Comparing the Consistency of Atom Probe Tomography Measurements of Small-Scale Segregation 229
Table 1. Summary of Atom Probe Tomography Analysis Conditions Used for Each of the Four Materials Used in This Study, for Both the LEAP 3000 and LEAP 5000 Instruments.
Materials
SG RPV steel ODS steel
Analysis Mode Voltage
Laser
M50 bearing steel Laser Silicon
Laser ODS, oxide-dispersion-strengthened steel. The LEAP 5000 samples of the SG steel were prepared
standard reactive ion etched silicon flat top micro post coupons used for LEAP analysis. The coupons were HF dipped and the micro posts were implanted directly with 14 keV phosphorus ions, with the fluence calculated to give 0.2 at% peak concentration at a depth of ~22 nm. Following implantation the sample was HF dipped again before capping with Ni (LEAP 3000 sample) or directly coated in Co without an HF dip (LEAP 5000 sample), in both cases using a thermal evaporator with 99.99% pure metal sources in an initial vacuum of ~10−6Torr, before tungsten protec- tion in the FIB instrument as for the reactor pressure vessel (RPV) specimens. Micro posts were then polished into a sharp tip using a Zeiss NVision Focused Ion Beam instru- ment (Carl Zeiss AG, Jena, Germany). These capped micro posts were annular milled at 30kV until the capping layer/ silicon interface was ~200nm from the top of the specimen, and then polished using a 2kV ion beam to remove all the
tungsten cap and all but ~15nm of the Ni or Co cap, finishing with a final tip radius below 100nm. All four materials were analyzed in both the LEAP 3000
and LEAP 5000. Although some of the older LEAP 3000 instruments have a two anode delay line detector, the Oxford instrument has the later three anode delay line detector, making comparisons between the 3000 and 5000 more straightforward. Settings were kept as similar as possible for ease of comparison, bearing in mind that due to the smaller laser spot size on the LEAP 5000, the laser energy of the UV laser on the LEAP 5000 is approximately one order of magnitude smaller to produce evaporation conditions that are comparable to that of the green laser on the LEAP 3000. Table 1 summarizes the specimen temperature, pulse energy/ fraction, and frequency used for each material on both instruments. For the ODS steel, the pulse energy was varied between 0.2 and 0.6 nJ on the LEAP 3000 and from 0.01 to
using standard FIB lift-out procedures (Thompson et al., 2007) because the quantity of heat-treated material available was insufficient for electropolishing. Tungsten was deposited onto the region selected for lift-out via electron beam using a Gas Injection System for 2min. This was followed by a further 2min of tungsten deposition using the ion beam at 30 kV, to provide additional protection fromGa damage duringmilling. The phosphorus-implanted silicon samples used the
0.16 nJ on the LEAP 5000 to observe the effect of laser energy on cluster composition. The reported results are the average from multiple
specimen tips, so that the data is not unduly affected by compositional heterogeneity. In the case of the SG steel, four data sets were obtained using the LEAP 3000 and five data sets were obtained using the LEAP 5000. The carbide and ODS steel cluster data was also obtained over several data sets. While the data presented for the phosphorus- implanted silicon was only from one data set, it was com- pared with other repeated data and the same results were observed—only one profile from each machine is shown, as aligning the exact position of the start of the implantation profile results in averaging that smooths out the signal. It should be noted that due to sample yield the number of atoms collected for each instrument varied, and as such the total number of clusters detected is not directly comparable for each material, but as the statistics of all clusters are being studied, this should not affect the results of this study. Data sets were analyzed using the commercial software
IVAS, version 3.6.12. Clusters in the SG and ODS steels were identified in the data using the method of maximum separa- tion distance of “solute” atoms (Hyde et al., 2011). Ions within a maximum separation (Dmax) of each other are considered clustered. The Dmax parameter was kept the same between data sets from both instruments, but the Nmin parameter (the minimum number of ions in a cluster) was varied to suit the number of clusters observed. In the bearing steel and the silicon sample, isoconcentration surfaces were used to identify the carbide precipitates and capping layer, respectively. Where appropriate, the cumulative distribution of
clusters F(t) was compared between the two instruments using the empirical distribution function
Fn t ^ ðÞ= Number of elements in the sample≤t n
= 1 n
X n
i=1
where 1z is the indicator of event z. Comparing the two data sets in this manner allows small changes in size distribution to be more easily distinguished. Assuming a LEAP 3000 efficiency of 37% and a LEAP 5000 efficiency of 52%, we should expect that multiplying the LEAP 3000 cumulative distribution by the ratio between the two efficiencies
1xi ≤t;
Pulse Fraction (Voltage Mode) Laser Energy (Laser Mode) Temperature (K) Pulse Frequency (kHz)
20% N/A N/A
N/A N/A Varied
0.4 nJ (LEAP 3000) 44 pJ (LEAP 5000)
0.4 nJ (LEAP 3000) 50 pJ (LEAP 5000)
50 50 50
50
200 200 200
200
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