432 Zirong Peng et al.


applications (Andrén, 2001), has been chosen to study the above-mentioned effects. Laser-pulsed APT was used as the acquisitionmode as according to a previous study (Thuvander et al., 2011), voltage-pulsed APT leads to large errors in quantitative analysis of carbon concentration in carbides. Two different local electrode atom probe (LEAP) systems were used to study the impacts of experimental parameters, including laser pulse energy, pulse repetition rate, specimen base temperature and specimen geometry, as well as instrumental parameters such as laser type and detector performance on the final data quality. The quality of the mass spectra, the accuracy of the resulting chemical compositions, and the proportions of multiple detection events have been evaluated.Aspecial emphasis was placed on the assessment of the significance of the detector pile-up effect. All these results provide useful guidelines for optimizing the laser-pulsed APT acquisition parameters with regard to carbon detection and quantification.


MATERIALS AND METHODS


The studied material is nano-grain-sized cemented tungsten carbide with a cobalt binder (Ceratizit S.A., Mamer, Luxembourg). Its nominal composition determined by chemical analysis is given in Table 1. The carbon content was measured by infrared (IR) absorption spectroscopy after combustion, whereas the N content was checked by thermal conductivity measurement. Quantitative chemical analyses of other elementswere performed by inductively coupled plasma optical emission spectroscopy (ICP-OES). The atomic ratio between carbon and tungsten is calculated to be 0.99, very close to the stoichiometric value of 1. APTspecimenswere prepared using a dual beamfocused-


ion beam (FIB) instrument (FEI Helios Nanolab 600/600i, Hillsboro, OR, USA) by an in situ lift-out method (Thompson et al., 2007). Commercial flat-top Si MicrotipTM arrays (CAMECA Instruments,Madison,WI,USA) were used as the support posts. To minimize the damage areas induced by the high-energy Ga ion beam during the specimen fabrication, a final cleaning step was carried out at 2kV and 28 pA. Laser-pulsed APT experiments were conducted on both a LEAPTM 3000XHRand a LEAPTM5000 XS device(CAMECA Instruments). The 3000X HR instrument uses a 532-nm- wavelength green laser, whereas the 5000 XS instrument uses a 355-nm-wavelength ultraviolet (UV) laser with a more tightly focused laser spot. As the 3000X HR system is equipped with a reflectron lens, which contains a field-defining mesh, the detection efficiency is about 37% and the flight path length is ~382mm. For the 5000 XS system, the straight flight path is


fixed at 100mm and the detection efficiency is increased to 80%, according to the instrument supplier. To investigate the influence of the laser pulse energy,


pulse repetition rate and specimen base temperature on the final data quality including mass spectrum, chemical com-


position, and multiple events, six different test series were carried out, denoted as 3000PEA, 3000PED, 5000PRRA, 5000PRRD, 3000BTA, and 3000BTD in the following. Here 3000 or 5000, respectively, indicates the instrument with which the test was performed, i.e. LEAP 3000X HR or LEAP 5000 XS; PE, PRR, and BT represent laser pulse energy, pulse repetition rate, and base temperature, respectively; A or D indicate the sequence in which the acquisition parameter was changed during testing. Each acquisition parameter was var- ied in ascending (A) and descending (D) sequence in two individual test series. Thereby, on the one hand, the potential influence of the specimen-to-specimen deviations can be excluded, and on the other hand, the impact of the specimen geometry can be inferred. The detailed acquisition conditions and the standing DC voltage range of each measurement are listed in Table 2. The target detection rate of the 3000X HR system was set to be 0.2% (two ions detected every 1,000 pulses), whereas for the 5000 XS system, the target detection rate was increased to 0.5% (five ions detected per 1,000 pulses), to compensate the difference in detection efficiency and maintain a comparable evaporation rate in both systems. For all conditions, the total number of ions considered for data evaluation was >1 million. To compare the laser energy setting between 3000XHR and 5000 XS, an additional test, denoted as 5000PEA, was performed, as listed in Table 2. The commercial software IVASTM 3.6.10 (CAMECA Instruments) was used to reconstruct and analyze the three-dimensional APT data sets. MATLAB was adopted to examine the exported *.ePos files and study the multiple detector events.


RESULTS


The quality of the collected mass spectrum, the accuracy of the resulting chemical composition, as well as the multi-hit proportion, are commonly used to assess APT data quality. In this part, different experimental conditions were evaluated and compared based on these three aspects.


Mass Spectrum


Figure 1 shows four representative, normalized mass spectra acquired using the 3000X HR (Figs. 1a, 1b) and 5000 XS (Figs. 1c, 1d) metrology systems, respectively. The mass


Table 1. Chemical Composition of the Studied Cemented Tungsten Carbide Measured by Chemical Analysis. W C


Co


at% wt%


Balance Balance


47.41 5.92


3.79 2.32


V 0.63 0.333 Si 0.28 0.081 Cr 0.06 0.0323 Fe 0.05 0.0267 Cu 0.02 0.0141


N 0.02 0.0025


Ca 0.003 0.0013


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