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Type Angseryd, J.; Liu, F.; Andrén, H.-O.; Gerstl, S.S.A.; Thuvander, M.
  Publication Quantitative APT analysis of Ti(C,N) Volume Journal Article
Pages 2011
  Abstract Ultramicroscopy  
  Corporate Author Special Issue: 52nd International Field Emission Symposium  
Publisher 111  
Editor 6
  Summary Language 609-614 Series Editor Atom probe tomography; Apt; Ti(C,N); Quantification; Pcbn  
Abbreviated Series Title A specially produced Ti(C,N) standard material, with a known nominal composition, was investigated with laser assisted atom probe tomography. The occurrence of molecular ions and single/multiple events was found to be influenced by the laser pulse energy, and especially C related events were affected. Primarily two issues were considered when the composition of Ti(C,N) was determined. The first one is connected to detector efficiency, due to the detector dead-time. The second one is connected to peak overlap in the mass spectrum. A method is proposed for quantification of the C content in order to establish the C/N ratio. A correction was made to the major C peaks, C at 6 and 12 Da, with the 13C isotopes, at 6.5 and 13 Da, according to the known natural abundance. In addition, a correction of the peak at 24 Da, where C and Ti overlap, is proposed based on the occurrence of single/multiple events for respective element. The results were compared to the results from other techniques such as electron energy loss spectroscopy, chemical analysis and X-ray diffraction. After applying the corrections, atom probe tomography results were satisfactory. Furthermore, the content of dissolved O in Ti(C,N) was successfully quantified.
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0304-3991 no NU @ karnesky @ 11233
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Benedek, R.; van de Walle, A.; Gerstl, S. S. A.; Asta, M.; Seidman, D. N.; Woodward, C. Partitioning of solutes in multiphase TiAl alloys Journal Article 2005 Physical Review B Phys. Rev. B 71 9 094201: 1-12 First-principles calculations based on a plane-wave pseudopotential method, as implemented in the VASP code, are presented for the formation energies of several transition-metal and non-transition-metal dopants in TiAl alloys. Substitution for either Ti or Al in gamma -TiAl , alpha 2 - Ti3 Al , Ti2 AlC , and Ti3 AlC are considered. Calculated (zero-temperature) defect formation energies exhibit clear trends as a function of the periodic-table column of transition metal solutes. Early transition metals in TiAl prefer the Ti sublattice, but this preference gradually shifts to the Al sublattice for late transition metals; the Ti sublattice is preferred by all transition metal solutes in Ti3 Al . Partitioning of solutes to Ti3 Al is predicted for mid-period transition elements, and to TiAl for early and late transition elements. A simple Ising model treatment demonstrates the plausibility of these trends, which are in excellent overall agreement with experiment. The influence of temperature on formation energies is examined with a cluster expansion for the binary TiAl alloys and a low temperature expansion for dilute ternary alloys. Results for Nb-doped alloys provide insight into the relative sensitivity of solute partitioning to individual contributions to the free energy. Whereas the calculated formation energy of Nb (substitution) at zero temperature favors partitioning to alpha 2 - Ti3 Al , temperature-dependent contributions to the formation free energy, evaluated at 1075 K , favor partitioning to gamma -TiAl , in agreement with experiment. no NU @ karnesky @ 39
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Chad M Eichfeld and Stephan S A Gerstl and Ty Prosa and Yue Ke and Joan M Redwing and Suzanne E Mohney Local electrode atom probe analysis of silicon nanowires grown with an aluminum catalyst Journal Article 2012 Nanotechnology 23 21 215205 Local electrode atom probe (LEAP) tomography of Al-catalyzed silicon nanowires synthesized by the vapor-liquid-solid method is presented. The concentration of Al within the Al-catalyzed nanowire was found to be 2 × 10 20 cm -3 , which is higher than the expected solubility limit for Al in Si at the nanowire growth temperature of 550 °C. Reconstructions of the Al contained within the nanowire indicate a denuded region adjacent to the Al catalyst/Si nanowire interface, while Al clusters are distributed throughout the rest of the silicon nanowire. 0957-4484 no NU @ karnesky @ 11353
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Dmitrieva, O.; Choi, P.; Gerstl, S.S.A.; Ponge, D.; Raabe, D. Pulsed-laser atom probe studies of a precipitation hardened maraging TRIP steel Journal Article 2011 Ultramicroscopy Special Issue: 52nd International Field Emission Symposium 111 6 623-627 Pulsed-laser atom probe tomography; Local electrode atom probe; Microanalysis; Field evaporation; Precipitation hardened steels; Solute clustering 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. 0304-3991 no NU @ karnesky @ 11236
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Gerstl, Stephan S. A. Three-Dimensional Nanometer Scale Analyses of Precipitate Structures and Local Compositions in TiAl Engineering Alloys Book Whole 2006 231 Titanium aluminide (TiAl) alloys are among the fastest developing class of materials for use in high temperature structural applications. Their low density and high strength make them excellent candidates for both engine and airframe applications. Creep properties of TiAl alloys, however, have been a limiting factor in applying the material to a larger commercial market. In this research, nanometer scale compositional and structural analyses of several TiAl alloys, ranging from model Ti-Al-C ternary alloys to putative commercial alloys with 10 components are investigated utilizing threedimensional atom probe (3DAP) and transmission electron microscopies. Nanometer sized borides, silicides, and carbide precipitates are involved in strengthening TiAl alloys, however, chemical partitioning measurements reveal oxygen concentrations up to 14 at. % within the precipitate phases, resulting in the realization of oxycarbide formation contributing to the precipitation strengthening of TiAl alloys. iv The local compositions of lamellar microstructures and a variety of precipitates in the TiAl system, including boride, silicide, binary carbides, and intermetallic carbides are investigated. Chemical partitioning of the microalloying elements between the a2/g lamellar phases, and the precipitate/g–matrix phases are determined. Both W and Hf have been shown to exhibit a near interfacial excess of 0.26 and 0.35 atoms nm-2 respectively within ca. 7 nm of lamellar interfaces in a complex TiAl alloy. In the case of needle-shaped perovskite Ti3AlC carbide precipitates, periodic domain boundaries are observed 5.3±0.8 nm apart along their growth axis parallel to the TiAl[001] crystallographic direction with concomitant composition variations after 24 hrs. at 800°C. Northwestern University Ph.D. thesis no NU @ karnesky @ 803
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Gerstl, Stephan S. A.; Kim, Young-Won; Seidman, David N. Atomic scale chemistry of alpha(2)/gamma interfaces in a multi-component TiAl alloy Journal Article 2004 Interface Science Interface Sci. 12 2-3 303-310 TiAl - interface - partitioning - proxigram - interfacial excess We report on the distribution of micro-alloying elements in a multi-component TiAl-based alloy. The specimen contains 3 at.% Nb, 1.5 at.% Cr, 0.5 at.% Mn, 0.6 at.% (W + Hf + Zr), and 0.2 at.% each of B, C, and O. The distributions of all micro-alloying elements with respect to the heterophase interface between agr2 and gamma lamellae are analyzed with a three-dimensional atom-probe (3DAP) microscope. All the elements partition except boron, which resides primarily in boride precipitates. Oxygen, C, Mn, and Cr partition to the agr2-phase, whereas Nb and Zr partition to the gamma-phase. Both W and Hf exhibit excess concentration values within ca. 7 nm of the lamellar interface in the agr 2-phase, and their near interfacial excesses are 0.26 and 0.35 atoms nm–2, respectively. no 9295
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Gerstl, Stephan S. A.; Seidman, David N. Chemical and Structural Investigation of Internal Domains of Needle-Like Ti3AlC Carbide Precipitates in γ-TiAl with 3-D Atom-Probe Tomography Journal Article 2006 Microscopy & Microanalysis Microsc. Microanal. 12 S2 1570-1571 Titanium aluminide (TiAl) alloys have shown improvements in high temperature creep resistance, due in large part to the addition of carbides in the microstructure. Two types of carbide precipitates increase the creep resistance: Ti2AlC [1] and Ti3AlC [2]. There is evidence, however, that hexagonal Ti2AlC carbides (Htype) are formed from the metastable perovskite Ti3AlC carbides (P type). Therefore, the nucleation and growth of Ti3AlC precipitates is of interest due to their subsequent transformation into the Ti2AlC phase, which is a stable phase above 800C. Domains within the needle-like carbide precipitates have been observed via strain-field contrast in transmission electron microscopy (TEM), Fig. 1, and high-resolution TEM, Fig. 2. The latter provides evidence that structurally coherent atomic columns traverse the domain boundaries. The chemical makeup of these domains has not been previously investigated. In the current work, three dimensional atom probe (3DAP) microscopy is utilized for resolving the chemical nature of these boundaries [3]. Applying one-dimensional analysis cylinders along the primary axis of Ptype carbides in 3D reconstructions, Fig. 3, makes possible the measurement of the chemical composition variations inside the carbide precipitates. The nominal composition of the γ-TiAl alloy is Ti50Al48CMo, with C added for carbide formation and Mo providing increased ductility [4]. After 24 hrs. at 800C (Fig. 4a)the domains have a mean composition of Ti67Al21C11O1Mo0.4. At the domain boundaries, the Ti, Al, and Mo concentrations are ca. 57, 30, and 2.5 at.%, respectively. After 48 hrs. at 800C (Fig. 4b), the domains have a mean composition of Ti67Al20C11O1.3Mo0.7, and the composition fluctuations at the domain boundaries are less well defined, particularly in the case of the Mo concentration profile, and are indistinguishable from the average domain concentration. The alteration of domain boundary chemical composition reveals the initial steps in the transformation from a P-type to an H-type carbide. [1] P.I. Gouma, K. Subramanian, Y-M Kim, and M.J. Mills, edited by M.J. Mills E.P. George, M. Yamaguchi (MRS, 1999), Vol. 552. [2] J. Muellauer and F. Appel., Mat. Res. Soc. Symp. Proc. 5.1.1 (2003) 753. [3] S. S. A. Gerstl. PhD Thesis, Northwestern University (2006). [4] S. Kim, G. D. W. Smith, S. G. Roberts, and A. Cerezo., Mater. Sci. Eng. A 250(1), (1998) 78. [5] This research was supported by the DOE Basic Energy Sciences. no NU @ karnesky @ 754
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Gerstl, Stephan S. A.; Seidman, David, N.; Gribb, Amy A.; Kelly, Thomas F. LEAP Microscopes look at TiAl Alloys Journal Article 2004 Advanced Materials & Processes 162 10 31-33 LEAP The Local-Electron Atom-Probe (LEAP) microscope helps to investigate how nanoscale precipitates strengthen TiAl alloys at elevated temperatures and stresses in proposed jet engine and aerospace applications. no NU @ karnesky @ 657
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Larson, D.J.; Geiser, B.P.; Prosa, T.J.; Gerstl, S.S.A.; Reinhard, D.A.; Kelly, T.F. Improvements in planar feature reconstructions in atom probe tomography Journal Article 2011 Journal of Microscopy 243 1 15-30 Atom probe microscopy; data analysis; reconstruction Summary Standard atom probe tomography spatial reconstruction techniques have been reasonably successful in reproducing single crystal datasets. However, artefacts persist in the reconstructions that can be attributed to the incorrect assumption of a spherical evaporation surface. Using simulated and experimental field evaporation, we examine the expected shape of the evaporating surface and propose the use of a variable point projection position to mitigate to some degree these reconstruction artefacts. We show initial results from an implementation of a variable projection position, illustrating the effect on simulated and experimental data, while still maintaining a spherical projection surface. Specimen shapes during evaporation of model structures with interfaces between regions of low- and high-evaporation-field material are presented. Use of two-and three-dimensional projection-point maps in the reconstruction of more complicated datasets is discussed. Blackwell Publishing Ltd 1365-2818 no NU @ karnesky @ 10992
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Moszner, F.; Gerstl, S.S.A.; Uggowitzer, P.J.; Löffler, J.F. Structural and chemical characterization of the hardening phase in biodegradable Fe-Mn-Pd maraging steels Journal Article 2014 Journal of Materials Research 29 09 1069-1076 X-ray diffraction, atom probe tomography, maraging steels Fe–Mn–Pd alloys are promising candidates as biodegradable material for use in temporary implant applications. In this study, the hardening phase of Fe-rich martensitic alloys containing 1, 3, and 6 wt.% Pd and a fixed Mn content of 10 wt.% was investigated. All of these alloys show considerable age-hardening upon isothermal aging at 500 °C, exhibiting a behavior characteristic of maraging steels. Atom probe tomography (APT) and x-ray diffraction (XRD) measurements were performed to characterize the composition and crystallography of nanometer-sized precipitates forming in the overaged region of the Fe–Mn–Pd alloys. The precipitates consist mainly of Mn and Pd and the peaks of the intermetallic particles observed in the XRD spectra can be ascribed to the face-centered tetragonal β1-MnPd phase. The precipitation sequence for Fe–Mn–Pd is revealed to be similar to that reported for Fe–Mn–Ni and Fe–Mn–Pt maraging steels. no NU @ karnesky @ 11506
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