Grain-boundary diffusion and precipitate trapping of hydrogen in ultrafine-grained austenitic stainless steels processed by high-pressure torsion

This study was conducted to clarify the effects of grain boundaries and precipitates on room-temperature hydrogen transport in two types of austenitic stainless steels with ultrafine-grained structures produced by high-pressure torsion (HPT) and subsequent annealing. The grains in the Fe-25Ni-15Cr (in mass%) alloy containing Ti and the Fe-25Cr-20Ni alloy were refined by the HPT-processing to ∼150 and ∼85 nm, respectively. The high-temperature annealing after the HPT processing led to the precipitation of η-Ni₃Ti for the former and σ-FeCr for the latter. In the HPT-processed specimens, hydrogen diffusivity was enhanced through short-circuit diffusion because of the increased population of grain boundaries in comparison with the increased opportunity of hydrogen trapping on dislocations. As for the post-HPT-annealed specimens having the precipitates, the hydrogen diffusion was hindered by the hydrogen trapping on η-Ni₃Ti precipitates, but was not affected by σ-FeCr precipitation. This depends on the affinity between hydrogen and constituting elements.