Hydrogen damage in high strength metals can be reduced by engineering the microstructure so that hydrogen can be irreversibly trapped locally at preferential sites, thus preventing its diffusion within the lattice, in particular, to sites where cracks can initiate. Typical trap sites include second phase particles, phase interfaces and grain boundaries, each having different trapping capabilities. Analytically, it is very challenging to spatially distinguish between the different trap sites and their capacity to trap hydrogen; mainly due to the resolution and sensitivity limitations imposed by conventional characterisation techniques. The NanoSIMS is a high-resolution secondary ion mass spectrometry technique capable of spatially resolving hydrogen and its isotope deuterium thus revealing any hydrogen/deuterium enriched features.
In this study, type 303 stainless steel specimens were electrochemically charged with deuterium. Deuterium was used to minimise the effect of experimental artefacts on NanoSIMS data, notably surface adsorption of residual hydrogen present in the analysis vacuum chamber. Following charging, samples were left to outgas for 10 days in ambient conditions to allow diffusible deuterium (lattice and reversibly trapped) to effuse from the matrix, leaving behind only the irreversible trapped deuterium in the microstructure.
Deuterium enriched regions in 303 stainless steel were detected via NanoSIMS, in particular trapped deuterium associated with deformation features. The limitations of this technique are discussed, being primarily associated with the destructive nature of the ion beam during image acquisition, which leads to surface damage and topography effects. X-ray diffraction (XRD) and Secondary electron microscopy (SEM) were used to validate the observations.