Structural integrity assessment of reactor pressure vessels during pressurized thermal shock

A comparative assessment study is performed for the deterministic fracture mechanics approach of the pressurized thermal shock of a reactor pressure vessel. Round robin problems consisting of two transients and two defects are solved. Their results are compared to suggest some recommendations of best practices and to assure an understanding of the key parameters of this type of approach, which will be helpful not only for the benchmark calculations and results comparisons but also as a part of the knowledge management for the future generation. Seven participants from five organizations solved the problem and their results are compiled in this study.     

Atom probe tomography and transmission electron microscopy characterisation of precipitation in an Al-Cu-Li-Mg-Ag alloy

State-of-the art atom probe tomography (APT) combined with transmission electron microscopy (TEM) were used to investigate the microstructure at different stages of the ageing process of an alloy of composition (at%) Al-1.68%Cu-4.62%Li-0.33%Mg-0.1%Ag. These alloys were shown to exhibit a complex microstructure of T₁ plates and several metastable phases, including θ′ and S. We will highlight the early stages of clustering, precipitate interactions and possible solute segregation at the matrix/precipitate interfaces and detail the chemical composition of the different phases.     

Advances in the reconstruction of atom probe tomography data

Key to the integrity of atom probe microanalysis, the tomographic reconstruction is built atom by atom following a simplistic protocol established for previous generations of instruments. In this paper, after a short review of the main reconstruction protocols, we describe recent improvements originating from the use of exact formulae enabling significant reduction of spatial distortions, especially near the edges of the reconstruction. We also show how predictive values for the reconstruction parameters can be derived from electrostatic simulations, and finally introduce parameters varying throughout the analysis.     

The influence of voxel size on atom probe tomography data

A methodology for determining the optimal voxel size for phase thresholding in nanostructured materials was developed using an atom simulator and a model system of a fixed two-phase composition and volume fraction. The voxel size range was banded by the atom count within each voxel. Some voxel edge lengths were found to be too large, resulting in an averaging of compositional fluctuations; others were too small with concomitant decreases in the signal-to-noise ratio for phase identification. The simulated methodology was then applied to the more complex experimentally determined data set collected from a (Co_0.95Fe_0.05)₈₈Zr₆Hf₁B₄Cu₁ two-phase nanocomposite alloy to validate the approach. In this alloy, Zr and Hf segregated to an intergranular amorphous phase while Fe preferentially segregated to a crystalline phase during the isothermal annealing step that promoted primary crystallization. The atom probe data analysis of the volume fraction was compared to transmission electron microscopy (TEM) dark-field imaging analysis and a lever rule analysis of the volume fraction within the amorphous and crystalline phases of the ribbon.     

Atom probe tomography and transmission electron microscopy of a Mg-doped AlGaN/GaN superlattice

The electronic characteristics of semiconductor-based devices are greatly affected by the local dopant atom distribution. In Mg-doped GaN, the clustering of dopants at structural defects has been widely reported, and can significantly affect p-type conductivity. We have studied a Mg-doped AlGaN/GaN superlattice using transmission electron microscopy (TEM) and atom probe tomography (APT). Pyramidal inversion domains were observed in the TEM and the compositional variations of the dopant atoms associated with those defects have been studied using APT. Rarely has APT been used to assess the compositional variations present due to structural defects in semiconductors. Here, TEM and APT are used in a complementary fashion, and the strengths and weaknesses of the two techniques are compared.     

Multivariate statistical analysis of atom probe tomography data

The application of spectrum imaging multivariate statistical analysis methods, specifically principal component analysis (PCA), to atom probe tomography (APT) data has been investigated. The mathematical method of analysis is described and the results for two example datasets are analyzed and presented. The first dataset is from the analysis of a PM 2000 Fe–Cr–Al–Ti steel containing two different ultrafine precipitate populations. PCA properly describes the matrix and precipitate phases in a simple and intuitive manner. A second APT example is from the analysis of an irradiated reactor pressure vessel steel. Fine, nm-scale Cu-enriched precipitates having a core-shell structure were identified and qualitatively described by PCA. Advantages, disadvantages, and future prospects for implementing these data analysis methodologies for APT datasets, particularly with regard to quantitative analysis, are also discussed.     

Quantitative atom probe analysis of carbides

Compared to atom probe analysis of metallic materials, the analysis of carbide phases results in an enhanced formation of molecular ions and multiple events. In addition, many multiple events appear to consist of two or more ions originating from adjacent sites in the material. Due to limitations of the ion detectors measurements generally underestimate the carbon concentration. Analyses using laser-pulsed atom probe tomography have been performed on SiC, WC, Ti(C,N) and Ti₂AlC grains in different materials as well as on large M₂₃C₆ precipitates in steel. Using standard evaluation methods, the obtained carbon concentration was 6-24% lower than expected from the known stoichiometry. The results improved remarkably by using only the ¹³C isotope, and calculating the concentration of ¹²C from the natural isotope abundance. This confirms that the main reason for obtaining a too low carbon concentration is the dead time of the detector, mainly affecting carbon since it is more frequently evaporated as multiple ions. In the case of Ti(C,N) and Ti₂AlC an additional difficulty arises from the overlap between C₂⁺, C₄²⁺ and Ti²⁺ at the mass-to-charge 24 Da.     

Atom probe analysis of early-stage strengthening behaviour in an Al-Mg-Si-Cu alloy

The strengthening of an Al-Mg-Si-Cu alloy during natural ageing and subsequent short artificial ageing was investigated using three-dimensional atom probe (3DAP) analysis and tensile testing. The contingency table and Markov chain analyses confirmed that non-random arrangements of atoms already exist after a natural ageing time of only 3.5 h. Extensive use of particle analysis tools in the IVAS and PoSAP software packages revealed that whilst the commonly used minimum aggregate size (N_min) of 10 is a reasonable choice, much more useful information about the system can be gained by additionally employing a wide range of larger and smaller N_min values. In particular, it was found that the density and volume fraction of solute aggregates increased with increasing natural ageing time in the T4 condition. After a 0.5 h artificial ageing treatment at 170 °C (designated as T6), the size, volume fraction and Mg/Si ratio of the aggregates were all found to decrease with increasing prior natural ageing time. These findings are used to discuss the detrimental effect of natural ageing, where the T6 strength has been observed to decrease rapidly with increasing prior natural ageing time before stabilising after several hours of natural ageing.     

Atom probe crystallography: characterization of grain boundary orientation relationships in nanocrystalline aluminium

Spatial Distribution Maps (SDM) in their various forms have previously been used to identify and characterize crystallographic structure within APT reconstructions. Importantly, it has been shown that such SDM analyses can also provide the crystallographic orientation of the specimen with respect to the direction of the detector in the original experiment. In this study, we investigate the application of SDMs to the analysis of APT reconstruction of a nanocrystalline Al film. We demonstrate that significant intra-granular crystallographic information is retained in the reconstruction, even in the x-y plane perpendicular to the direction of the detector. Further, the crystallographic orientation of the grains can be characterized highly accurately not only with respect to the bulk specimen but also their misorientation with respect to neighbouring grains.     

Atom probe tomography characterization of heavily cold drawn pearlitic steel wire

Atom Probe Tomography (APT) was used to analyze the carbon distribution in a heavily cold drawn pearlitic steel wire with a true strain of 6.02. The carbon concentrations in cementite and ferrite were separately measured by a sub-volume method and compared with the literature data. It is found that the carbon concentration in ferrite saturates with strain. The carbon concentration in cementite decreases with the lamellar thickness, while the carbon atoms segregate at dislocations or cell/grain boundaries in ferrite. The mechanism of cementite decomposition is discussed in terms of the evolution of dislocation structure during severe plastic deformation.     

Optimisation of specimen temperature and pulse fraction in atom probe microscopy experiments on a microalloyed steel

This paper details the effects of systematic changes to the experimental parameters for atom probe microscopy of microalloyed steels. We have used assessments of the signal-to-noise ratio (SNR), compositional measurements and field desorption images to establish the optimal instrumental parameters. These corresponded to probing at the lowest possible temperature (down to 20 K) with the highest possible pulse fraction (up to 30%). A steel containing a fine dispersion of solute atom clusters was used as an archetype to demonstrate the importance of running the atom probe at optimum conditions.     

Atomic scale investigation on the distribution of boron in medium carbon steels by atom probe tomography and EELS

The medium carbon (0.5 wt% C) steels containing various boron contents were studied to observe the distribution of boron using atom probe tomography and electron energy loss spectroscopy. APT revealed the segregation of boron atoms at retained austenite for 100 ppm boron added steels and the trapped carbon atoms at micro-twins for 50 ppm boron treated steels. Moreover, it was also found that boron was randomly distributed for 20 ppm boron added steels regardless of the interactions between carbon and boron.     

Evaporation mechanisms of MgO in laser assisted atom probe tomography

In this paper the field evaporation properties of bulk MgO and sandwiched MgO layers in Fe are compared using laser assisted Atom Probe Tomography. The comparison of flight time spectra gives an estimate of the evaporation times as a function of the wavelength and the laser energy. It is shown that the evaporation takes place in two steps on two different time scales in MgO. It is also shown that as long as the MgO layer is buried in Fe, the evaporation is dominated by the photon absorption in Fe layer at the tip apex. Eventually the evaporation process of MgO is discussed based on the difference between the bulk materials and the multilayer samples.     

Effect of analysis direction on the measurement of interfacial mixing in thin metal layers with atom probe tomography

The accuracy and precision of thin-film interfacial mixing as measured with atom probe tomography (APT) are assessed by considering experimental and simulated field-evaporation of a Co/Cu/Co multilayer structure. Reconstructions were performed using constant shank angle and Z-scale reordering algorithms. Reconstruction of simulated data (zero intermixing) results in a 10-90% intermixing width of ∼0.2 nm while experiential intermixing (measured from multiple runs) was 0.47±0.19 and 0.49±0.10 nm for Co-on-Cu and Cu-on-Co interfaces, respectively. The experimental data were collected in analysis orientations both parallel and anti-parallel to film growth direction and the impact of this on the interfacial mixing measurements is discussed. It is proposed that the resolution of such APT measurements is limited by the combination of specimen shape and reconstruction algorithms rather than by an inherent instrumentation limit.     

Quantitative comparison of energy-filtering transmission electron microscopy and atom probe tomography

Whereas transmission electron microscopy (TEM) is a well established method for the analysis of thin film structures down to the sub-nanometer scale, atom probe tomography (APT) is less known in the microscopy community. In the present work, local chemical analysis of sputtered Fe/Cr multilayer structures was performed with energy-filtering transmission electron microscopy (EFTEM) and APT. The single-layer thickness was varied from 1 to 6 nm in order to quantify spatial resolution and chemical sensitivity. While both the methods are able to resolve the layer structure, even at 2 nm thickness, it is demonstrated that the spatial resolution of the APT is about a factor of two, higher in comparison with the unprocessed EFTEM data. By calculating the influence of the instrumental parameters on EFTEM images of model structures, remaining interface roughness is indicated to be the most important factor that limits the practical resolution of analytical TEM.     

Thermal stability of TiAlN/CrN multilayer coatings studied by atom probe tomography

This study is about the microstructural evolution of TiAlN/CrN multilayers (with a Ti:Al ratio of 0.75:0.25 and average bilayer period of 9 nm) upon thermal treatment. Pulsed laser atom probe analyses were performed in conjunction with transmission electron microscopy and X-ray diffraction. The layers are found to be thermally stable up to 600 °C. At 700 °C TiAlN layers begin to decompose into Ti- and Al-rich nitride layers in the out-of-plane direction. Further increase in temperature to 1000 °C leads to a strong decomposition of the multilayer structure as well as grain coarsening. Layer dissolution and grain coarsening appear to begin at the surface. Domains of AlN and TiCrN larger than 100 nm are found, together with smaller nano-sized AlN precipitates within the TiCrN matrix. Fe and V impurities are detected in the multilayers as well, which diffuse from the steel substrate into the coating along columnar grain boundaries.     

Atom probe tomography investigation of assisted precipitation of secondary hardening carbides in a medium carbon martensitic steels

A medium carbon martensitic steel containing nanometer scale secondary hardening carbides and intermetallic particles is investigated by field ion microscopy and atom probe tomography. The interaction between the concomitant precipitations of both types of particles is investigated. It is shown that the presence of the intermetallic phase affects the nucleation mechanism and the spatial distribution of the secondary hardening carbides, which shifts from heterogeneous on dislocations to heterogeneous on the intermetallic particles.     

Analysis of clustering in Al-Mg-Si alloy by density spectrum analysis of atom probe data

Early stages of cluster formation in an Al-Si-Mg alloy were investigated by atom probe tomography and evaluated by a newly developed statistical method based on the nearest neighbour distributions. After solutionising and quenching, an alloy sample was naturally aged for one week. The atom probe data then measured was analysed for Mg, Si or Mg-Si clusters. For comparison specimen artificial aged with well developed precipitates was also investigated. A general approach for the analysis of density spectra was set up, which reduced the problem to the solution of an integral equation. Application of the method to the atom probe data set allowed us to detect clusters and to evaluate the atomic fractions within these clusters. This is also possible for an arbitrary number of nucleated phases. The higher-order next nearest neighbour distributions were used for the estimation of cluster sizes. Combining the density distribution method with a Monte Carlo simulation we found very small Si-Si and Mg-Mg clusters consisting of only a few atoms in the naturally aged state.     

反应堆压力容器接管安全端焊接工艺研究

介绍一套经焊接工艺评定及实际产品制造验证的压力容器接管与安全端不锈钢焊接工艺方法。     

Correlated ion analysis and the interpretation of atom probe mass spectra

Several techniques are presented for extracting information from atom probe mass spectra by investigating correlations within multiple-ion detector events. Analyses of this kind can provide insights into the origins of noise, the shape of mass peaks, or unexpected anomalies within the spectrum. Data can often be recovered from within the spectrum noise by considering the time-of-flight differences between ions within a multiple event. Correlated ion detection, particularly when associated with shifts in ion energies, may be used to probe the phenomenon of molecular ion dissociation, including the questions of data loss due to ion pile-up or the generation of neutrals in the dissociation process.