A Study of the Anomalous Field Evaporation of Sm(Co{sub}0.68Fe{sub}0.20Cu{sub}0.10Zr{sub}0.02){sub}7.5 Alloy by 3D-Atom Probe

摘要

Three-dimensional, atom-probe, field-ion microscopy, has been shown to be a unique technique to analyze the compositions of nanometer precipitates and clusters in various alloys and composites [1,2]. However, to do quantitative, compositional analysis using the 3D atom probe (3DAP) for a new alloy system, the experimental conditions have to be selected carefully to avoid any anomalous field evaporation, i.e. the preferential evaporation or retention of one species from the specimen surface. It is well known that the measured composition varies with the following important experimental parameters: (a) the temperature of the specimen; (b) the pulse fraction; (c) orientation of the specimen; and (d) field-evaporation rate. Previous studies on various alloys [1] have revealed that for the pulse fraction it is safe to use >15% to achieve an equal detection efficiency for each element in the alloys; for the orientation of the specimen, the probing direction could be close to but not on the low-index poles to obtain correct compositions; for the field-evaporation rate, it is suggested that a low rate (< 0.02 ions per pulse) should be used to avoid possible ion pile-up at the detector. Our atom probe work on the Sm(Co{sub}0.68Fe{sub}0.20Cu{sub}0.10Zr{sub}0.02){sub}7.5 alloy suggested that an anomalous field evaporation occurred in this alloy. Figure 1 is the measured compositions of the alloy as a function of the temperature of the specimen from 65 to 10.6K. The results show that with decreasing temperature to about 20K, the preferential evaporation of Sm could be prevented from occurring.