A quantitative atom-probe field-ion microscope study of the compositions of dilute Co(Nb) and Co(Fe) alloys

The compositions of Co-1.0at%Nb and Co-1.0at%Fe alloys have been measured using the atom-probe field-ion microscope. The main purpose of this experimental work was to find a set of optimum conditions which gives the correct solute concentrations in these alloys. This information was necessary for measuring the absolute compositions, in our extensive investigations of solute atom segregation effects to individual stacking faults in these alloys. The dependence of each alloy's composition on the following parameters was investigated: (1) the specimen temperature; (2) the pulse fraction; (3) the field-evaporation rate; (4) the crystallographic plane; (5) the effect of the presence of hydrogen in the atom-probe on the measured Nb composition in a Co-1.0at%Nb alloy. It is shown that the composition determined by the atom-probe FIM is very sensitive to some of the parameters listed above. The effects of these experimental parameters on the measured Nb and Fe concentrations are discussed in terms of possible field-evaporation models. Under certain experimental conditions drastically different behavior has been observed in these alloys; preferential field-evaporation of solute atoms in a Co-1.0at%Fe alloy and preferential retention of solute atoms in a Co-1.0at%Nb alloy was observed. Correct solute concentrations were obtained by using the following experimental parameters: (1) a specimen temperature of less than or equal to 60 K; (2) a pulse fraction of greater than or equal to 0.10.