Voltage-Pulsed and Laser-Pulsed Atom Probe Tomography of a Multiphase High-Strength Low-Carbon Steel Michael D. Mulholland 1 and David N. Seidman 1,2, * 1 Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108, USA 2 Northwestern University Center for Atom Probe Tomography (NUCAPT), Evanston, IL 60208-3108, USA Abstract: The differences in artifacts associated with voltage-pulsed and laser-pulsed ~wavelength = 532 or 355 nm! atom-probe tomographic ~APT! analyses of nanoscale precipitation in a high-strength low-carbon steel are assessed using a local-electrode atom-probe tomograph. It is found that the interfacial width of nanoscale Cu precipitates increases with increasing specimen apex temperatures induced by higher laser pulse energies ~0.6–2 nJ pulse -1 at a wavelength of 532 nm!. This effect is probably due to surface diffusion of Cu atoms. Increasing the specimen apex temperature by using pulse energies up to 2 nJ pulse -1 at a wavelength of 532 nm is also found to increase the severity of the local magnification effect for nanoscale M 2 C metal carbide precipitates, which is indicated by a decrease of the local atomic density inside the carbides from 68 6 6 nm -3 ~voltage pulsing! to as small as 3.5 6 0.8 nm -3 . Methods are proposed to solve these problems based on comparisons with the results obtained from voltage-pulsed APT experiments. Essentially, application of the Cu precipitate compositions and local atomic density of M 2 C metal carbide precipitates measured by voltage- pulsed APT to 532 or 355 nm wavelength laser-pulsed data permits correct quantification of precipitation. Key words: atom probe tomography, precipitate analysis, Cu precipitation, M 2 C carbides I NTRODUCTION The addition of laser pulsing to atom-probe tomography ~APT! expands the range of materials that can be analyzed by APT ~Tsong, 1978; Kellogg & Tsong, 1980; Kellogg, 1987; Gorelikov, 2000!. Research projects that would be impossi- ble to conduct using voltage-pulsed APT are now becoming numerous ~Larson et al., 2004; Cerezo et al., 2007a; Gault et al., 2007; Kelly et al., 2007; Kelly & Miller, 2007; Chiara- monti et al., 2008; Yoon et al., 2008; Chen et al., 2009; Mulholland & Seidman, 2009; Li et al., 2010; Schreiber et al., 2011a, 2011b; Moutanabbir et al., 2011!. Advances in APT technology ~Bunton et al., 2007; Kelly & Miller, 2007; Seidman, 2007! permit the pulse repetition rates of laser- pulsed APT to exceed significantly those of voltage-pulsed APT, which increases the rate of data collection. Addition- ally, laser-pulsing generally results in a smaller specimen failure rate than does voltage pulsing ~Tsong, 1978!. This increased yield arises because the Maxwell mechanical stresses induced by the alternating electric field associated with voltage pulsing are absent for laser pulsing, which instead uses thermal pulses to induce evaporation of ions. The increased dataset size obtained utilizing laser pulsing is beneficial, especially for materials with high failure rates such as carbide-containing steels and metallic oxides. The effects of thermal pulses on the data quality obtained are, however, the subject of scientific debate. Many studies of the differences between laser-pulsed and voltage-pulsed APT have been conducted on single- phase materials or single-phase regions of multiphase mate- rials ~Kellogg & Tsong, 1980; Smith et al., 1982; Cerezo et al., 2007b; Zhou et al., 2008!. For instance, Zhou et al. ~2008! conducted a thorough study on the effect of green laser pulsing ~532 nm wavelength! on the measured concen- trations in an as-solutionized single-phase Ni-Al-Cr alloy. Few studies exist, however, on the effects of laser pulsing on multiphase materials. Sha and Ringer ~2009! observed only small effects of varying pulse energies ~0.2–1.5 nJ, wave- length = 532 nm! and base temperatures ~20–80 K! on the composition of both the solute clusters and matrix of an Al-Mg-Si-Cu alloy. One reason for the limited number of studies on multiphase materials is that these systems are significantly more complicated to study. Field evaporation of multiple phases with different evaporation fields leads to artifacts even in the voltage pulsing mode. Specifically, preferential field evaporation between voltage pulses, local magnifica- tion, or demagnification effects, and trajectory overlap of the field-evaporated ions are known to be artifacts that arise because of evaporation-field differences ~Goodman et al., 1973; Miller, 2000; Vurpillot et al., 2000!. Since laser-pulsing produces a temperature increase to induce evaporation of ions, which may be on the order of 300 K for a material with a small thermal diffusivity ~Cerezo et al., 2007b!, additional artifacts are anticipated. For example, Cerezo et al. ~2007b! observed the loss of the ability to resolve atomic planes in elemental tungsten due to enhanced surface diffusion of atoms at large values of the laser energy ~2 mJ pulse -1 , beam diameter = 100 mm, wavelength = 515 nm, and pulse duration = 500 fs!. Received December 29, 2010; accepted May 24, 2011 *Corresponding author. E-mail: d-seidman@northwestern.edu Microsc. Microanal. 17, 950–962, 2011 doi:10.1017/S1431927611011895 Microscopy AND Microanalysis © MICROSCOPY SOCIETY OF AMERICA 2011