Mining information from atom probe data

Whilst atom probe tomography (APT) is a powerful technique with the capacity to gather information containing hundreds of millions of atoms from a single specimen, the ability to effectively use this information creates significant challenges. The main technological bottleneck lies in handling the extremely large amounts of data on spatial-chemical correlations, as well as developing new quantitative computational foundations for image reconstruction that target critical and transformative problems in materials science. The power to explore materials at the atomic scale with the extraordinary level of sensitivity of detection offered by atom probe tomography has not been not fully harnessed due to the challenges of dealing with missing, sparse and often noisy data. Hence there is a profound need to couple the analytical tools to deal with the data challenges with the experimental issues associated with this instrument. In this paper we provide a summary of some key issues associated with the challenges, and solutions to extract or "mine" fundamental materials science information from that data.

Grain boundary segregation in Fe-Mn-C twinning-induced plasticity steels studied by correlative electron backscatter diffraction and atom probe tomography

We report on the characterization of grain boundary (GB) segregation in an Fe-28Mn-0.3C (wt.%) twinning-induced plasticity (TWIP) steel. After recrystallization of this steel for 24 h at 700 °C, ∼50% general grain boundaries (GBs) and ∼35% Σ3 annealing twin boundaries were observed (others were high-order Σ and low-angle GBs). The segregation of B, C and P and traces of Si and Cu were detected at the general GB by atom probe tomography (APT) and quantified using ladder diagrams. In the case of the Σ3 coherent annealing twin, it was necessary to first locate the position of the boundary by density analysis of the atom probe data, then small amounts of B, Si and P segregation and, surprisingly, depletion of C were detected. The concentration of Mn was constant across the interface for both boundary types. The depletion of C at the annealing twin is explained by a local change in the stacking sequence at the boundary, creating a local hexagonal close-packed structure with low C solubility. This finding raises the question of whether segregation/depletion also occurs at Σ3 deformation twin boundaries in high-Mn TWIP steels. Consequently, a previously published APT dataset of the Fe-22Mn-0.6C alloy system, containing a high density of deformation twins due to 30% tensile deformation at room temperature, was reinvestigated using the same analysis routine as for the annealing twin. Although crystallographically identical to the annealing twin, no evidence of segregation or depletion was found at the deformation twins, owing to the lack of mobility of solutes during twin formation at room temperature.

Wootz steel project by electron backscatter diffraction

The data is from an electron backscatter diffraction (EBSD) study of the microstructure of high carbon ‘Wootz’ steel. The objective of the study is to infer an unknown thermomechanical history from observation and analysis of the final microstructure in various ancient artefacts (swords and tools), and then compare the findings with heat treatments of the ancient artefacts and modern attempts at duplication of the structure. Electron backscatter data reveals the orientation relationships between various phases in the material, particularly cementite and ferrite.

Wootz steel project by electron backscatter diffraction

The data is from an electron backscatter diffraction (EBSD) study of the microstructure of high carbon ‘Wootz’ steel. The objective of the study is to infer an unknown thermomechanical history from observation and analysis of the final microstructure in various ancient artefacts (swords and tools), and then compare the findings with heat treatments of the ancient artefacts and modern attempts at duplication of the structure. Electron backscatter data reveals the orientation relationships between various phases in the material, particularly cementite and ferrite. The dataset is randomly structured and organised. The data is automatically generated by an electron backscattered diffraction system attached to a field emission scanning electron microscope. The dataset uses proprietary software (cannot be copied or distributed without complying with licensing agreements): Oxford HKL Channel 5. As the native formats are binary they cannot be read with standard software.

Atomic scale analysis of light alloys using atom probe tomography

The present paper reviews recent progress in atomic-scale characterisation of composition and nanostructure of light alloy materials using the technique of atom probe tomography. In particular, the present review will highlight atom-by-atom analysis of solid solution architecture, including solute clustering and short-range order, with reference to current limitations of spatial resolution and detector efficiency of atom probe tomography and methods to address these limitations. This leads to discussion of prediction of mechanical properties by simulation and modelling of the strengthening effect exerted by solute clusters and the role of experimental atom probe data to assist in this process. The unique contribution of atom probe tomography to the study of corrosion and hydrogen embrittlement of light alloys will also be discussed as well as a brief insight into its potential application for the investigation of solute strengthening of twinning in Mg alloys.

Correlation between microstructure and phosphorus segregation in a hypereutectoid Wootz steel

The influence of bands rich in phosphorus on the microstructure of hypereutectoid Wootz steel implement is described. Electron probe micro-analysis is combined with optical microscopy. Phosphorus-rich bands are seen to correspond to regions of internal cracking, carbon depletion, and enhanced frequency of spheroidized cementite in place of pearlite. A rationale for the findings is presented in terms of the influence of phosphorus on the Fe–C phase diagram and on the rate of the eutectoid reaction.

Electrooxidation of [(η5-C5H5)Fe(CO)2]2 as a probe of the nucleophilic properties of ionic liquid anions

The oxidative electrochemistry of [CpFe(CO)2]2, 1 (Cp = [η5-C5H5]–), was examined in detail in ionic liquids (ILs) composed of ions of widely varying Lewis acid−base properties. Cyclic voltammetric responses were strongly dependent on the nucleophilic properties of the IL anion, but all observations are consistent with the initial formation of 1+ followed by attack from the IL anion. In [NTf2]–-based ILs ([NTf2]– = bis(trifluoromethylsulfonyl)amide), the process shows nearly ideal chemical reversibility as the reaction between 1+ and [NTf2]– is very slow. This is highly significant, as 1+ is known to be highly susceptible to nucleophilic attack and its stability indicates a remarkable lack of coordinating ability of these ILs. In 1-methyl-3-butylimidazolium hexafluorophosphate, [bmim][PF6], the oxidation of 1 is still largely reversible, but there is more pronounced evidence of [PF6]– coordination. In contrast, 1 exhibits an irreversible two-electron oxidation process in a dicyanamide-based IL. This overall oxidation process is thought to proceed via an ECE mechanism, details of which are presented. Rate constants were estimated by fitting the experimental data to digital simulations of the proposed mechanism. The use of [NTf2]–-based ILs as a supporting electrolyte in CH2Cl2 was examined by using this solvent/electrolyte as a medium in which to perform bulk electrolyses of 1 and 1*, the permethylated analogue [Cp*Fe(CO)2]2 (Cp* = [η5-C5(CH3)5]–). These cleanly yielded the corresponding binuclear radical-cation species, 1+ and 1*+, which were subsequently characterized by electron paramagnetic resonance (EPR) spectroscopy. In addition to the above oxidation studies, the reduction of 1 was studied in each of the ILs; differences in cathodic peak potentials are attributed, in part, to ion-pairing effects. This study illustrates the wide range of electrochemical environments available with ILs and demonstrates their utility for the investigation of the redox properties of metal carbonyls and other organometallic compounds.

Three-dimensional atom probe analysis of solute distribution in thermomechanically processed TRIP steels

Complex multiphase microstructures were obtained in transformation induced plasticity C–Mn–Si–(Nb–Al–Mo) steels by simulated controlled thermomechanical processing. These microstructures were characterized using transmission electron microscopy, X-ray diffraction and three-dimensional atom probe tomography (APT), which was used to determine the partitioning of elements between different phases and microconstituents. The measured carbon concentration (not, vert, similar0.25 at%) in the ferrite of carbide-free bainite was higher than expected from para-equilibrium between the austenite and ferrite, while the concentrations of substitutional elements were the same as in the parent austenite suggesting that incomplete bainite transformation occurred. In contrast, the distribution of substitutional elements between the ferrite lath and austenite in carbide-containing bainite indicated a complete bainite reaction. The average carbon content in the retained austenite (3.2 ± 1.6 at%) was somewhat higher than the T0 limit. On the basis of the APT measured composition, the calculated Ms temperatures for retained austenite were above room temperature, indicating its low chemical stability.

Precipitate characterisation of an advanced high-strength low-alloy (HSLA) steel using atom probe tomography

Increased fuel economy, combined with the need for the improved safety has generated the development of new hot-rolled high-strength low-alloy (HSLA) and multiphase steels such as dual-phase or transformation-induced plasticity steels with improved ductility without sacrificing strength and crash resistance. However, the modern multiphase steels with good strength-ductility balance showed deteriorated stretch-flangeability due to the stress concentration region between the soft ferrite and hard martensite phases [1]. Ferritic, hot-rolled steels can provide good local elongation and, in turn, good stretch-flangeability [2]. However, conventional HSLA ferritic steels only have a tensile strength of not, vert, similar600 MPa, while steels for the automotive industry are now required to have a high tensile strength of not, vert, similar780 MPa, with excellent elongation and stretch-flangeability [1]. This level of strength and stretch-flangeability can only be achieved by precipitation hardening of the ferrite matrix with very fine precipitates and by ferrite grain refinement. It has been suggested that Mo [3] and Ti [4] should be added to form carbides and decrease the coiling temperature to 650 °C since only a low precipitation temperature can provide the precipitation refinement [4]. These particles appeared to be (Ti, Mo)C, with a cubic lattice and a parameter of 0.433 nm, and they were aligned in rows [4]. It was reported [4] that the formation of these very fine carbides led to an increase in strength of not, vert, similar300 MPa. However, the detailed analysis of these particles has not been performed to date due to their nanoscale size. The aim of this work was to carry out a detailed investigation using atom probe tomography (APT) of precipitates formed in hot-rolled low-carbon steel containing additions Ti and Mo.

The investigated low-carbon steel, containing Fe–0.1C–1.24Mn–0.03Si–0.11Cr–0.11Mo–0.09Ti–0.091Al at.%, was produced by hot rolling. The processing route has been described in detail elsewhere [5] European Patent Application, 1616970 A1, 18.01.2006.[5]. The microstructure was characterised by transmission electron microscopy (TEM) on a Philips CM 20, operated at 200 kV using thin foil and carbon replica techniques. Qualitative energy dispersive X-ray spectroscopy (EDXS) was used to analyse the chemical composition of particles. The atomic level of particle characterisation was performed at the University of Sydney using a local electrode atom probe [6]. APT was carried out using a pulse repetition rate of 200 kHz and a 20% pulse fraction on the sample with temperature of 80 K. The extent of solute-enriched regions (radius of gyration) and the local solute concentrations in these regions were estimated using the maximum separation envelope method with a grid spacing of 0.1 nm [7]. A maximum separation distance between the atoms of interest of dmax = 1 nm was used.

The microstructure of the steel consisted of two types of fine ferrite grains: (i) small recrystallised grains with an average grain size of 1.4 ± 0.2 μm; and (ii) grains with a high dislocation density (5.8 ± 1.4 × 1014 m−2) and an average grain size of 1.9 ± 0.1 μm in thickness and 2.7 ± 0.1 μm in length (Fig. 1a). Some grains with high dislocation density displayed an elongated shape with Widmanstätten side plates and also the formation of cells and subgrains (Fig. 1a). The volume fraction of recrystallised grains was 34 ± 8%.

Emotional habituation to filmed violence employing the startle probe response

This thesis reports on research examining the habituation of emotional variables to filmed violence. The following subjective emotional variables were assessed: positive feelings, anxiety, disgust, entertainment and anger. In addition, an objective measure of emotional response was recorded physiologically, using the startle eyeblink response. The mediating influence of personality, individual differences and contextual features on an individuals’ perception of and reaction to filmed violence were also explored. Study one was exploratory in nature served to identify and select the film stimuli to be employed in the subsequent studies. The primary aim of the study was to allow for the identification of violent stimuli considered to be most socially and culturally relevant. The present research builds on existing scientific literature which has began to appreciate that differences in the context of a portrayal holds important implications for its impact on viewers. Thus, a secondary and more specific desire of study one was to obtain an evaluation of the contextual features of the violent film segments. A sample of 30 participants viewed and rated the film stimuli on the variables of realism, fantasy and violence. From this exploratory study four violent film stimuli were subsequently employed. Study 2 used the eyeblink startle response proposed by Vrana, Spence and Lang (1988) to explore the habituation of emotional variables to a realistic depiction of filmed violence. Emotional response was assessed both objectively, using the eyeblink startle response and subjectively through individuals self-reports. In addition the study investigated the significance of individual differences as mediators of emotional response. Questionnaire and physiological data were obtained from 30 participants. Overall, repeated exposure to filmed violence resulted in a decline in both objective and subjective emotional response. Differences were identified in the manner in which men and women responded to the film. Women reacted initially and over time with more intense physiological and psychological reactions to the violent film than males. Specifically, men displayed more curiosity and reported greater entertainment and positive feelings in response to the film, whilst women found the violence more disgusting and reported higher levels of anger and anxiety. It was found that the eyeblink startle magnitude paralleled the subjective emotional processing of the violent film, thus providing further confirmatory evidence of its validity in the investigation of emotional reactions to a stimulus. Personality factors were found to mediate emotional response to filmed violence, with neuroticism most powerfully implicated. High levels of neuroticism were found to be associated with greater anger and anxiety and less positive feelings whilst viewing violent film. A high score on extraversion was correlated with higher levels of anger and lower levels of curiosity and entertainment. Whilst the aims of study 3 were identical to that of study 2, a variation in the contextual feature of the violent film stimuli under investigation was the distinguishing and pertinent feature. Study 2 provided data on the habituation of emotional response to a realistic depiction of filmed violence. Study 3 was concerned with emotional response over repeated exposure to a fantasized violent depiction. Therefore, allowing for a comparison regarding the effects of context on emotional response. The results of study 3, with respect to habituation of emotional response, personality and individual differences, were similar to that obtained in study 2. A comparison of the two studies, however, revealed that individuals responded significantly different to the contextual features of the violent portrayal. Compared to the fantasised portrayal the realistic film stimulus was reported to be more digusting, anxiety provoking and less entertaining. In addition, and not surprisingly respondents reported that the realistic portrayal resulted in the production of more anger and less positive feelings.

Using polyatomic primary ions to probe an amino acid and a nucleic base in water ice

In this study on pure water ice, we show that protonated water species [H₂O]_nH⁺ are more prevalent than (H₂O)_n⁺ ions after bombardment by Au+ monoatomic and Au₃⁺ and C₆₀⁺ polyatomic projectiles. This data also reveals significant differences in water cluster yields under bombardment by these three projectiles. The amino acid alanine and the nucleic base adenine in solution have been studied and have been shown to have an effect on the water cluster ion yields observed using an Au₃⁺ ion beam.

Atom probe microscopy of self-assembled monolayers : preliminary results

We have achieved three-dimensional imaging of decanethiol self-assembled monolayers (SAMs) on metal surfaces by atom probe tomography (APT). The present Letter provides preliminary results on Ni [001] and Au [111], shows the analytical potential of APT analysis of SAMs, and details developments in specimen preparation and in data-treatment methodologies. Importantly, the investigation of the mass spectra from analysis of the SAMs revealed no combination of sulfur and hydrogen at the interface between the metal substrates and the organic materials, potentially providing insight about the bonding of the thiols on the substrate.

Application of advanced analytical techniques to study structure-property relationship of hot rolled high strength low alloy steel

The microstructure-property relationship in conventional high strength low alloy (HSLA) steel was evaluated using data obtained from transmission electron microscopy (TEM) and atom probe tomography (APT). Atom probe tomography allowed the characterisation of fine TiC particles with average radius of 3±1·2 nm that were not observed by TEM. The increase in the yield strength of steel due to the presence of fine precipitates was calculated to be 128 MPa.

Colloid probe AFM study of thermal collapse and protein interactions of poly(N-isopropylacrylamide) coatings

Graft coatings of poly(N-isopropylacrylamide) (pNIPAM) are of considerable interest for the reversible control of bio-interfacial interactions. In this study, graft coatings were prepared by free radical polymerisation from surface-bound polymerisable groups, on silicon wafers and quartz crystal microbalance (QCM) sensors. QCM with dissipation monitoring showed a gradual, extended phase change as the temperature increased. Colloid probe atomic force microscopy (CP-AFM) revealed a marked change in the compressibility of the coating below and above the lower critical solution temperature (LCST). Force curves showed an approximate 9-fold reduction in thickness between 24 °C and 38 °C, yet the collapsed coating at 38 °C still had a thickness significantly higher than the ellipsometrically determined dry thickness, indicating a residual extent of hydration above the LCST. At all temperatures, interaction force curves showed steric repulsion, though over different distances. There was little hysteresis between approach and retract force curves, which is evidence for almost instantaneous relaxation of the coating upon decompression. CP-AFM using a probe coated with bovine serum albumin (BSA) showed repulsive interactions with little approach/retraction hysteresis below the LCST, indicating lack of adhesion between the pNIPAM coating and the BSA-coated probe. In contrast, above the LCST the force curves on retraction were characteristic of adhesion, while the approach curves were still repulsive, and the adhesion increased in strength as the temperature was increased further beyond the LCST. Thus, QCM-D and CP-AFM data correlated well in showing a gradual nature of the phase transition and a concomitant gradual change in the interaction force with BSA.