Position-sensitive Atom Probe Research Articles

Characterisation of the early stages of solute clustering in 1Ni–1.3Mn welds containing Cu

Microstructural characterisation of neutron irradiated low alloy steels is important for developing mechanistic understanding of irradiation embrittlement. This work is focused on the early stages of irradiation-induced clustering in a low Cu (0.03 wt%), high Ni (∼1 wt%) weld. The weld was irradiated at a very high dose rate and then examined by atom probe (energy-compensated position-sensitive atom probe (ECOPoSAP) and local electrode atom probe (LEAP)) with supporting microstructural information obtained by small angle neutron scattering (SANS) and positron annihilation (PALA). It was demonstrated that extreme care must be taken optimising parameters used to characterise the extent of clustering. This is particularly important during the early stages of irradiation-damage when the clusters are poorly defined and significant compositional variations are present in what is traditionally described as matrix. Analysis of the irradiated materials showed increasing clustering of Cu, Mn, Ni and Si with dose. In the low Cu steel the results showed that initially the irradiation damage results in clustering of Mn, Ni and Si, but at very high doses, at very high dose rates, redistribution of Si is significantly more advanced than that for Mn and Ni.

Fracture Toughness and Atom Probe Characterization of a Highly Embrittled RPV Weld

Abstract The Heavy-Section Steel Irradiation Program at Oak Ridge National Laboratory (ORNL) includes a task to investigate the shape of the fracture toughness master curve for reactor pressure vessel steel highly embrittled as a consequence of irradiation exposure. A radiation-sensitive reactor pressure vessel (RPV) weld with intentionally enhanced copper content, designed KS-01, is characterized in terms of static initiation (KIc, KJc) and Charpy impact toughness in the unirradiated and irradiated conditions. The objective of this project is to investigate the ability of highly embrittled material to maintain the shape of the unirradiated transition fracture toughness curve, as well as to examine the ability of the Charpy 41-J shift to predict the fracture toughness shift at such a high level of embrittlement. Irradiation of this weld was performed at the University of Michigan Ford Reactor. Specimens of KS-01 weld were irradiated to about 0.74 × 1019 neutron/cm2 at 288°C. Irradiation resulted in Charpy ductile-to-brittle transition temperature (DBTT) shift of 169°C. It was anticipated that this shift would result in a fracture toughness transition temperature (at 100 MPa√m) in the irradiated condition near or slightly above the pressurized thermal shock screening criterion for weld metals, ToPTS = 129°C. The fracture toughness characterization of KS-01 weld in the unirradiated and irradiated conditions was mainly performed by testing 1T C(T), although some 0.5T C(T) and precracked Charpy specimens were used in this study. The master curve analysis showed that this material exhibited shift of reference fracture toughness transition temperature, To, of 165°C as result of radiation, which is in remarkable agreement with Charpy DBTT shift. The absolute value of To in the irradiated condition was determined to be equal to 139°C. This weld exhibited a low ductile initiation toughness (JQ) after irradiation. It left a relatively narrow temperature window to examine the shape of the transition region. Irradiated median fracture toughness values up to 148 MPa√m follow the master curve shape. However, low toughness brittle fractures occurred at temperatures further above To (To + 61°C) than expected with a leveling of the KJc data from the master curve shape. The microstructure of the weld was characterized with the ORNL's energy-compensated optical position-sensitive atom probe. Atom probe tomography revealed a high number density (∼3 × 1024 m−3) of Cu-, Mn-, Ni-, Si-, and P-enriched precipitates and a lower number density (∼1 × 1023 m−3) of P clusters.

Microstructural evolution in a PH13-8 stainless steel after ageing

The precipitation process in a wrought PH13-8 steel during ageing was investigated using a position sensitive atom probe. The precipitates formed are enriched in Ni and Al, and depleted of Fe and Cr, yet the composition is far from the stoichiometric NiAl phase. They may take on different shapes at different temperatures. The hardening effects observed during early stages of ageing should be due to the redistribution of atoms such as Fe, Cr, Ni, and/or Al. Particle coarsening takes place simultaneously with the development of the composition of the NiAl-enriched precipitates. Mo segregation at the precipitate/matrix interface was not found.

A model of irradiation damage in high nickel submerged arc welds

A model has been produced to describe irradiation damage in high nickel submerged arc welds (1.6% Ni, 1.5% Mn, 0.5% Mo). It is fitted to a database of hardness change results obtained for 12 such welds with copper contents from about 0.02 to about 0.6% irradiated in materials test reactors. Irradiation dose rates ranged from about 6×10 −9 dpa/s down to about 1×10 −10 dpa/s. In addition to the hardness change data, Charpy shift data were also available. For some specimens irradiation induced changes in microstructure have been characterised using small angle neutron scattering (SANS) and energy compensated position sensitive atom probe (ECoPoSAP) techniques. The model was empirically fitted to the data, but available physical understanding and the evidence from the microstructural studies were used to guide selection of the equations fitted. Physical understanding and microstructural evidence were also used to help choose between alternative models with similar statistical goodness of fit. The model has two components. First, ‘matrix’ damage, which is insensitive to copper content and irradiation dose rate, and which appears to increase with phosphorus and sulphur. Second, ‘precipitation damage’, which increases with copper and manganese content, and reduces with increasing silicon. Precipitation damage also increases with reducing irradiation dose. The ECoPoSAP data show that the precipitates are composed of primarily of Ni, Mn, Cu and Si, with a substantial amount of iron entrained within them. In the low copper welds the same instrument shows significant compositional fluctuations in these elements, but no visible clusters. The Charpy data have been used to develop an empirical correlation between transition shift and hardness change. This is linear up to about 250 °C shift.

Fabrication of specimens of metamorphic magnetite crystals for field ion microscopy and atom probe microanalysis

Field ion specimens have been successfully fabricated from samples of metamorphic magnetite crystals (Fe 3O 4) extracted from a polymetamorphosed, granulite-facies marble with the use of a focused ion beam. These magnetite crystals contain nanometer-scale, disk-shaped inclusions making this magnetite particularly attractive for investigating the capabilities of atom probe field ion microscopy (APFIM) for geological materials. Field ion microscope images of these magnetite crystals were obtained in which the observed size and morphology of the precipitates agree with previous results. Samples were analyzed in the energy compensated optical position-sensitive atom probe. Mass spectra were obtained in which peaks for singly ionized 16O, 56Fe and 56FeO and doubly ionized 54Fe, 56Fe and 57Fe peaks were fully resolved. Manganese and aluminum were observed in a limited analysis of a precipitate in an energy compensated position sensitive atom probe.

Microstructural characterization of irradiation-induced Cu-enriched clusters in reactor pressure vessel steels

The effect of irradiation on microstructure of four irradiated reactor pressure vessel steels (a low copper A533B-1 plate, a low copper A508-3 forging, a high copper Linde 80 flux weld and a high copper Linde 1092 flux weld) was determined by using complementary microstructural techniques such as optical position-sensitive atom probe (OPoSAP), field emission gun scanning transmission electron microscopy (FEGSTEM) and small angle neutron scattering (SANS). In the low copper steels, irradiation resulted in small shifts in transition temperature and small changes in hardness increments. The microstructural analyzes showed that this response was dominated by matrix damage. In contrast, both copper-enriched clusters and matrix damage formed in the high copper welds. This information was then used as input to the Russell–Brown model to predict the change in hardness resulting from copper-enriched clusters. The calculated hardness increments were found to be consistent with the experimental data.

Three dimensional atom probe investigation of vanadium nitride precipitates and the role of oxygen and boron in rapidly solidified 316 stainless steel

A 316 stainless steel that was rapid solidification processed (RSP) by gas atomization and hot extrusion of the powder has been studied previously by conventional atom probe field ion microscopy (APFIM). Vanadium, nitrogen, and oxygen have been introduced intentionally as alloying elements for the purpose of improving the mechanical properties through fine-scale precipitation. Previous efforts to locate the oxygen in the structure using APFIM were inconclusive, largely due to poor counting statistics. An energy-compensated three-dimensional position-sensitive atom probe (3DAP) has been used in the present study to search for oxygen in cavities, precipitates, and grain boundaries in this alloy. As a direct result of the much greater number of atoms detected and the three-dimensional imaging, oxygen concentrations on the order of 1 atomic% have been found both inside nitride precipitates and in the vicinity of grain boundaries. Boron was also found unexpectedly in concentrations of up to 2 atomic% inside nitride precipitates and in the vicinity of grain boundaries. These findings are important to developing an understanding of the grain growth resistance and precipitate stability both in this alloy and in this processing method in general.

The Position-Sensitive Atom Probe - A New Dimension In Atom Probe Analysis

A possible description of the ideal microscope would be an instrument which was able to reconstruct, with atomic resolution and in 3 dimensions, both the position and the chemical identity of atoms in a material. The 3-dimensional atom probe (3DAP) is the technique which comes closest to this goal.The position-sensitive atom probe (PoSAP) was the first 3DAP. In the PoSAP, the high magnification of the field-ion microscope is combined with the time-of-flight mass spectroscopy of the atom probe, and position-sensitive detection based on a wedge-and-strip anode, Fig.1. This combination allows the chemical identity and the original surface position to be determined for single atoms removed from a field-ion specimen by pulsed field evaporation. Continued field evaporation and analysis builds up a 3D image of the distribution of all the atomic species originally present in the material, Fig. 2.

Towards 3D lattice reconstruction with the Position Sensitive Atom Probe

3-dimensional atom probe (3DAP) analysis enables 3D reconstruction of the chemical variations inside materials with near atomic resolution. This technique has enabled important insights into the phase chemistry and microstructure of a range of complex engineering materials. Although individual planes can be resolved when analysing in a direction close to a low-index pole, the spatial resolution of this technique has not been sufficient for reconstruction of all the original lattice positions. The spatial resolution is thought to be fundamentally limited by small lateral displacements of the atom on the surface of the specimen during the field evaporation process due primarily to interactions with the neighbouring surface atoms. It has therefore been considered that it will be impossible to correctly reconstruct the crystal lattice from 3DAP data. Recently, we have found that a 3DAP reconstruction of a region near a grain boundary in pure tungsten (an element renown for its good imaging and evaporation behaviour) revealed several different sets of crystallographic planes visible at angles up to 45° from the direction of analysis.

A Transparent Anode Array Detector for 3d Atom Probes

In a three dimensional atom probe, the identity and spatial coordinates of the atoms field evaporated from the specimen are determined. Their identity is calculated from the flight time from the specimen to the single atom detector. The x and y coordinates of the atom in the specimen are determined from the coordinates of its impact position on the position-sensitive detector and the z coordinate is determined from its position in the evaporation sequence. These data may then be reconstructed to visualize and quantify the distribution of all the elements in the specimen. Several types of position-sensitive detectors have been used including a wedge-and-strip detector (position-sensitive atom probe), a 10 by 10 array of anodes (tomographic atom probe), and a gateable CCD camera (optical atom probe). The wedge-and strip and the CCD camera detectors both suffer from the limitation that if more than one atom strikes the detector on a field evaporation pulse then the impact positions cannot be determined in many cases.

Atomic Structure, Composition, Mechanisms and Dynamics of Transformation Interfaces in Diffusional Phase Transformations

This overview describes progress that has been made in understanding the atomic structure, composition, mechanisms and dynamics of transformation interfaces in diffusional transformations involving plate-shaped transformation products in the past eight years. Some of the important developments in the area of structure include recognition that diffusional phase transformations obey many of the same crystallographic principles developed for martensite, widespread application of high-resolution transmission electron microscopy (HRTEM) and atomistic modelling to understand interphase boundary structure and energetics, and determination of the atomic structure and energetics of high-index ledged interfaces. In the area of composition, recent development of thermal field-emission and energy-filtering HRTEM allows compositional profiling at precipitate interfaces with subnanometer spatial resolution. In addition, the position sensitive atom-probe (PoSAP), which can determine both the mass and position of individual atoms, makes it possible to map elemental distributions in some materials with atomic resolution. These techniques are expected to provide an abundance of data on composition at transformation interfaces approaching the atomic level in the near future. In terms of the dynamics of transformation interfaces, application of in situ hot-stage HRTEM to precipitate growth and dissolution has provided information on the atomic mechanisms and kinetics of ledge motion and interaction, the mechanisms and kinetics of kink nucleation, and mechanisms of faceting/roughening of precipitate plates at the atomic level. Also, kinetic models for ledge growth which are able to treat multiple-ledge interactions and time-dependent growth have been developed and are available for comparison with experimental data. In addition to discussion of these and other major developments, effort is made to emphasize current strengths and limitations in each area and indicate possible directions for future research.

Co-deposition of Co-Ni alloys and their microstructure analysis by PoSAP

ion concentration in the depositing baths on the Co content and the microstruc-ture in the corresponding deposit was studied by the position sensitive atom probe （PoSAP）,TEM and X-ray diffraction. The Co-Ni phase diagram shows that, at room temperature,there are two phases in Co-Ni alloys, one is eCo with hcp lattice, the other is αCo of fcc lat-tice. A Co-Ni alloy with a Ni content below 27% consists of single eCo, beyond 36% consistsof single αCo, in between consists of eCo + αCo. The examination results of TEM, SEM andX-ray diffraction indicated that the Co-Ni deposits with average grain size of 70 nm and thick-

Atomic Scale Study Of Precipitate / Matrix Interfaces in a Metallic Alloy

ABSTRACTThe study of phase transformations involves making assumptions about interfaces within a material in order to apply mathematical models. An example of this is the Gibbs classical theory of nucleation and growth which is based upon the assumption that the precipitate / matrix interface is sharp. Recent developments in atom probe microscopy have made it possible for the first time to characterize complex three-dimensional internal interfaces within materials to sub-nanometre accuracy. We have used the OPoSAP (Optical Position Sensitive Atom Probe) to characterize the precipitate / matrix interface in the dilute Cu-Co system which is a model alloy for the study of homogeneous nucleation and growth. Interface widths derived from radial composition profiles were measured to be 0.9 ± 0.2 nm in size. The effects of thermal energy, positioning inaccuracies, and statistical limitations on the measurement of interface widths are examined through the use of mathematical models and computer simulations. Similar widths are measured for precipitates in the coarsening regime which shows the interface velocity does not affect the interface width.

Materials Applications of an Advanced 3-Dimensional Atom Probe

An advanced 3-dimensional atom probe system has been constructed, based on an optical position-sensitive atom probe (OPoSAP) detector with energy compensation using a reflectron lens. The multi-hit detection capability of the OPoSAP leads to significant improvements in the efficiency of the instrument over the earlier serial position-sensing system. Further gains in efficiency are obtained by using a biassed grid in front of the detector to collect secondary electrons generated when ions strike the interchannel area. The improvement in detection efficiency gives enhanced performance in the studies of ordered materials and the determination of site occupation. Energy compensation leads to a much improved mass resolution (m/Δm=500 full width at half maximum) making it possible to map out the 3-dimensional spatial distributions of all the elements in complex engineering alloys, even when elements lie close together in the mass spectrum. For example, in the analysis of a maraging steel, this allows separation between the 61 Ni 2+ and 92 Mo 3+ peaks, which are only 1 / 6 of a mass unit apart.

Interfacial diffusion studies in CoPd layered films

Position-sensitive atom probe microanalysis has been used to study diffusion in CoPd bilayers and multilayer films. Diffusion coefficients have been determined and the effect of grain boundaries on diffusion rates have been assessed.

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 precipitates in high speed steel

The microstructure of a high speed steel specimen alloy with nominal composition of 1.3% C, 3.8% Cr, 4.3% Mo, 5.2% W, 4.1% V and 0.75% Nitrogen (in wt. %, balance Fe), was studied using a three-dimensional atom probe. From the position-sensitive analysis the variations of composition present in the samples can be reconstructed in three dimensions with sub-nanometer resolution. In alloy samples, annealed, quenched and tempered to peak hardness, the three-dimensional reconstruction of volumes analysed shows spherical and needle-like precipitates with extensions of about 1–3 nm as well as chain-like accumulations. The precipitates contain predominantly Cr, V, Cr-X and V-X (X stands for C and N). In addition Mo is found embedded and in close proximity to these clusters. The role of W in the precipitates is minor but W shows some tendency to form chain-like clusters near Cr-X and V-X precipitates.

Production by gas atomization and properties of nitrided stainless steel : S.D. Ridder et al (National Inst. of Standards and Technology, Gaithersburg, USA)

A 316 stainless steel that was rapid solidification processed (RSP) by gas atomization and hot extrusion of the powder has been studied previously by conventional atom probe field ion microscopy (APFIM). Vanadium, nitrogen, and oxygen have been introduced intentionally as alloying elements for the purpose of improving the mechanical properties through fine-scale precipitation. Previous efforts to locate the oxygen in the structure using APFIM were inconclusive, largely due to poor counting statistics. An energy-compensated three-dimensional position-sensitive atom probe (3DAP) has been used in the present study to search for oxygen in cavities, precipitates, and grain boundaries in this alloy. As a direct result of the much greater number of atoms detected and the three-dimensional imaging, oxygen concentrations on the order of 1 atomic% have been found both inside nitride precipitates and in the vicinity of grain boundaries. Boron was also found unexpectedly in concentrations of up to 2 atomic% inside nitride precipitates and in the vicinity of grain boundaries. These findings are important to developing an understanding of the grain growth resistance and precipitate stability both in this alloy and in this processing method in general.

The performance of the IMR three-dimensional atom probe

This paper reports the design and the performance of a three-dimensional atom probe which was recently constructed at the Institute for Materials Research, Tohoku University. The new atom probe consists of both a reflectron type time-of-flight atom probe (ToFAP) and a position sensitive atom probe (PoSAP). These are controlled with a CAMAC based controlling system interfaced with a Macintosh computer. The performance of the reflectron type atom probe and the position sensitive atom probe is demonstrated.

A study of the effect of ageing temperature on phase separation in Fe45%Cr alloys

Atom probe and position-sensitive atom probe microanalyses have been performed to examine the microstructures resulting from spinodal decomposition in a series of FeCr alloys thermally aged at 400°C. A comparison with microstructures from Fe45%Cr alloys aged at 500°C showed that the activation energy for diffusion was approximately 2.3 eV. The increase in the scale and composition of the α α′ domains was measured as a function of ageing time for both heat treatment temperatures. These parameters were also related to the increase in microhardness and development of interface structure between the domains. Simulations using the dynamic Ising model show that the interfaces between the α and α′ phases should be smoother at lower ageing temperatures for a given scale of decomposition. A comparison between the experimental results and simulations is discussed.