The accuracy of compositional measurements using atom probe tomography is often reduced because some ions are not recorded when several ions hit the detector in close proximity to each other and within a very short time span. In some cases, for example in analysis of carbides, the multiple hits result in a preferential loss of certain elements, namely those elements that frequently field evaporate in bursts or as dissociating molecules. In this paper a method of reducing the effect of multiple hits is explored. A fine metal grid was mounted a few millimeters behind the local electrode, effectively functioning as a filter. This resulted in a decrease in the overall detection efficiency, from 37% to about 5%, but also in a decrease in the fraction of multiple hits. In an analysis of tungsten carbide the fraction of ions originating from multiple hits decreased from 46% to 10%. As a result, the measured carbon concentration increased from 48.2 at.% to 49.8 at.%, very close to the expected 50.0 at.%. The characteristics of the multiple hits were compared for analyses with and without the grid filter.

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Resum6 -Un acier 2.25Cr -lMo, soude sous flux, est Cte etudie par microscopie electronique, microscopie ionique B emission de champ et la sonde B atomes. La microstructure bainite de l'acier soude consiste en ferrite et martensite. En traitment thermique B 690°C martensite se transforme en ferrite et cementite, et carbures (Cr,Mo)nC en forme d'aiguille se forment. Avec une reduction de la densite de dislocations, cela a result6 d'une amelioration de la tenacite de l'acier.Abstract -A submerged arc welded 2.25Cr -1Mo steel has been investigated using electron microscopy and atom probe field ion microscopy. The bainitic microstructure of the as-welded steel consisted of ferrite and martensite. During heat treatment at 690°C the martensite transformed to ferrite and cementite and needle-shaped (C~,MO)~C carbides precipitated. Together with a substantial decrease in dislocation density, this resulted in an improvement of the toughness.

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