The analyzing area of a conventional atom probe is an apex of a long, sharp tip. However, the fabrication of such a filamentary tip is extraordinary difficult for most materials, such as organic materials, ceramics and semiconductors with multi-layer structures. Accordingly, the application of the atom probe are severely restricted In order to overcome this difficulty, Nishikawa proposed to develop a scanning atom probe (SAP) introducing a funnel-shaped microextraction electrode to the conventional AP in order to confine the high field into a small space between a tip apex and an open end of the electrode. Thus, the SAP allows to analyze not only an apex of a sharp slender tip but also an apex of a few micron high minute cusp which normally exists on an unsmoothened specimen surface.

The microextraction electrode scans over a rugged specimen surface at a bias voltage and stands still right above an apex of a micro cusp, FIG. 1.

The Need for Improvements. Though atom probe techniques have impressive atomic-scale analytical capabilities, there are some definite needs for improvement. The needle shape of APFIM specimens is a difficult geometry to make for many types of materials, especially technologically important thin-film materials. Even though 3DAPs collect more data than traditional APs, the analysis volumes are still small by analytical standards: typically there are fewer than 1 million atoms in an image which is about 15nm x 15nm x 50nm. The data collection rate (up to tens of atoms per second) limits the practical size of analyzed volumes to about 1 million atoms. Poor mass resolution makes it difficult to separate ions of similar mass-to-charge ratio in spectra of approximately 20% of samples. Thus, there are three principal areas where improvements are desirable: 1) specimen geometry, 2) data collection rates, and 3) mass resolving power.

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.

Due to environmental concerns, there is a trend to avoid the use of chemicals needed to develop negatives and to process photographic paper, and to use digital technologies instead. Digital technology also offers the advantages that it is convenient, as it enables quick access to the endresult, allows image storage and processing on computer, allows rapid hard copy output, and simplifies electronic publishing. Recently significant improvements have been made to the performance and cost of camera-sensors and printers. In this paper, field ion images recorded with two digital cameras of different resolution are compared to images recorded on standard 35 mm negative film. It should be noted that field ion images exhibit low light intensity and high contrast.

Field ion images were recorded from a standard microchannel plate and a phosphor screen and had acceptance angles of ∼60°.

Over the last 30 years, the atom probe has proved to be a powerful tool for studying nanometer-sized compositional fluctuations in a wide range of metallic alloys but has had only limited applications to semiconductors and ceramics. One of the primary reasons for this difference is the higher resistivity of semiconducting and ceramic specimens. Because of this high resistivity, the high voltage field evaporation pulse is attenuated before it reaches the apex of the specimen thereby making the pulse ineffective for field evaporation. Experiments have demonstrated that both variants of the voltagepulsed atom probe (i.e., those instruments in which the field evaporation pulse is applied directly to the specimen and those in which the negative pulse is applied to a counter electrode in front of the specimen) are equally affected. In this overview, the limits of applicability of the voltage-pulsed atom probe to high resistivity materials are examined.

Ni-Mo alloys containing at least 26 wt.% Mo have a negligible corrosion rate in boiling 10% hydrochloric acid and are therefore used in corrosive environments. A series of commercial Ni-Mo alloys has been developed with subtle variations in chemical composition. These alloys usually contain ∼28 wt.% Mo with additions of up to 5 % Fe and Cr. A significant amount of research has been performed to understand the microstructure and properties of these alloys, although most of the effort has concentrated on the Ni-Mo binary system. In some alloys with low Fe and Cr contents, a severe embrittlement problem has been observed due to the formation of the Ni4Mo (D1a-ordered) phase within the microstructure. The calculated section of the ternary Ni-Mo-Fe phase diagram in Fig. 1 illustrates that the addition of Fe stabilises Ni3Mo with respect to the brittle Ni4Mo phase.

Weldability of single-crystal and directionally solidified Ni-base alloys is important for repairwelding of failed components. During welding, phase decomposition and subsequent microstructure development may occur under non-equilibrium conditions. The final microstructure and phase composition of γ and γ' phases in these welds control the weld properties. Previous work used atom-probe field-ion microscopy (APFIM) for characterizing commercial single-crystal alloys. This work summarizes the microstructural development in PWA-1480 [Ni - 11.5 at. % Cr - 11.0% A1 - 5.1% Co - 1.3% W - 1.9% Ti - 4.0% Ta] electron beam (EB) welds and CMSX-4 [Ni - 7.5 at. % Cr - 12.7% A1 - 9.9% Co - 0.38% Mo -2.1% W - 1.3% Ti - 2.2% Ta - 0.95% Re - 0.034% Hf| pulsed laser (PL) welds.

In both PL and EB welds, a dendritic mode of weld solidification was observed.

Precipitation-hardened stainless steels are important structural materials in a variety of industrial applications in aircraft, chemical and power plants. These steels have a combination of good mechanical properties, corrosion resistance and simple fabrication process. Precipitation reaction in a PH 17-4 stainless steel which contains approximately 3 wt.% of Cu begins with formation of coherent copper-rich precipitate, which subsequently transform to incoherent fee Cu by further aging. By prolonged aging at 400 °C, embrittlement occurs as hardening progresses. Both the coarsening of Cu and the decomposition of the martensite matrix could be the reasons for this embrittlement. Thus, this study attempted to clarify the phase separation and the precipitation processes in a PH 17-4 stainless steel on prolonged aging by atom probe field ion microscopy (APFIM), in order to understand the mechanism of the degradation of mechanical properties by long term medium temperature aging.

Recently, Inoue et al. succeeded in fabricating ultrahigh-strength Al-based alloys consisting of a nanoscale mixture of α-Al and amorphous phases or a mixture of a-Al, amorphous and icosahedral phases in Al-TM-Ce, Al-TM-Ln (TM: transition metals) and Al-Cr-Co-Ce systems by rapid solidification [1-3]. In order to understand the mechanism of the nanoscale microstructural evolution during the rapid solidification processes in these nanocomposite alloys, we have characterized the microstructures of rapidly solidified Al94.5Cr3Co1.5Ce1 and Al96V4Fe2 alloys by atom probe field ion microscopy (APFIM) and high resolution transmission electron microscopy (HREM).

TEM investigations have revealed that the as-quenched Al94.5Cr3Co1.5Ce1 alloy is composed of a nanoscale mixture of amorphous and α-Al. A typical TEM bright field micrograph is shown in Fig. 1. The microdiffraction patterns taken at various locations in the darkly contrasted region have shown that the region consists of a few interconnected α-Al grains and many localized amorphous regions which are trapped within the Al grains.

Titanium aluminides have a number of potential high temperature applications due to their good elevated-temperature mechanical properties, low density, and good creep and oxidation resistance. However, fabrication of commercial components of these materials has been impeded by their poor mechanical properties at ambient temperatures. Significant efforts with various degrees of success have been made to improve the mechanical properties of these TiAl alloys by doping them with a variety of different elements including B, C, Cr, Er, Fe, Mn, Mo, Ni, Nb, P, Si, Ta, V and W. One of the optimum analytical tools for investigating the effects of these additions on the microstructure is the atom probe field ion microscope. However, relatively few studies of titanium aluminides, compared to some other intermetallic compounds, have performed by atom probe field ion microscopy. This lack of attention can be attributed to the brittle nature of the material, in-situ transformations that occur during the field ion microscopy and preferential evaporation problems that were encountered in some of the early studies.

Two phase γ-based TiAl alloys are attractive for structural applications at high temperatures because they possess good elevated-temperature mechanical properties, low density, and good creep and oxidation resistance. The microstructures of these alloys consist of plates of the near equiatomic γ phase (L10-ordered structure) and the Ti3Al α2 phase (D019-ordered structure). It is of great interest to study the details of the lamellar α2+γ microstructure because the interface stability is the key to providing a usable high temperature material.

Polysynthetically twinned (PST) TiAl crystals have been developed in order to systematically study the lamellar microstructure. These PST materials contain no high angle grain boundaries and have an single set of aligned lamellae of a α2 and γ phases, as shown in Fig. 1. Therefore, PST samples facilitate the study of the dependence of mechanical properties on lamellar structure by providing a known, consistent set of aligned lamellae.

Steels represent by far the most widely used class of engineering alloys, and comprise almost 95% of total world metal production. Yet despite their familiarity and ubiquitous distribution, many fundamental questions remain unanswered regarding the nature of the solid state chemical reactions occurring during the heat treatment of steels, the mechanisms by which alloy additions affect the kinetics of these processes, and the role of trace impurities on properties.

The difficulties involved in studying such processes experimentally arise from two main sources. Firstly, the effects involved are often on an extremely fine scale. Secondly, one of the key elements involved is carbon, which is extremely difficult to study by conventional microanalytical techniques.

Remarkable progress has been achieved in these areas through the application of the atom probe method. Much of this work has been reviewed in a recent monograph by Miller et al.

One of the advantages of Atom Probe Field Ion Microscopy (APFIM) as compared to other nanoanalytical instruments is its very high spatial resolution and its quantitativity in composition measurement. A few tenths of a nanometer at the specimen surface as well as an ultimate depth resolution of one atomic resolution can routinely be attained. Among the numerous improvements brought about since its early developments in the seventies, the advent of three-dimensional atomprobes (see papers of G.D.W Smith and A. Cerezo, this issue) has been probably one of the most important breakthroughs, the Tomographic Atom Probe (TAP) being the French prototype [1]. As shown in a recent publication, this instrument is able to map out both composition and order 3D fields on a scale close to that of the unit cell [2], In this contribution, we shall focus on the application of 1D and 3D atom-probes to the study of nickel base superalloys.

Atom probe field ion microscopy is the most suitable technique for determining local chemical composition changes during nanocrystallization processes of amorphous alloys. In this talk, we report atom probe field ion microscopy (APFIM) and high resolution electron microscopy (HREM) studies on nanocrystallization processes in Fe-Si-B-Nb-Cu soft magnet and Fe-Nd-B-Co-Ga exchange spring magnet.

Fe or Co based alloys with nanocrystalline microstructure show excellent permeability because the net magnetocrystalline anisotropy is significantly reduced when the grain size becomes smaller than the ferromagnetic exchange length. Fe-Si-B-Nb-Cu alloy is the pioneering nanocrystalline soft magnetic material invented by Yoshizawa et al. in 1988 [1]. Our previous works [2,3] reported evidence for clustering of Cu prior to the onset of the crystallization reaction. However, in the previous studies, it was not confirmed that these Cu clusters work as heterogeneous nucleation sites for a-Fe primary crystals.

The ability to locate an individual atom on a surface, remove it in a controlled fashion, and determine its chemical identity makes the atom-probe field-ion microscope an extremely powerful tool for the analysis of solid surfaces. By itself, the field ion microscope has contributed significantly to our understanding of surface atomic structure, single-atom surface diffusion, and the detailed interactions that occur between atoms and defects on surfaces.1 When used in combination with the atom-probe mass spectrometer there have been several additional areas within the traditional definition of "surface science" where the chemical identification capability of the atom probe has led to new insights. In this paper these applications are reviewed focusing on two specific areas: surface segregation in intermetallic alloys and chemical reactions on metal surfaces.

The equilibrium distribution of component species in the near surface region of solid solution alloy may be different from the distribution in the bulk.

Control and optimization of materials’ properties will increasingly depend on the ability to tailor both the structure and chemical composition of the materials at the nanoscale level (100 nm and less). One specific class of “nanostructured” materials is the multilayer film (MLF) structure, which is formed by alternate deposition of two or more different elements or compounds and has a wide range of applications in, for example, the field of data recording and storage. In order to understand MLF structures well enough to improve and optimize their properties, characterization at the atomic level with regard to both structure and chemistry is essential. Interface quality is one of the most important parameters due to the possibility of intermixing between layers. It is thus desirable to obtain information about the interface morphology in these materials at the highest possible resolution.

The distribution of small amounts of alloying elements or impurities to interfaces can have a major influence on the properties of a material and the atom probe field ion microscope technique is well suited for the study of such segregation effects, where structural and compositional changes often take place over regions only a few nanometers wide.

Many features of the atom probe technique make it very useful in studies of interfaces:

1. i) high spatial resolution, about lnm laterally and 0.2 nm in depth

2. ii) equal sensitivity for all elements, including the lightest

3. iii) no specimen contamination, surface artifacts from the specimen preparation are removed by field evaporation,

4. iv) low temperature operation, negligible diffusion during analysis

A ncessary prerequisite for any systematic analysis is the ability to prepare suitable specimens. The volume accessible for analysis by APFIM is orders of magnitude smaller than a typical grain size, so a specimen preparation technique was developed that could bring features of interest sufficiently close to the apex (see Fig. la).

Trace additions of Ag and Mg in Al-Cu and Al-Li-Cu alloys change the precipitation processes drastically. In Al-Cu-Mg-Ag alloy, plate-like Ω phase precipitates on the {111} matrix planes. Similarly, additions of Ag and Mg to Al-Li-Cu alloy cause uniform dispersion of the T1 precipitate on the {111} matrix planes. Ω and T1 phases are structurally and morphologically similar, and a similar mechanism is expected to work in both alloy systems to enhance uniform dispersion of the plate-like precipitates on the {111} planes. A previous atom probe work [1] reported that Ag and Mg are segregated at the αΩ. interfaces in an Al-Cu-Mg-Ag alloy. In an Al-Li-Cu-Mg-Ag alloy, Ag and Mg atoms were found to be incorporated with the T, phase [2]; however, because of the thinness of the T1, plate, the exact location of these atoms with respect to the T1, phase could not be determined sucessfully by a conventional atom probe.

Segregation of boron, carbon and nitrogen to grain boundaries in a nickel based model alloy has been investigated using atom probe field ion microscopy (APFIM). The material corresponds to a commercial alloy (Inconel 600), but contains lower levels of alloy additives and impurities. The major composition was Ni-16Cr-10Fe (wt.%). The alloy was solution annealed at 950°C for 10 min, which resulted in a grain size of 20 μ. Subsequently heat treatments for 1 h at temperatures of 550°C, 600°C and 700°C were applied. TEM investigation showed that the heat treatment at 700°C resulted in precipitation of intergranular chromium-rich carbides. The other temperatures were obviously too low and the aging times too short to cause precipitation, since carbides were not observed in the materials heat treated at 550°C and 600°C.

As the grain size was about 100 times larger than the accessible depth of APFIM analysis (≃200 nm), much care had to be taken in preparing samples containing a grain boundary close to the tip apex.

The last several decades have seen an explosion in the number of experimental techniques being developed to characterize materials. In particular, there has been exponential growth in the coupling of microscopic methods with conventional spectroscopic characterization tools. This has been due mainly to the renaissance in microscopy over the last quarter century and the advent of significant computational power “on a bench”.

Compositional imaging is just a new term for what was over a half century ago called “Chemical Microscopy”. Although then the term referred to optical microscopy, today the notion of compositional imaging refers to a method by which we can correlate chemical or elemental composition with a microscopic image feature.

Because many of these microcharacterization methods have developed from many different disciplines, the “local” language used often appears to be untranslatable from one technique to the next without considerable pain and difficulty.