Microsc. Microanal. 23, 431–442, 2017 doi:10.1017/S1431927616012654
© MICROSCOPY SOCIETY OF AMERICA 2017
Evaluation of Analysis Conditions for Laser-Pulsed Atom Probe Tomography: Example of Cemented Tungsten Carbide
Zirong Peng,1,* Pyuck-Pa Choi,1,2 Baptiste Gault,1,* and Dierk Raabe1
1Department of Microstructure Physics and Alloy Design, Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany 2Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-338, Republic of Korea
Abstract: Cemented tungsten carbide has been analyzed using laser-pulsed atom probe tomography (APT). The influence of experimental parameters, including laser pulse energy, pulse repetition rate, and specimen base temperature, on the acquired data were evaluated from different aspects, such as mass spectrum, chemical composition, noise-to-signal ratio, and multiple events. Within all the applied analysis conditions, only 1MHz pulse repetition rate led to a strong detector saturation effect, resulting in a largely biased chemical composition. A comparative study of the laser energy settings showed that an ~12 times higher energy was required for the less focused green laser of the LEAPTM 3000X HR system to achieve a similar evaporation field as the finer spot ultraviolet laser of the LEAPTM 5000 XS system.
Key words: atom probe tomography, laser-pulsing, data quality, tungsten carbide INTRODUCTION
Atom probe tomography (APT) is an advanced elemental analysis technique, in which atoms are successively field evaporated, a process involving their ionization and deso- rption, from the surface of a sharp, needle-shaped specimen. During an APT measurement, in addition to a standing electric field applied to the specimen, high-voltage (HV) pulses or laser pulses are used to trigger the field evaporation. After each pulse, the time-of-flight (TOF) of the ions which arrive at the detector within a certain period, known as the detection window, is recorded, thereby the mass-to-charge ratio m/n of each ion can be determined (Müller, 1968; Blavette et al., 1993; Kelly & Miller, 2007; Gault et al., 2012). As the elemental identification of APT is based on TOF
mass spectrometry, unlike energy-dispersive X-ray spectro- scopy, APT essentially exhibits equal detection sensitivity for all species regardless of their mass. Therefore, it is capable of carbon detection and quantification. Over the last decades, APT has been widely applied to the analyses of carbides and various kinds of materials containing carbon such as steels and superalloys (Miller & Smith, 1977; Bhadeshia & Waugh, 1982; Miller et al., 1983, 1989; Li et al., 2011, 2014; Thuvander et al., 2011; Tytko et al., 2012; Seol et al., 2013; Herbig et al., 2014; Isik et al., 2015; Yao et al., 2016). However, the accuracy of carbon quantification using APT has been a matter of debate for a long time (Sha et al., 1992). Field-evaporated carbon ions are preferentially detected
*Corresponding authors.
z.peng@
mpie.de;
b.gault@
mpie.de Received June 9, 2016; accepted December 8, 2016
during so-called multiple events, where two or more ions emitted during the same HV or laser pulse are detected within a single detection window, and/or as molecular ions (Rolander & Andrén, 1989a; Yao et al., 2010; Angseryd et al., 2011; Kobayashi et al., 2011; Takahashi et al., 2011; Thuvander et al., 2011; Miyamoto et al., 2012; Marceau et al., 2013; Kitaguchi et al., 2014). Although delay line detectors currently used in commercial APT instruments show very good multi-hit detection capability, ions may not be properly detected under certain circumstances, e.g. if two or more ions are too closely correlated in detector impact position and time. This is due to the existence of a detector dead time and dead zone after an ion impact (Jagutzki et al., 2002;Da Costa et al., 2005; Meisenkothen et al., 2015). As a consequence of this effect, known as ion pile-up or
detector saturation, the concentration of carbon atoms may be underestimated and the resulting chemical composition may be biased. To mitigate this artifact, suggestions on how to choose the experimental parameters have been made by several research groups. Acquisition conditions, for which the average hit multiplicity is low and ions show an even distribution to different charge states and molecular species, are reported to be desirable (Tang et al., 2010; Meisenkothen et al., 2015). Decreasing the evaporation rate will also be helpful as the statistical occurrence of multiple events is reduced (De Geuser et al., 2007). On the other hand, diverse approaches have been proposed to correct this signal loss from a probabilistic point of view (Rolander & Andrén, 1989b; Thuvander et al., 2011; Stephan et al., 2015). In this work, cemented tungsten carbide, an industrial material particularly important for cutting and drilling tool
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