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The effect orientation of features in reconstructed atom probe data onthe resolution and measured composition of T1 plates in an A2198aluminium alloy

Maria A. Mullin a,b, Vicente J. Araullo-Peters a,b, Baptiste Gault c, Julie M. Cairney a,b,n

a School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW 2006, Australiab Australian Centre for Microscopy and Microanalysis, The University of Sydney, Madsen Building F09, NSW 2006, Australiac Department of Materials, University of Oxford, Parks Road, OX13PH Oxford, UK

a r t i c l e i n f o

Article history:Received 29 August 2014Received in revised form27 March 2015Accepted 2 April 2015

Keywords:Atom probe tomographyAluminium alloysData analysisMicroscopy

x.doi.org/10.1016/j.ultramic.2015.04.00591/& 2015 Elsevier B.V. All rights reserved.

esponding author at: School of Aerospace, Mring, The University of Sydney, NSW 2006, Au

e cite this article as: M.A. Mullin, etured composition of..., Ultramicrosco

a b s t r a c t

Artefacts in atom probe tomography can impact the compositional analysis of microstructure in atomprobe studies. To determine the integrity of information obtained, it is essential to understand how thepositioning of features influences compositional analysis. By investigating the influence of feature or-ientation within atom probe data on measured composition in microstructural features within anAA2198 Al alloy, this study shows differences in the composition of T1 (Al2CuLi) plates that indicatesimperfections in atom probe reconstructions. The data fits a model of an exponentially-modifiedGaussian that scales with the difference in evaporation field between solutes and matrix. This in-formation provides a guide for obtaining the most accurate information possible.

& 2015 Elsevier B.V. All rights reserved.

1. Introduction

Atom probe tomography is increasingly used to investigate themicrostructure of both conducting and non-conducting solid ma-terials [1–5]. As a whole, it provides excellent compositional andspatial resolution [6,7]. However, imperfections and artefacts inthe tomographic reconstruction prevent fully-atomically resolvedanalytical imaging and limit its true potential. Atomic positions arereconstructed based on the detector-hit positions of the atomsfield-evaporated from the surface of the specimen combined withthe depth coordinate determined by the field evaporationsequence.

The state-of-the-art reconstruction protocol makes the as-sumption that ions fly following straight trajectories all crossing ata projection point, and that the specimen tip is perfectly spherical.These assumptions are rarely fulfilled and this introduces in-accuracies into the final reconstruction [8–11]. The changing shapeof the specimen throughout the evaporation process in the atomprobe produces variations in the magnification factor [12] thatlead to trajectory aberration artefacts. Similarly, differences in theevaporation field between species are known to induce pre-ferential evaporation or retention of some atomic species [13–17].This translates into a modification of the sequence of detection,

echanical and Mechatronicstralia.

al., The effect orientation ofpy (2015), http://dx.doi.org/1

whereby atoms of lower evaporation field are field evaporated“too early” and hence appear at a higher depth in the re-construction, whereas high evaporation field species that are re-tained at the surface appear too deep. Furthermore, the change inthe specimen topography arising from the presence (preferentialretention) or absence (preferential evaporation) of an atom at thesurface triggers the appearance of local curvatures that, in turn,induce trajectory aberrations. These prevent the specimen fromreaching a self-similar shape that is necessary for optimal spatialperformance [18]. Such effects ultimately combine to limit thespatial resolution of the technique, and can become critical in theanalysis of materials exhibiting complex microstructures, such asindustrial alloys.

The aim of the present work is to examine the influence ofthese artefacts on the atom probe characterisation of plate-shapedprecipitates. An Al–Cu–Li–Mg–Ag alloy was chosen that has adispersion of plate-shaped, approx. 2 nm thick, T1 precipitates thatconstitute the main strengthening phase within this series of al-loys that is currently used in the aeronautical industry [19]. Thedata was obtained as part of a study of the precipitation sequencein this alloy [20]. T1 precipitates have been extensively char-acterised by a variety of techniques: they have a well-establishedAl2CuLi stoichiometry, grow on {111} planes [21–23], and theyusually require prior deformation and/or alloying of Mg and Ag topromote their nucleation. Atom probe has already demonstratedthat Mg and Ag segregate to the T1/matrix interface [24] and are

features in reconstructed atom probe data on the resolution and0.1016/j.ultramic.2015.04.005i

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not incorporated in the T1 structure after nucleation [25]. Due totheir extremely high aspect ratios, T1 precipitates in these alloysare highly amenable to the application of atom probe crystal-lography methods [7,26–29], and their orientation with respect toa reference direction can be determined very accurately. In thisstudy, we have examined how the measured size and compositionof T1 plates varies as a function of their [30] orientation relative tothe analysis direction.

Fig. 2. Schematic depicting the inclination of a T1 plate relative to the probingdirection.

2. Materials and methods

2.1. Experimental

Analysis was conducted on an AA2198 Al–Li–Cu–Mg–Ag–Zralloy. The specimens were heated at 20 °C per hour (heating ramp)from room temperature until reaching a plateau at 155 °C for 15 or84 h. Specimens for the atom probe were prepared by electro-chemical polishing with 25% perchloric acid in 75% glacial aceticacid at 14 V, followed by a second-stage fine polishing under anoptical microscope using a solution of 2% perchloric acid in2-butoxyethanol [31]. Atom probe analyses were performed on aCameca (formerly Imago Scientific) LEAPs 3000X Si instrumentusing high voltage pulsing mode at a temperature of 2075 K anda pulse fraction of 20%. Three 15 h specimens and six 84 h speci-mens were analysed, containing a total of 65 T1 precipitates.

2.2. Data treatment and visualisation

Data was reconstructed and visualised using IVASTM (Cameca)and reconstructions were calibrated using the methods describedin Ref. [32,33]. For visualisation of the precipitates, we have usedthree-dimensional density maps that display the variation in thenumber of atoms of a given species (here Cu) measured locally ona grid superimposed to the dataset (1�1�1 nm3 voxels,3�3�1.5 nm delocalisation [34]). Fig. 1 illustrates how these 3Ddensity maps allow T1 plates lying on three different orientationswithin the same dataset to be simultaneously imaged. The in-clination of each precipitate was manually extracted by fitting adisc to each plate and recording the normal to the disc's habitplane using IVAS, as depicted in Fig. 2. One-dimensional con-centration profiles were obtained along the normal direction tothe T1 precipitates. To ensure consistency, features such as otherprecipitates, crystallographic poles and zone axes were avoided

Fig. 1. Three-dimensional volume rendering of the Cu concentration within thetomographic reconstruction showing T1 plates at different orientations (redcorresponds to high Cu concentration). (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

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when extracting concentration profiles.

3. Results and discussion

3.1. Composition profiles

Concentration profiles similar to those in Fig. 3 were obtainedfor each T1 precipitate and subsequently analysed. The full widthat half maximum (FWHM) was obtained for each element. To beconsistent, the average concentration of each element was ob-tained within the same interval, defined by the FWHM obtainedon the composition profile of copper.

Fig. 3a and b was obtained from T1 precipitates oriented withthe plate normal (a) at 3.1°, which is close to parallel to theprobing direction and (b) at 87.8°, which is close to perpendicularto the probing direction. Clearly, the measured concentrationprofile is altered by the difference in the inclination angle. At a lowangle (3.1°), the profile is much sharper and higher concentrationsare reached, while the composition of the profile at high angle(87.8°) appears much wider and is less well-defined. A draggingtail, associated with preferential retention [14], is observed whenthe plate is at a low angle, and is marked by an arrow in Fig. 3a. Nosuch tail was observed for the high-angle plate, Fig. 3b. Further-more, at high angles the concentration appears lower and thedistribution of atoms is much more diffuse, which indicates thatthe spread of atoms due to trajectory aberration is more pro-nounced and hence the loss of resolution is greater than at smallangles.

Fig. 4 shows the change in the measured thickness of T1 pre-cipitates with respect to the inclination angle. At low inclinationangles the full-width at half-maximum is the lowest. This in-creases gradually to approximately double for precipitates at an-gles close to 90°. Although T1 precipitates have been proposed tooccur as both single and double unit cells, two unit cells do notappear to be present here as there is a close dispersion of pointswith no large outliers. Furthermore no obvious difference in thecomposition of precipitates at 15-h and 84-h ageing times wasobserved. Therefore the variations observed are attributed directlyto variations in the spatial resolution with the inclination angle.

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Fig. 3. Concentration profiles through T1 precipitates taken in the direction normal to the plate with (a) the plate normal near parallel to the probing direction (3.1°) and(b) the plate normal near perpendicular to the probing direction (87.8°).

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3.2. Modelling

The difference in the resolution with respect to orientation islikely to arise from artefacts in the data. The most important factoris thought to be trajectory aberrations, which influence the lateralresolution. Although reconstruction protocols assume a lineartrajectory, this is an incorrect assumption, and evidence of al-terations to this in the initial stages of flight have been observed[35]. Irregularities on the surface such as edges of atomic terraces

Fig. 4. Full width at half maximum of the peaks of T1 precipitates as a function of the inthat the plate is oriented parallel to the probing direction and 90° indicates that the plat(c) Li (d) Mg and (e) Ag for specimens aged at 155 °C for both 15 h (red) and 84 h (blackreferred to the web version of this article.)

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result in a slight bending of the ion's trajectory, and the effects areamplified in the case of high-evaporation-field precipitates thatprotrude from the surface.

When the precipitates are oriented closer to perpendicular tothe probing direction, the profiles are less dependent on the lateralcoordinates and increasingly dependent on the depth coordinate,which has previously been shown to have a better resolution [36].While the depth has a resolution of E 0.1 nm, the lateral resolu-tion is E0.5 nm [37]. Here, in an effort to establish a model that

clination angle between the plate and the probing direction. Zero degrees indicatede is oriented perpendicular to the analysis direction. Data is shown for (a) Al, (b) Cu,). (For interpretation of the references to colour in this figure legend, the reader is

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Fig. 5. (a) Subset of the model plate-shape precipitate. (b) Example of a compo-sition profile, with a secondary axis displaying the evolution of the atomic density.(c) Average FWHM vs. angle for a series of five simulations at each angle.

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would describe the observed variations in the apparent thickness,we have generated a plate-shaped precipitate in 2D, on a face-centred cubic lattice, with an Al2CuLi stoichiometry with alter-nating pure-Al planes and mixed Cu/Li planes, on which Cu and Liare randomly distributed. The precipitate is 2 unit-cells in thick-ness and, to ensure good statistics, the precipitate is made of 100unit-cells along the z axis. At the interface with the matrix, a singlelayer of Ag was added. This is summarised in Fig. 5(a). Surroundingthe precipitate is a matrix of Al atoms that are displayed in a dif-ferent colour in this figure to distinguish them from the Al thatoriginates from within the precipitate. Marquis et al. [38] haveshown that, when modelling trajectory aberrations, atoms fromwithin precipitates, but of a similar species to the matrix, shouldbe considered as having a different evaporation field. This is due tothe difference in the bonding energy originating from the close-neighbourhood of the atom in the two different environments[39]. Lefebvre et al. [40] demonstrated that this effect could betraced back by looking at the evolution of the charge-state-ratio ofmatrix atoms in the vicinity of high-evaporation-field precipitates.

The spatial resolution has often been modelled as a three-di-mensional Gaussian dithering of the atomic positions, with a lat-eral resolution of the order of 0.1–0.3 nm and a depth resolution of0.05–0.1 nm [36,41–46], attributed to a combination of measure-ment errors and trajectory aberrations. So as to account for thevariation of the spatial resolution as a function of the inclination,we have used the formula introduced by Vurpillot et al.:

cos( ) sin( )depth lateralδ δ α δ α= + where δdepth and δlateral are thedepth and lateral resolution respectively, and α is the inclinationangle. We used δdepth¼0.05 nm and δlateral¼0.2 nm, which arevalues reported in the literature in the analysis of pure aluminium

Please cite this article as: M.A. Mullin, et al., The effect orientation ofmeasured composition of..., Ultramicroscopy (2015), http://dx.doi.org/1

[36].In the case of a high-field precipitate, ions are known to be

repelled away from the location of the precipitate itself [10,47–50].The contribution of the trajectory aberrations would therefore takethe form of an additional, non-symmetrical contribution to thespatial resolution, which results in intermixing between the atomsfrom the precipitate and the matrix inside of the matrix. Non-symmetrical distortions of composition profiles affected by fieldevaporation artefacts were recently modelled using exponentiallymodified Gaussian (EMG) function [15], and we decided to makeuse of a similar function to model the effect of trajectory aberra-tions on the atomic positions. We made use of the formulationintroduced in Ref. [51],

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where A is the amplitude, x0 the position of the centre, w thewidth of the peak, and t0 is a modification factor that accounts forthe skewing of the distribution. So as to account for the differencein trajectory aberrations for the different species, we defined t0 asthe ratio between the evaporation field of each solute and that ofthe matrix. The former was taken from the image-hump model[52], while the latter was determined based on the ratio of thecharge-states of Al through the whole experiment (21 V nm�1) asextracted from the post-ionisation curves introduced by Kingham[53]. A probability distribution function corresponding to eachelement was thus computed.

Finally, the two contributions to the resolution combine and anorientation-dependent term and a non-symmetrical random termare added to each atomic position of the simulated 2D-structure. Itis worth noting that neither the potential limited spatial resolutionof the detector [54,55], nor its limited detection efficiency, wereaccounted for in this model. From the final atomic positions re-sulting from this simple method, we computed a compositionprofile, an example of which is shown in Fig. 5(b) for the case of aprecipitate with a 0° inclination. Also displayed in the profile is theevolution of the atomic density (black dashed line, right axis),which is lower within the precipitate, as expected for high-fieldprecipitates. Simulations were run with increasing inclinationangle, and the evolution of the average full-width-half-maximumof the Cu and Li are plotted in Fig. 5(c). Average values and errorbars in this graph, corresponding to the standard deviation, wereextracted from a set of 5 simulations.

Despite the simplicity of the model introduced here, the resultsfrom the simulations are in good qualitative agreement with theexperimental data presented in Fig. 4: the width of the pre-cipitates almost double as the inclinations increase from 0 to 90°;there is a significant variability in the width for similar inclinationdue to the statistical nature of the problem; and the model pre-dicts a different extent of the intermixing region for the differentspecies. It is worth noting that, although it is not shown here forthe sake of clarity, the trend exhibited by the Ag is similar, whichmakes the assessment of the presence of species at the interfacemuch more difficult with increasing inclination and, consequently,intermixing. This is in agreement with our recent results in whicha surface Ag layer could only be observed in T1 plates that wereoriented perpendicular to the analysis direction [20].

In Fig. 6, we demonstrate that the average concentration of thealloying elements through the T1 plates decreased as the pre-cipitate became oriented towards the probing direction. This is inagreement with Blavette et al. [56], who also found a dependency

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Fig. 6. The average concentration of (a) Al, (b) Cu, (c) Li, (d) Mg and (e) Ag in the T1 precipitates as a function of inclination angle of the precipitate to the probing direction.The composition is averaged between the points where the copper half maxima occur.

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of the composition of grain boundaries in a nickel based superalloyon the orientation along which the composition was measured,together with a decrease in the width of grain boundaries withincreasing tilt angle. Studies have shown that the approximatecomposition of T1 precipitates is Al2CuLi [19]. This is accurate forthe precipitates that were at low inclination angles, where theaverage concentration for aluminium is E54% and 25% for copper.The scattered values for Li approximate to 13%, which is lowerthan the stoichiometric value. This value may have been under-estimated due to aberrations [57] or may be due to Mg/Ag occu-pying the Li sites in the T1 structure. It was recently demonstratedthat Mg occupies the Li-sites in δ′ precipitates in ternary Al–Mg–Lialloys [58].

With increasing inclination angle, the concentrations divergeaway from the expected values. The composition through theseprecipitates is dependent on lateral positioning of atoms, whichwe know is more affected by trajectory aberrations at high in-clination angles. In turn, analysis from precipitates parallel to theprobing direction is influenced by the depth positioning of atomsand therefore is expected to be more accurate. For Al, Cu and Li,the average concentrations were 74%, 13% and 8.5% respectively atinclination angles of 85–90°. Furthermore, Mg and Ag enrichmentwas found in all T1 precipitates, but these were present at muchlower concentrations. The measurement of Al in the T1 plates inthe concentration profiles with the large inclination angle isoverestimated because of matrix Al being falsely included. This isdue in part to the intermixing of atoms from the precipitate andmatrix, and in part to the increasing difficulty to precisely define

Please cite this article as: M.A. Mullin, et al., The effect orientation ofmeasured composition of..., Ultramicroscopy (2015), http://dx.doi.org/1

the interface of the precipitate as the spatial resolution worsens.This not seen in plates at low inclination due to both the higherspatial resolution when concentration profiles are taken parallel tothe probing direction, and the fact that concentration profiles wereobtained from regions away from the edge of the T1 plates.

4. Conclusion

The measured composition of T1 precipitates was found tochange considerably as a function of the orientation. The variationis attributed to trajectory aberrations during the atom probeanalysis that affect atom probe data [10], even using improvedreconstruction protocols [59] that still rely on the assumption ofstraight ions trajectories to determine the atom position withinthe reconstruction [9,11,12,59]. We have confirmed that the spatialresolution is best when a planar feature is oriented perpendicularto the analysis direction and have quantified how it degrades withincreasing inclination. This can be modelled by application of anexponentially-modified Gaussian that scales with the difference inevaporation field between solutes and matrix. These artefacts havebeen shown to affect the atom probe's capacity to accuratelymeasure the thickness and composition of T1 precipitates in Alalloys, and is likely to play a role for any microstructural featurethat is sufficiently small.

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Acknowledgements

The authors would like to acknowledge the scientific andtechnical input and the provision of all specimens from the Aus-tralian Microscopy and Microanalysis Research Facility (AMMRF)at the University of Sydney. The authors would like to thank AlexisDeschamps, Frederic de Geuser and Christophe Sigli for provisionof the aged AA2198 specimens and for their discussions. BG ac-knowledges that he is a full time employee of Elsevier Ltd. butdeclares no conflict of interest as his contribution to this articlecorresponds to his activity out of office hours.

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