Pulsed-laser atom probe studies of a precipitation hardened maraging TRIP steel O. Dmitrieva a,n , P. Choi a,n , S.S.A. Gerstl b , D. Ponge a , D. Raabe a a Max-Planck-Institute for Iron Research, Max-Planck-Str. 1, 40237 D¨ usseldorf, Germany b Imago Scientific Instruments, Madison, WI 53711, USA article info Available online 15 December 2010 Keywords:: Pulsed-laser atom probe tomography Local electrode atom probe Microanalysis Field evaporation Precipitation hardened steels Solute clustering abstract A precipitation hardened maraging TRIP steel was analyzed using a pulsed laser atom probe. The laser pulse energy was varied from 0.3 to 1.9 nJ to study its effect on the measured chemical compositions and spatial resolution. Compositional analyses using proximity histograms did not show any significant variations in the average matrix and precipitate compositions. The only remarkable change in the atom probe data was a decrease in the++/+ charge state ratios of the elements. The values of the evaporation field used for the reconstructions exhibit a linear dependence on the laser pulse energy. The adjustment of the evaporation fields used in the reconstructions for different laser pulse energies was based on the correlation of the obtained cluster shapes to the TEM observations. No influence of laser pulse energy on chemical composition of the precipitates and on the chemical sharpness of their interfaces was detected. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Over the past decade, there has been a rapidly growing interest in atom probe tomography (APT) as a technique for spatially resolved chemical analyses with near atomic resolution [1–8]. Recent developments in APT such as energy compensating wide angle reflectrons and ultrafast pulsed lasers have significantly improved the mass resolution and extended the applicability of this method to complex engineering alloys and materials with low electrical conductivity [1–8]. APT analyses of metallic samples have been traditionally carried out by applying high voltage pulses to the specimen with a typical pulse to base voltage ratio of about 20%. However, a common problem arising during voltage pulse APT analysis is premature specimen fracture. The exact mechanisms of this phenomenon remain unknown, but mechanical stresses associated with the high electric field at the specimen are believed to be its origin [9–11]. Pulsing with ultra-fast lasers has recently emerged as an attractive alternative to voltage pulsing. Laser pulsing offers certain advan- tages such as the capability of analyzing low-conductivity materials as well as an enhancement in mass resolution [4–6]. It has also been reported that laser pulsing leads to fewer specimen fractures due to a standing electric field, and thus constant mechanical stress applied to the specimen, whereas in the voltage mode the sample is exposed to a cyclically varying electric field. Moreover, the electric field is usually kept about 10–20% lower than for voltage pulsing [4]. However, the interaction between laser pulses and matter is not yet fully understood [12–16]. Temperature rises that occur during laser pulsing can deteriorate the spatial and mass resolution owing to surface diffusion and slow specimen cool-down [16,17]. Further- more, standing field evaporation can occur for specimens of low thermal conductivity and low shank angle as a result of an increased specimen base temperature [16]. Ions that are field evaporated between the voltage/laser pulses are lost, which at best will lead to a reduction in the overall detection efficiency, and at worst lead to an error in composition measurement due to preferential evaporation of lower evaporation field elements. The studies of Sha et al. [18] and Vurpillot et al. [19] indicate that thermal effects are considered to play a major role in ion emission [18,19]. Vurpillot et al. [19] have shown that the observed fast anomalous cooling rate of the tip can only be related to a confined heating zone at the tip apex smaller than the wavelength of the laser. Considerable effort has been put into the estimation of tempera- ture rises during pulsed laser APT analyses and modeling the temperature profile [16–23]. A detailed work of Tang et al. [24] was performed on the influence of laser pulse energy on the mass resolution of APT [24]. However, in our work we investigated the influence of laser pulsing energy on the measured composition of complex precipitation-hardened alloy and on the chemical content and spatial detection of the intermetallic nano-precipitates. Com- positional studies as a function of laser experiment parameters have been performed on selected Al- and Ni-based engineering alloys [25,26]. Laser energy dependent phenomena such as preferential evaporation, elemental surface diffusion, and local magnification could lead to deviations in the composition of the precipitates in complex steels. To our knowledge however, systematic pulsed laser Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ultramic Ultramicroscopy 0304-3991/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2010.12.007 n Corresponding authors. Tel.: + 49 211 6792325; fax: + 49 211 6792333. E-mail addresses: o.dmitrieva@mpie.de (O. Dmitrieva), p.choi@mpie.de (P. Choi). Ultramicroscopy 111 (2011) 623–627