Images obtained from an analog SEM are traditionally viewed and recorded from a cathode-ray tube (CRT). Many laboratories use instant film (e.g. Polaroid #52, #55 instant film) to justify image quality and obtain permanent image quickly. Digital imaging provides an alternative approach for image acquisition and recording. One major advantage of digital SEM is image averaging that allows one to improve the signal-to-noise ratio (SNR) from a noisy quick-scan image to reduce charging. SEM signal yield is proportional to incident beam intensity, image acquisition time or duration of beam interaction with specimen (dwell time). The higher beam intensity, or longer the dwell time, the more signal generated. However, for high-resolution SEM imaging, the beam dose and dwell time are limited by drafting, radiation damage, and contamination. Therefore high-resolution biological SEM images invariably have poor SNR.

Modem technical imaging devices acquire digital image data at a precision level that often exceeds that of available display technologies so that parts of the data information may not be revealed in the displayed image. Data resolution and display resolution are closely related to image information and digital image processing provides several useful tools for compensation of such display deficiencies.

Typical precision data may be captured by a CCD camera, a scanning microscope or flatbed digitizer from photographic film and may exceed a size of 1,000×1,000 data points and a data point depth of 12 bit. Digital data has two general characteristics that deal with spatial resolution and intensity resolution. The distance represented between data points determines the spatial data resolution. Equally important is the bit-resolution which describes the maximum number of intensity levels possible in each point.

The Electron Microscope Unit at the University has a good selection of microscopes: two tungsten SEMs, two FESEMs, a microprobe analysis system, a 125 kY TEM, a 200 kV TEM, an AFM, an FIB miller, a Zeiss Photomicroscope and a Leica Macroscope. We service around 300 clients’ projects a year in every field of experimental science and engineering, logging over 8,000 hrs of beam time annually.

But funding constraints have always kept us short staffed and our laboratory has been working towards complete digital image capture for the past ten years to enhance our productivity. The perceived benefits of digitisation for us are:

• Photographic processing of negatives eliminated.

• Archiving of (bulky) photographic negatives eliminated.

• Need for special darkroom & graphics skills eliminated

• Responsibility for archiving and indexing images devolved to individual users

• Responsibility for image processing devolved to individual users.

• Rapid turnaround of images.

• Rapid sharing of results.

The Atom-Probe Field Ion Microscope was introduced in 1967 at the 14th Field Emission Symposium in Gaithersburg, Maryland. The Atom-Probe was, and remains, the only instrument capable of determining “the nature of one single atom seen on a metal surface and selected from neighboring atoms at the discretion of the observer”. The development of the Atom-Probe is a story that highlights Erwin Muller's strong and sometimes volatile personality. It is a story of an instrument that one NSF proposal reviewer called “impossible” because “single atoms could not be detected”. It is also the story of the Field Emission Laboratory at Penn State in the late 1960s and the contributions of two superb technicians, Gerald Fowler and Brooks McLane, and two graduate students, Douglas Barofsky and John Panitz. The anecdotes from this time are colorful and reflect Erwin's pedigree as Gustav Hertz's student in the Berlin of the 1930s.

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.

Today material science requires the use of increasingly powerful tools in material analysis. The last twenty years have witnessed the development of a number of analytical techniques. However, among these techniques, only a few allow observation and analysis of materials at the nanometer level ; one can mention scanning transmission electron microscopy coupled with electron energy loss spectrometry (STEM-EELS), SIMS and atom-probe techniques. In the STEM combined with electron energy loss spectrometry the spatial resolution can be better than one nanometer on a wide scanning area. However the in-depth spatial resolution is limited by the specimen thickness and therefore strongly depends on samples preparation. On the contrary, In SIMS techniques the in-depth spatial resolution can be as good as one nanometer but the lateral resolution is set by the ion beam diameter.

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.

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.