Some Applications of Field-Ion Atom-Probe Analysis to Iron and Steels

Abstract

Small-scale analyses have been made by the field-ion atom-probe technique on high-purity iron containing 0.05 at.-% carbon, on an Fe-4%Mo-0.2%C steel, and an Fe-5%Mo-0.1%N steel. Analysis of the iron–carbon alloy at 50 K showed the presence of iron only (together with hydrogen as a surface impurity) and background levels for all other elements of less than one part per thousand. Determinations made near a grain boundary in the same. material showed a carbon concentration of 0.4 at.-%, indicating segregation but at a level well below monolayer coverage. Analysis of precipitates in the iron–molybdenum–carbon alloy gave a composition of Mo₂C, in agreement with previous investigations. Analysis of precipitates in the iron–molybdenum–nitrogen alloy gave a composition of Mo₂N with 10% of the molybdenum replaced by iron. These latter two results indicate that the response of the instrument to the various ions is not significantly biased and that the unexpectedly low number of carbon signals from the grain-boundary analyses is not a detection artefact.