annealing, the average diameters of the bubbles vary from ~1 nm at T_room to 10–20 nm at T_ann 1420 K. The mechanisms of bubbles growth either by migration and coalescence, or by Ostwald ripening – dissolution and re-trapping are considered. Since each of these mechanisms corresponds to a certain trend of bubbles size and density dependence on the annealing temperature, the temperature dependences of average diameters and densities of helium bubbles for a dose of 1·10²¹ m^-2 have been constructed and analyzed. Experimental data are characterized by three temperature ranges: 1 – from 300 to 760 K, 2 – from 760 to 1030 K, and 3 – from 1030 to 1350 K with clearly differing trends. In the low-temperature region the diameter and density of the bubbles virtually does not change. Their size increases and the density decreases at annealing in the temperature range 760-1030 K. This tendency intensifies in the temperature range of 1030-1420 K. An estimation of activation energy of the processes controlling the mechanism of bubble growth in the temperature range of 1000-1420 K has been done. An obtained value of ~3.7 eV correlates well with the theoretically calculated value of the activation energy of the dissociation process (E_He^diss) of the Ostwald ripening mechanism.