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Ultramicroscopy ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 1 Contents lists available at ScienceDirect 2 3 4 Ultramicroscopy 5 6 journal homepage: www.elsevier.com/locate/ultramic 7 8 9 10 11 12 Specimen preparation for correlating transmission electron microscopy 13 and atom probe tomography of mesoscale features 14 a b,c b,c a,n Q115 Matthew I. Hartshorne , Dieter Isheim , David N. Seidman , Mitra L. Taheri 16 a 17 Department of Materials Science and Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA b 18 Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, IL 60208-3108, USA c Northwestern University Center for Atom-Probe Tomography, 2220 Campus Drive, Evanston, IL 60208-3108, USA 19 20 21 article info abstract 22 23 Article history: Atom-probe tomography (APT) provides atomic-scale spatial and compositional resolution that is ideally 24 Received 20 November 2013 suited for the analysis of grain boundaries. The small sample volume analyzed in APT presents, however, 25 Received in revised form a challenge for capturing mesoscale features, such as grain boundaries. A new site-specific method 26 8 May 2014 utilizing transmission electron microscopy (TEM) for the precise selection and isolation of mesoscale Accepted 24 May 2014 27 microstructural features in a focused-ion-beam (FIB) microscope lift-out sample, from below the original 28 surface of the bulk sample, for targeted preparation of an APT microtip by FIB–SEM microscopy is 29 Keywords: presented. This methodology is demonstrated for the targeted extraction of a prior austenite grain 30 Atom probe tomography boundary in a martensitic steel alloy; it can, however, be easily applied to other mesoscale features, such Focused ion beam 31 as heterophase interfaces, precipitates, and the tips of cracks. Transmission electron microscopy & 32 2014 Elsevier B.V. All rights reserved. 33 34 67 35 68 36 69 37 1. Introduction features of interest within the microtip in materials that do not 70 38 pulse electropolish reliably, such as cemented carbides [19,26]. 71 39 Atom-probe tomography (APT) and atom-probe field-ion These methodologies may involve many steps depending on the 72 40 microcopy (APFIM) and have been used to analyze point, line feature being investigated, and relies heavily on the desired 73 41 and planar imperfections in materials since the beginning of their feature occurring within a reasonable distance from the apex of 74 – 42 development in the late 1960s [1 11]. The use of APT to study the electropolished microtip. With the development of focused 75 43 grain boundaries (GBs) in metallic alloys is of particular interest, as ion-beam (FIB) microscopes, it became possible to produce APT 76 44 their structure and composition can determine many properties of specimens with their microtips containing GBs and other features 77 45 the bulk material, such as ductility, toughness and corrosion of interest, with both predictability and accuracy [27–31]. Various 78 – – 46 resistance [12 18]. FIB SEM-microscope based methodologies have been developed 79 47 In the early years of the FIM and the APFIM techniques, to lift-out sections of GBs and place them on single microtips or on 80 48 specimens were prepared by electropolishing to form a microtip silicon microrods residing on microtip arrays for analyses [27,28]. 81 49 geometry, which would permit both field ionization and field Alternatively, techniques involving FIB–SEM microscopy ion-beam 82 50 evaporation of specimens [6,19]. While successful, electropolishing milling and analyzing specimens in situ have also been successful 83 51 alone does not readily permit site specific targeting of imperfec- for the study of different features, such as GBs and heterophase 84 52 tions or mesoscale features. To address this problem, methods interfaces [29,32], which are central to the control of materials 85 53 have been developed using transmission electron microscopy properties. In steels, for example, it has been demonstrated that 86 54 (TEM) to identify features of interest and then electropolishing segregation of elements such as copper, tin, antimony, phosphorus 87 55 repeatedly to position the feature of interest in the microtip, which and sulfur to austenite GBs during hot working leads to a loss of 88 – – 56 is subsequently analyzed [19 25]. In addition to the pulse electro- ductility [12 14]. 89 57 polishing approach, broad ion beams have been used to position Despite their importance in determining material properties, 90 58 microstructural features with a spacing in the 5–40 μmrange 91 (corresponding to ASTM grain size 6–12 in metals [33])havenot 92 59 n Corresponding author. Tel.: þ1 215 895 6618. been analyzed readily with APT, due to difficulties in preparation of 93 60 E-mail addresses: [email protected] (M.I. Hartshorne), microtips using either electropolishing or existing FIB–SEM micro- 94 61 [email protected] (D. Isheim), 62 [email protected] (D.N. Seidman), scopy lift-out methods. An important example of these mesoscale 95 63 [email protected] (M.L. Taheri). features is prior austenite GBs, which serve as nucleation sites for 96 64 97 http://dx.doi.org/10.1016/j.ultramic.2014.05.005 98 65 0304-3991/& 2014 Elsevier B.V. All rights reserved. 66 99 Please cite this article as: M.I. Hartshorne, et al., Specimen preparation for correlating transmission electron microscopy and atom probe tomography of mesoscale features, Ultramicroscopy (2014), http://dx.doi.org/10.1016/j.ultramic.2014.05.005i 2 M.I. Hartshorne et al. / Ultramicroscopy ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 1 carbides and intermetallic precipitates, further reducing ductility, This methodology as demonstrated in this paper, can be used to 67 2 toughness and corrosion resistance [13,15–18]; therefore, character- select desired mesoscale features from among large numbers of 68 3 ization of compositional changes at these features is critical for microscale features at depths of several microns below the surface 69 4 understanding the evolution of microstructures in steels. During an of the sample, which has applications beyond grain boundaries. 70 5 electropolishing procedure, a large number of specimens and Irradiated materials [40,41], such as nuclear reactor steels, can be 71 6 repeated electropolishing steps are required to locate an interface. analyzed at a specific depth with the specimen’s z-axis parallel to 72 7 For example, in the methods presented by Henjered and Norden the surface of the sample to ensure that all features within the 73 8 [19] and Krakauer and Seidman [20,22–25], an approximately dataset received the same dose or to analyze a section of a GB at a 74 9 1–1.5 μm long section of the specimen is rendered electron trans- specific depth from the irradiated surface; current methods only 75 10 parent after each electropolishing step [19,22]. As such, if 500 nm of allow for microtips to be prepared parallel to the bulk sample 76 11 the viewed volume is removed with each polishing step, 20 surface if they are extracted from a region very close to the bulk 77 12 electropolishing steps will be required to view 10 μm of the speci- sample’s surface. Thin-films and semiconductor device structures 78 13 men. In addition to this large number of steps, materials with a [42,43], such as electrode-gate-substrate triple points, can also be 79 14 mixture of microscale and mesoscale features, such as the lath prepared with the device-substrate interface parallel to the ana- 80 15 boundaries and prior austenite GBs in martensitic steels, require lysis direction in the APT to minimize the probability of the 81 16 more intensive analyses employing selected area diffraction (SAD), specimen rupturing during analysis. 82 17 using a TEM, of each boundary encountered to determine whether 83 18 or not it is of interest. While this approach is certainly possible it 84 19 requires a significant amount of TEM analysis. 2. Samples and experimental apparatus 85 20 More recently, specimen preparation has been performed using 86 21 FIB–SEM microscope lift-out procedures, which are more site- A martensitic ultrahigh strength steel, currently marketed as 87 22 specific than conventional electropolishing techniques; standard PremoMet™ by Carpenter Technology Corporation, was used to 88 23 lift-out techniques require, however, a straight and several micron validate the methodology presented in this article. This steel was 89 24 long section of a GB to extract a specimen [27]. Cairney et al., Felfer austenitized at 1191 K for 1.5 h, quenched, cryogenically treated at 90 25 et al. and Tytko et al. have demonstrated methods that have 200 K for 8 h and tempered at 533 K for 2 h. The microstructure 91 26 successfully analyzed grains in the 5–40 μm size range, which contains a prior austenite grain diameter of approximately 10 μm 92 27 involves lifting out a rod of material with a GB located near its end, and a sub-micron martensitic lath diameter. As such, the features 93 28 then shaping the end of the rod into an APT specimen [34–38]. of interest, the prior austenite GBs, must be precisely selected 94 29 These techniques provide an excellent methodology for the fast, and distinguished carefully from among the more numerous lath 95 30 reliable and relatively damage-free preparation of samples con- boundaries. 96 31 taining GBs, but they have two major limitations when considering FIB–SEM microscopy ion-beam milling was performed using a FEI 97 32 preparation methods for materials containing mesoscale features. Strata DB-235 dual-beam FIB microscope equipped with an Omnip- 98 33 First, it does not permit TEM observation of the broader micro- robe Model 100.7 micromanipulator. The ion-beam column in this 99 34 structure in the vicinity of the tip to be prepared prior to shaping instrument permits Ga þ ion beam milling at either 5 or 30 kV.

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