A study of the anomalous field evaporation of Sm(Co0.68Fe0.20Cu0.10Zr0.02)7.5 alloy by 3D atom probe
Introduction
Atom-probe tomography (APT) 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 analysis of the composition with the 3D atom probe for a new alloy system, appropriate experimental conditions must be selected to avoid possible preferential evaporation or retention of one species from the specimen surface, because different elements have different values of field-evaporation strength in the alloy. 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], [2], [3], [4], [5] have demonstrated that the specimen temperature can be selected from below 15 to 70 K; the pulse fraction is usually higher than 15% to achieve an equal detection efficiency for each element in the alloy; for the orientation of the specimen, the probing direction should not be on the poles or zone lines, to obtain correct compositions; as for the field-evaporation rate, it is suggested that a low rate (<0.02 ions per pulse for the optical position-sensitive detectors) should be used to avoid possible ion pile-up at the detector. A combination of the above parameters suitable for a specific alloy system needs to be determined.
The SmCo-based permanent magnets have superior high temperature magnetic properties, which are dependent on heat treatments, involving the compositional changes in the nanometer scale features in the complicated microstructures. The mechanism involved is not yet fully understood. The 3D atom probe makes it possible to analyze the changes in the compositions of nanometer scale features in alloys. The purpose of this work is to determine suitable probing conditions for analysing a Sm(Co0.68Fe0.20Cu0.10Zr0.02)7.5 alloy. This alloy is in a single phase state with the SmCo7 structure, so that the compositions of more complicated microstructures of this alloy after heat treatment, can be determined correctly in the further investigations of the effects of heat treatment on alloy constitution and phase compositions. The effects of analysis conditions on the measured composition are discussed, based on the evaporation fields.
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Experimental
An alloy ingot of the composition Sm(Co0.68Fe0.20Cu0.10Zr0.02)7.5 was prepared using an Ar arc-furnace. Ribbon samples were made using a single roller melt-spinning technique under a controlled inert gas atmosphere. The chemical analysis of the composition for the ribbon samples confirmed that the actual composition of the alloy was similar to the nominal composition, shown in Table 1. The microstructure was examined by using X-ray diffraction (XRD) and transmission electron microscopy (TEM,
Results and discussion
The XRD analysis of the as-spun ribbons showed that the alloy comprised a single phase with the SmCo7 structure. A TEM micrograph of the as-spun ribbons is given in Fig. 1, showing fairly large grain structure, exceeding 0.5 μm in size. The atom probe analysis showed uniform distribution of atoms for each element. The raw data size of each test was around 900,000 ions. A typical Sm atom map and composition profiles for the tip probed at 65 K are shown in Fig. 2, illustrating the desired
Conclusions
The effects of specimen temperature and pulse fraction on the compositional analysis results for a Sm(Co0.68Fe0.20Cu0.10Zr0.02)7.5 alloy using the 3D atom probe have been examined. When the specimen temperature was ∼20 K and pulse fraction ⩾15%, the quantitative analysis of the composition gave reasonably correct estimates. At higher specimen temperatures, serious preferential evaporation of Sm atoms occurred due to the very low evaporation field of Sm, estimated to be 15.2 V/nm by using the
Acknowledgment
This work was supported by an Australian Research Council (Discovery Project) Grant.
References (11)
Acta Mater.
(1999)- et al.
Surf. Sci.
(1982) Surf. Sci.
(1982)Atom probe tomography: analysis at the atomic level
(2000)- et al.
J. Vac. Sci. Technol.
(1982)
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Present address: School of Physics, The University of Melbourne, Vic. 3010, Australia.