High performance time-of-flight atom-probe for characterization of hydrogen in metals

Design improvements in the last decade make the atom-probe field ion microscope (FIM) an exciting research tool for the study of surface and solid state phenomena, some of which are extremely important in the steel industry. For instance, with the greatly improved resolution now available in energy-focused atom probes, recent results show that hygdrogen can be easily resolved even when combined with metals having several isotopes. In addition to finding that H, H₂, FeH and TiH₂ accumulate at segregated grain boundaries in Fe-0.29 wt.% Ti, a striking observation was made—the formation and propagation of a microcrack when the (field ion microscope) tip was exposed to hydrogen gas at elevated temperatures. This crack could be easily grown by further H₂ exposure, or reduced in size by field evaporation of the surface. Hydrogen was identified in quantity in the fracture of the surface (former grain boundary), though not elsewhere. Other promising applications include the study of segregation at grain boundaries, in part because the atom probe is the only instrument available for the study of nonembrittled grain boundaries at the atomic level, surface segregation, cluster formation and the initial stages of precipitation.