Application of atom probe tomography to fundamental issues of steel materials

Although steel research has a very long history, there are still various questions and uncertainties on the fundamental issues of steel materials. Our aim is to clarify the issues by atomic-scale characterization using atom probe tomography (APT). This paper introduced our developed techniques in application of APT into steel materials. The first one is the specimen tip preparation technique from the site-specific regions of interest in steel, which significantly expanded the application field of steel analysis. The second one is the observation technique of trapped hydrogen in steel using our developed “deuterium charge cell,” which accomplished direct observation of the hydrogen trapping site in alloy carbide precipitation strengthened steels. Such atomic-scale analyses bring many new findings in steel materials science.     

Laser-assisted atom probe tomography of oxide materials.

Atom probe tomography provides a chemical analysis of nanostructured materials with outstanding resolution. However, due to the process of field evaporation triggered by nanosecond high voltage pulses, the method is usually limited to conductive materials. As part of recent efforts to overcome this limitation, it is demonstrated that the analysis of thick NiO and WO3 oxide layers is possible by laser pulses of 500 ps duration. A careful analysis of the mass spectra demonstrates that the expected stoichiometries are well reproduced by the measurement. The reconstruction of lattice planes proves that surface diffusion is negligible also in the case of thermal pulses.     

Spatial distribution maps for atom probe tomography.

A real-space technique for finding structural information in atom probe tomographs, spatial distribution maps (SDM), is described. The mechanics of the technique are explained, and it is then applied to some test cases. Many applications of SDM in atom probe tomography are illustrated with examples including finding crystal lattices, correcting lattice strains in reconstructed images, quantifying trajectory aberrations, quantifying spatial resolution, quantifying chemical ordering, dark-field imaging, determining orientation relationships, extracting radial distribution functions, and measuring ion detection efficiency.     

A quantitative model of preferential evaporation and retention for atom probe tomography

It is well known that the apparent concentration of a solute element in metal, detected by atom probe tomography analysis, depends on the measurement condition such as specimen temperature, pulse fraction, and pulse frequency. The dependence was qualitatively interpreted to be caused by preferential evaporation and retention in field evaporation. A quantitative physical model accounting for the preferential evaporation and retention was proposed herein for the first time. The proposed model was applied to a ferritic iron–copper (Fe–Cu) alloy for preferential evaporation and a ferritic iron–silicon (Fe–Si) alloy for preferential retention. The model explained the temperature dependence on the apparent concentration of the solute element and the unwindowed background noise in each alloy well, whereas the dependence of pulse fraction and pulse frequency was not completely explained. The cause of the difference between the experimental and calculated results based on the model was discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantification of dopant species using atom probe tomography for semiconductor application

Doping of semiconductors serve various purposes in metal-oxide-semiconductor (CMOS) technology, eg, increase carrier concentration and modify electric field distribution. With the scaling down of device and the introduction of three-dimensional fin field-effect transistors (FinFET), precise and reliable dopant quantification of concentration at the nano-scale is critical. Laser-assisted atom probe tomography (APT) provides a unique approach to characterize and quantify the dopant in three dimensions at sub-nanometer resolution. Nevertheless, quantification accuracy of APT is strongly influenced by the experimental conditions. Although B quantification has been widely studied, the correlation of B signal loss to B concentration is not yet established. In addition, no phosphorous quantification study has been reported. In this work, we found that, due to B multi-hit effect in APT, high B dose sample has larger B loading compared with low B dose sample. For standard calibration with minimized impact from multi-hit effect, we recommend B dose in the range of 1e14 atoms/cm². Despite the fact that B loading is dose dependent, APT quantification of B achieves precision within 2% to 6% relative standard deviation (RSD), which demonstrates that APT has good accuracy. On the other hand, P quantification suffers from mass interference of ³¹P⁺ and ³¹P₂²⁺ at 31 Da resulting in a large loading between APT and secondary ion mass spectrometry (SIMS). Nevertheless, we recommend that 31 Da to be labeled as ³¹P⁺ for smaller P variation for the APT analysis.     

A new approach to the determination of concentration profiles in atom probe tomography

Atom probe tomography (APT) provides three-dimensional analytical imaging of materials with near-atomic resolution using pulsed field evaporation. The processes of field evaporation can cause atoms to be placed at positions in the APT reconstruction that can deviate slightly from their original site in the material. Here, we describe and model one such process--that of preferential retention of solute atoms in multicomponent systems. Based on relative field evaporation probabilities, we calculate the point spread function for the solute atom distribution in the "z," or in-depth direction, and use this to extract more accurate solute concentration profiles.     

A reassessment of the metastable miscibility gap in Al-Ag alloys by atom probe tomography.

The evolution of Guinier-Preston zones in an Al-2.7 at.% Ag alloy was studied using atom probe tomography. The composition and morphology of the GP zones are time dependent, explaining discrepancies in previous work. This result requires the metastable miscibility gap for GP zones to be reevaluated, highlighting the importance of the temporal evolution of the GP zones. Preliminary results on the composition of gamma' and gamma plates are also presented.     

Application of focused ion beam to atom probe tomography specimen preparation from mechanically alloyed powders.

Focused ion-beam milling has been applied to prepare needle-shaped atom probe tomography specimens from mechanically alloyed powders without the use of embedding media. The lift-out technique known from transmission electron microscopy specimen preparation was modified to cut micron-sized square cross-sectional blanks out of single powder particles. A sequence of rectangular cuts and annular milling showed the highest efficiency for sharpening the blanks to tips. First atom probe results on a Fe95Cu5 powder mechanically alloyed in a high-energy planetary ball mill for 20 h have been obtained. Concentration profiles taken from this powder sample showed that the Cu distribution is inhomogeneous on a nanoscale and that the mechanical alloying process has not been completed yet. In addition, small clusters of oxygen, stemming from the ball milling process, have been detected. Annular milling with 30 keV Ga ions and beam currents >or=50 pA was found to cause the formation of an amorphous surface layer, whereas no structural changes could be observed for beam currents 相似文献   

New techniques for the analysis of fine-scaled clustering phenomena within atom probe tomography (APT) data.

Nanoscale atomic clusters in atom probe tomographic data are not universally defined but instead are characterized by the clustering algorithm used and the parameter values controlling the algorithmic process. A new core-linkage clustering algorithm is developed, combining fundamental elements of the conventional maximum separation method with density-based analyses. A key improvement to the algorithm is the independence of algorithmic parameters inherently unified in previous techniques, enabling a more accurate analysis to be applied across a wider range of material systems. Further, an objective procedure for the selection of parameters based on approximating the data with a model of complete spatial randomness is developed and applied. The use of higher nearest neighbor distributions is highlighted to give insight into the nature of the clustering phenomena present in a system and to generalize the clustering algorithms used to analyze it. Maximum separation, density-based scanning, and the core linkage algorithm, developed within this study, were separately applied to the investigation of fine solute clustering of solute atoms in an Al-1.9Zn-1.7Mg (at.%) at two distinct states of early phase decomposition and the results of these analyses were evaluated.     

Ar atom emission as a probe of craze formation and craze growth in polystyrene

A report of measurements of Ar emission during the loading of polystyrene and high impact polystyrene in vacuum is presented. Argon was introduced into the material prior to the experiment by storing the samples in an Ar atmosphere. The development of crazes during loading was monitored by videotaped visual observations and scattered light measurements. Increased Ar emission is observed at the onset of crazing, provided that the crazes intersect the surface. The strength of the Ar signal depends upon the extent of crazing; especially intense signals are observed from samples which display significant crazing prior to fracture. High-impact polystyrene shows intense emissions at yield which soon decay due to the depletion of Ar from the near surface material. The emission intensity rises again prior to fracture, when surface crazes become connected to crazes in the bulk. Thus the emission of volatile species during deformation reflects the growth of crazes intersecting the surface, as well as changes in the “connectivity” of the craze network. © 1993 John Wiley & Sons, Inc.     

First data from a commercial local electrode atom probe (LEAP).

The first dedicated local electrode atom probes (LEAP [a trademark of Imago Scientific Instruments Corporation]) have been built and tested as commercial prototypes. Several key performance parameters have been markedly improved relative to conventional three-dimensional atom probe (3DAP) designs. The Imago LEAP can operate at a sustained data collection rate of 1 million atoms/minute. This is some 600 times faster than the next fastest atom probe and large images can be collected in less than 1 h that otherwise would take many days. The field of view of the Imago LEAP is about 40 times larger than conventional 3DAPs. This makes it possible to analyze regions that are about 100 nm diameter by 100 nm deep containing on the order of 50 to 100 million atoms with this instrument. Several example applications that illustrate the advantages of the LEAP for materials analysis are presented.     

Protective properties of electrochemical polyaniline coatings on low-carbon steel

Protective properties of polyaniline coatings electrochemically deposited in the galvanostatic mode from oxalic acid solutions onto the surface of low-carbon steel were studied by measuring polarization curves.     

Evidence of superparamagnetic co clusters in pulsed laser deposition-grown Zn0.9Co0.1O thin films using atom probe tomography

Nanosized Co clusters (of about 3 nm size) were unambiguously identified in Co-doped ZnO thin films by atom probe tomography. These clusters are directly correlated to the superparamagnetic relaxation observed by ZFC/FC magnetization measurements. These analyses provide strong evidence that the room-temperature ferromagnetism observed in the magnetization curves cannot be attributed to the observed Co clusters. Because there is no experimental evidence of the presence of other secondary phases, our results reinforce the assumption of a defect-induced ferromagnetism in Co-doped ZnO diluted magnetic semiconductors.     

Review of atom probe FIB-based specimen preparation methods.

Several FIB-based methods that have been developed to fabricate needle-shaped atom probe specimens from a variety of specimen geometries, and site-specific regions are reviewed. These methods have enabled electronic device structures to be characterized. The atom probe may be used to quantify the level and range of gallium implantation and has demonstrated that the use of low accelerating voltages during the final stages of milling can dramatically reduce the extent of gallium implantation.     

Atom probe tomography: a technique for nanoscale characterization.

Atom probe tomography is a technique for the nanoscale characterization of microstructural features. Analytical techniques have been developed to estimate the size, composition, and other parameters of features as small as 1 nm from the atom probe tomography data. These methods are outlined and illustrated with examples of yttrium-, titanium-, and oxygen-enriched particles in a mechanically alloyed, oxide-dispersion-strengthened steel.     

Position sensitive atom probe study of segregation in molybdenum alloys

Three dimensional atom-probe measurements have been carried out to study the segregation behaviour of Mo(x)W((1-x)) alloys (x = 0.9; 0.75; 0.5; 0.25). As predicted from alloy thermodynamics, on in situ annealed specimens a significant surface enrichment of molybdenum has been observed. For alloys with tungsten contents above 25% the composition profiles measured are in a good agreement with the theoretical predictions. The positive regular solution parameter of this binary system gives reason for a random distribution of alloy components. In the three-dimensional reconstructions local agglomerations and combined features of tungsten atoms are to observe in alloys with Mo as major component.     

Position sensitive atom probe study of segregation in molybdenum alloys

Three dimensional atom-probe measurements have been carried out to study the segregation behaviour of Mo_xW_(1–x) alloys (x = 0.9; 0.75; 0.5; 0.25). As predicted from alloy thermodynamics, on in situ annealed specimens a significant surface enrichment of molybdenum has been observed. For alloys with tungsten contents above 25% the composition profiles measured are in a good agreement with the theoretical predictions. The positive regular solution parameter of this binary system gives reason for a random distribution of alloy components. In the three-dimensional reconstructions local agglomerations and combined features of tungsten atoms are to observe in alloys with Mo as major component.     

H atom plasma diagnostics: A sensitive probe of temperature and purity

Two-photon absorption laser-induced fluorescence (TALIF) has proven to be a convenient diagnostic for reactive light atoms in plasmas. We have carried out a series of TALIF experiments and report the first temperature measurements of ground state H atoms in an rf discharge. With reasonable care, measurements of the H atom linewidths, broadened by the Doppler effect, provide detailed information about the translational energy, i.e., temperature of the atoms. It is found that in pure H₂ plasmas, the H atom temperature is slighthy elevated with respect to ambient. In plasmas contaminated with the other H-containing molecules, Doppler-broadened linewidths corresponding to H atom temperatures in excess of 7000 K have been observed. The mechanisms leading to such high apparent temperatures are discussed.