Phase decomposition and ordering in Ni-11.3 at.% Ti studied with atom probe tomography

The decomposition behavior of Ni-rich Ni–Ti was reassessed using Tomographic Atom Probe (TAP) and Laser Assisted Wide Angle Tomographic Atom Probe. Single crystalline specimens of Ni-11.3 at.% Ti were investigated, the states selected from the decomposition path were the metastable γ″ and γ′ states introduced on the basis of small-angle neutron scattering (SANS) and the two-phase model for evaluation. The composition values of the precipitates in these states could not be confirmed by APT data as the interface of the ordered precipitates may not be neglected. The present results rather suggest to apply a three-phase model for the interpretation of SANS measurements, in which the width of the interface remains nearly unchanged and the L1₂ structure close to 3:1 stoichiometry is maintained in the core of the precipitates from the γ″ to the γ′ state.     

Three-dimensional atomic scale analysis of nanostructured materials

The 3-dimensional atom probe (3DAP) has been used to provide atomic-scale microcharacterisation of a number of nanostructured materials. Grain boundary segregation has been investigated in electrodeposited nanocrystalline nickel and Ni-P. In the nanocrystalline nickel, there was no observable grain boundary segregation in the as-deposited condition. After annealing, carbon and sulphur contamination was found at the boundary of an abnormally-grown grain. In the as-deposited Ni-P alloy, only limited grain boundary segregation of P is seen, but annealing produces significant segregation and the formation of Ni₃P precipitates at grain boundaries. The phase chemistry in a melt-spun amorphous Fe-Si-Cu-Nb-B-Al (FINEMET-type) alloy has also been studied, and the hetereogeneous nucleation of Fe-Si nanocrystals at Cu precipitates shown conclusively. It is found that at early stages of crystallisation, there is only limited partitioning of the Si between the nanocrystals and the amorphous matrix. Atom probe studies of thin layered films have historically been limited by specimen preparation problems, but recent advances have now yielded data on metallic multilayer films. This has allowed atomic-scale measurements of interface chemistry in these films for the first time.     

Contributions of atom probe tomography to the understanding of nickel-based superalloys

Atom probe tomography enables atomic level microstructural characterization to be performed on complex engineering materials such as superalloys. The technique provides information on the size, morphology and compositions of coexisting phases, the solute partitioning of the elements between the phases, and solute segregation to interfaces and grain boundaries. This information leads to a more complete understanding of nickel-based superalloys. The types of atomic level information that may be obtained with atom probe tomography are illustrated with examples of the formation of fine γ precipitates within the central region of the γ′ phase in PW 1480, the evolution of the dual γ′/γ″ nature of secondary precipitates in alloy 718, the interphase precipitation of the γ′ phase at the primary γ″–γ interface in alloy 718, and the quantification of the level and spatial extent of the boron segregation at grain boundaries in a nickel–molybdenum superalloy.     

Influence of field evaporation on Radial Distribution Functions in Atom Probe Tomography

This paper examines the extraction of structural information in the form of Radial Distribution Functions (RDFs) using Atom Probe Tomography (APT) data. These functions are generated in a highly efficient manner, thus allowing for the analysis of large data sets typical of APT. Experimental RDF calculations were performed for crystalline aluminium and a Mg₆₅Cu₂₅Y₁₀ bulk metallic glass. For the pure aluminium sample, significant pair distance information was extracted, the quality of which was found to vary throughout the data set. Through a novel analysis procedure, the measured total RDF was used to map the local pair distance quality about each reconstructed atom. Surprisingly, the RDF quality maps indicated improved pair distance quality around poles and zone lines. In the case of the metallic glass, however, significant pair correlations were not discernible within the data set, despite short-range ordering being observed using TEM diffraction. The lack of correlations is thought to be associated with a non-uniform ion desorption sequence, as observed in this study. This affects the uniform evaporation assumption that is implicit in current 3D APT reconstruction procedures.     

Size distribution and volume fraction of T1 phase precipitates from TEM images: Direct measurements and related correction

Transmission Electron Microscopy (TEM) can be used to measure the size distribution and volume fraction of fine scale precipitates in metallic systems. However, such measurements suffer from a number of artefacts that need to be accounted for, related to the finite thickness of the TEM foil and to the projected observation in two dimensions of the microstructure. We present a correction procedure to describe the 3D distribution of disc-like particles and apply this method to the plate-like T₁ precipitates in an Al–Li–Cu alloy in two ageing conditions showing different particle morphologies. The precipitates were imaged in a High-Angular Annular Dark Field Microscope (HAADF-STEM). The corrected size distribution is further used to determine the precipitate volume fraction. Atom probe tomography (APT) is finally utilised as an alternative way to measure the precipitate volume fraction and test the validity of the electron microscopy results.     

A study of threshold switching of NbO2 using atom probe tomography and transmission electron microscopy

Threshold switching is a phenomenon where the resistivity of an insulating material changes and the insulator exhibits metallic behavior. This could be explained by phase transformation in oxide materials; however, this behavior is also seen in amorphous insulators. In this study, through an ex-situ experiment using transmission electron microscopy (TEM), we proved that threshold switching of amorphous NbO₂ accompanies local crystallization. The change in I–V characteristics after electroforming was examined by evaluating the concentration profile. Atom probe tomography (APT) combined with in-situ TEM probing technique was performed to understand the threshold switching in amorphous NbO₂. The local crystallization in amorphous NbO₂ was validated by the observed difference in time-of-flight (ToF) between amorphous and crystalline NbO₂. We concluded that the slower ToF of amorphous NbO₂ (a-NbO₂) compared with crystalline NbO₂ (c-NbO₂) is due to the resistivity difference and trap-assisted recombination.     

The characterisation of silicon surfaces by time-of-flight atom probe analysis

New results are presented on the analysis of silicon surfaces in a Time-of-Flight Atom Probe. Comparison is made between the chemical data obtained from conventional voltage pulse and laser pulse desorption.     

Using atom interferometry to search for new forces

Atom interferometry is a rapidly advancing field and this Letter proposes an experiment based on existing technology that can search for new short distance forces. With current technology it is possible to improve the sensitivity by up to a factor of 10² and near-future advances may be able to rewrite the limits for forces with ranges from 1 mm to 100 m.     

Microtraps and Atom Chips: Toolboxes for Cold Atom Physics

Magnetic microtraps and Atom Chips are safe, small-scale, reliable and flexible tools to prepare ultra-cold and degenerate atom clouds as sources for various atom-optical experiments. We present an overview of the possibilities of the devices and indicate how a microtrap can be used to prepare and launch a Bose-Einstein condensate for use in an atom clock or an interferometer.     

Correlated field evaporation as seen by atom probe tomography

We have studied the correlation between field evaporated atoms during an atom probe tomography experiment. The evaporated atoms have been shown to be highly correlated both in time and in space. This correlation can be explained by the dynamic distribution of the electric field at the surface of the sample. The evaporation sequence follows the distribution of zones where the electric field intensity is higher. The distance of correlation between successively evaporated atoms corresponds to the mean size of these zones and the time of correlation corresponds to the mean time necessary to evaporate the entire zone.     

Atom probe tomography of 15Kh2MFA Cr–Mo–V steel surveillance specimens

An atom probe study has been performed on 15Kh2MFA base and 10KhMFT weld metal surveillance specimens from a VVER-440/213C reactor to investigate the mechanisms that produce embrittlement in low copper materials during service. The composition of the base metal was Fe-0.06 at.% Cu, 3.1% Cr, 0.34% V, 0.46% Mn, 0.35% Mo, 0.07% Ni, 0.34% Si, 0.74% C, 0.025% P, and 0.028% S. The base material was characterized after thermal aging for 10 years at 295°C and after neutron irradiation at 270°C for 10 years to a fluence of 1.0×10²⁵ m⁻² (E>0.5 MeV). The ductile-to-brittle transition temperatures (DBTT) of the base metal were −49, −70 and 141°C, for the unirradiated, thermally aged and neutron irradiated conditions, respectively. The composition of the weld metal was Fe-0.05 at.% Cu, 1.46% Cr, 0.22% V, 1.11% Mn, 0.29% Mo, 1.17% Si, 0.17% C, 0.02% P, and 0.029% S. The weld material was characterized after tempering for 18 h at 690°C plus a simulated stress relief treatment of 43.5 h at 680°C, after thermal aging for 5 years at 295°C, and after neutron irradiation at 275°C for 5 years to a fluence of 5.2×10²⁴ m² (E>0.5 MeV). The DBTTs were 7, 11 and 123°C, respectively, for these three conditions. A high number density of ultrafine manganese- and silicon-enriched regions was observed in both neutron-irradiated materials. Phosphorus segregation was observed at the VC-matrix interface and at grain boundaries.     

Atom probe tomography analysis of radiation-induced solute clustering in austenite stainless steels

Various types of defects are produced by the irradiation of energetic particles onto a structural material. The large number of mobile vacancies and self-interstitial atoms during irradiation induce defect fluxes and the diffusion of solute atoms in the matrix. The preferential interaction between the solute atoms and radiation-induced defects leads to the enrichment/depletion or clustering of the solutes at defect sinks. In the current work, atom probe tomography (APT) was used for the analysis of radiation-induced solute clustering in ion-irradiated austenite stainless steel 316. Quantitative analysis of the localised clustering of chemical elements was implemented and a parameter selection procedure was proposed. The number density and size distribution of Si clusters in APT specimens irradiated at various temperatures were examined. At high temperature, the number density of the clusters decreased and their size increased. The localized Si atoms in variously shaped defects were clearly identified. The APT method was demonstrated to be suitable for identifying defect structures and for the quantitative analysis of clustering in irradiated specimens.     

Correlated fluctuations of electron populations in high temperature plasmas

This paper presents a model for a simple treatment of the fluctuations of electron bound states populations in equilibrium plasmas at high temperature. Fluctuations in the various states are correlated by matrix elements of the Coulomb interaction. The model imposes to recover the average occupation numbers given by usual Average Atom (AA) calculations. The additional numerical work consists only in a matrix inversion. Examples of application are shown (calculation of correlation factors, fluctuations of the total bound charges of the ions), and comparison with other published results is made.     

离散效应对高强度激光三次谐波转换的影响

使用分步傅里叶变换和四阶龙格库塔法(R-K) ，对高强度激光以Ⅰ/Ⅱ类角度失谐方式，在KDP晶体中的谐波转换进行了研究，详细讨论了离散效应对三次谐波转换的影响。结果表明，离散效应不但降低了三次谐波转换效率，而且使谐波光束质量显著降低；初始入射基频光束腰半径较小时，离散效应是二次谐波转换和三次谐波转换效率降低的主要因素，失谐角对三次谐波转换的影响较小；随着束腰半径的增加，离散效应的影响逐渐减小，失谐角对三次谐波转换的影响逐渐增加。     

一种可用于原子全息光刻的二元计算全息新方法

 原子全息光刻即采用二元计算全息片掩模来操纵原子,实现微细结构的制作。传统二元计算全息产生的全息片在重现时会产生不止一个实像,这对于原子全息光刻的操作是不利的。提出了一种非相位编码的方法,该方法利用基元函数叠加方式产生实的编码前全息图,再利用类似罗曼Ⅲ型的编码方式产生二元计算全息图。模拟结果表明,利用该方法产生的掩模板可以产生单一的同原始图案相对应的微细结构。     

Electron trapping properties at HfO_2/SiO_2 interface,studied by Kelvin probe force microscopy and theoretical analysis

Electron trapping properties at the HfO_2/SiO_2 interface have been measured through Kelvin Probe force microscopy,between room temperature and 90 ℃.The electron diffusion in HfO_2 shows a multiple-step process.After injection,electrons diffuse quickly toward the HfO_2/SiO_2 interface and then diffuse laterally near the interface in two sub-steps:The first is a fast diffusion through shallow trap centers and the second is a slow diffusion through deep trap centers.Evolution of contact potential difference profile in the fast lateral diffusion sub-step was simulated by solving a diffusion equation with a term describing the charge loss.In this way,the diffusion coefficient and the average life time at different temperatures were extracted.A value of 0.57 eV was calculated for the activation energy of the shallow trap centers in HfO_2.     

用原子芯片操纵中性原子

原子芯片提供了一个稳定、精确且功能强大的实验平台来制备和操纵中性超冷原子。本文概述了近年来原子芯片的研究发展状况,并介绍了原子芯片上微势阱的设计原理以及几个典型的原子芯片实验,然后讨论了芯片实验中的原子损失、加热和退相干机制,最后对原子芯片可能的发展方向进行了预测。     

核反应堆压力容器模拟钢中富Cu原子团簇的析出与嵌入原子势计算

利用原子探针层析技术(APT)和热处理时效方法,研究了合金元素Ni对核反应堆压力容器模拟钢中富Cu原子团簇析出的影响.实验结果表明,添加合金元素Ni(0.84wt%)的样品中析出富Cu原子团簇的数量密度高于不添加Ni的样品,富Cu原子团簇内以及团簇和基体界面处都有Ni元素的富集现象,这说明合金元素Ni会促使富Cu原子团簇的析出.从多体势的角度出发,利用嵌入原子势理论,基于纯金属元素Fe,Cu,Ni的多体势参数,建立了Fe-Cu二元和Fe-Cu-Ni三元体系的嵌入原子多体势.计算结果表明,当模拟合金中存在1at%Ni时有利于富Cu原子团簇的析出,这与实验结果相符.     

A three-dimensional Markov field approach for the analysis of atomic clustering in atom probe data

Solute clustering is increasingly recognised as a significant characteristic within certain material systems that can be tailored to the optimization of bulk properties and performance. Atom probe tomography (APT) is emerging as a powerful tool for the detection of these nanoscale features; however, complementary to experiment, precise and efficient characterization algorithms are required to identify and characterise these nanoclusters within the potentially massive three-dimensional atomistic APT datasets. In this study, a new three-dimensional Markov field (3DMF) cluster identification algorithm is proposed. The algorithm is based upon an analysis of the direct atomic neighbourhood surrounding each atom, and the only input parameter required utilises known crystallographic properties of the system. Further, an array of statistical approaches has been developed and applied with respect to the results generated by the 3DMF algorithm including: an S _N statistic, a two-tailed z-test, a difference measure, the χ² test, and a direct evaluation of the Warren–Cowley parameter for short-range ordering. Finally, the methodologies have been applied to the characterization of the nanostructural evolution of an Al-1.1Cu-0.5Mg (at.%) alloy subjected to a variety of heat treatments.     

用原子芯片操纵中性原子

原子芯片提供了一个稳定、精确且功能强大的实验平台来制备和操纵中性超冷原子。本文概述了近年来原子芯片的研究发展状况，并介绍了原子芯片上微势阱的设计原理以及几个典型的原子芯片实验，然后讨论了芯片实验中的原子损失、加热和退相干机制，最后对原子芯片可能的发展方向进行了预测。