This paper presents an overview of duplex stainless steels (DSS) with particular emphasis on super DSS, i.e. steels containing sufficient amounts of chromium, molybdenum, and nitrogen to produce a pitting resistance equivalent greater than 40. Duplex stainless steels have an attractive combination of mechanical and corrosion properties and are thus suitable for many marine and petrochemical applications, particularly where chlorides are present. The paper covers the following aspects of DSS: physical metallurgy, mechanical properties, corrosion properties, metallurgy of welding, machinability, and applications. A large number of references to the literature are given to aid the reader who is interested in acquiring a deeper understanding of the behaviour of this family of steels.MST/1685

Super duplex stainless steels

1. Historical background and general introduction 2. Physical principles of field ion microscopy 3. FIM image interpretation and application 4. Physical principles of atom probe interpretation 5. Statistical analysis of atom probe data 6. Metallurgical applications 7. Atom probe studies of non-metallic materials, thin films and surface phenomena Epilogue: future directions

Atom Probe Field Ion Microscopy

The technique of atom probe tomography (APT) is reviewed with an emphasis on illustrating what is possible with the technique both now and in the future. APT delivers the highest spatial resolution (sub-0.3-nm) three-dimensional compositional information of any microscopy technique. Recently, APT has changed dramatically with new hardware configurations that greatly simplify the technique and improve the rate of data acquisition. In addition, new methods have been developed to fabricate suitable specimens from new classes of materials. Applications of APT have expanded from structural metals and alloys to thin multilayer films on planar substrates, dielectric films, semiconducting structures and devices, and ceramic materials. This trend toward a broader range of materials and applications is likely to continue.

Invited review article: Atom probe tomography.

The rapid development of electron tomography, in particular the introduction of novel tomographic imaging modes, has led to the visualization and analysis of three-dimensional structural and chemical information from materials at the nanometre level. In addition, the phase information revealed in electron holograms allows electrostatic and magnetic potentials to be mapped quantitatively with high spatial resolution and, when combined with tomography, in three dimensions. Here we present an overview of the techniques of electron tomography and electron holography and demonstrate their capabilities with the aid of case studies that span materials science and the interface between the physical sciences and the life sciences.

Electron tomography and holography in materials science

Atom Probe Field-Ion Microscopy

A position‐sensitive detector system based on a wedge‐and‐strip anode has been used to build a short flight‐path atom probe which identifies both the chemical nature and position of single atoms field evaporated from the surface of a field‐ion specimen. The detector also allows digitized field‐ion images to be obtained from the region being analyzed. The prototype instrument has a lateral resolution during analysis of substantially below 1 nm, and a depth resolution of one atomic layer. Initial applications of the instrument to the analysis of nanometer‐scale precipitates in metallic alloys has shown the capability of reconstructing the three‐dimensional microstructure and microchemistry of materials.

Application of a position-sensitive detector to atom probe microanalysis

A wide acceptance angle first-order reflectron lens has been incorporated into a three-dimensional atom probe (3DAP) to provide improved mass resolution. This new 3DAP instrument is capable of resolving isotopes in the mass spectrum, with resolutions better than m/Δm=500 full width at half maximum and 250 full width at 10% maximum. However, use of a reflectron for energy compensation within an imaging system means that improvements in mass resolution result in degradation of the spatial resolution. This article addresses the detailed design of the energy compensated 3DAP, and the minimization and compensation of chromatic aberrations in the imaging performance of the instrument. Some applications of the new instrument are included to illustrate its capabilities in the atomic-scale analysis of engineering alloys.

Performance of an energy-compensated three-dimensional atom probe

SUMMARY

A position-sensitive detector has been combined with time-of-flight mass spectrometry in the atom probe field-ion microscope to yield a system in which both chemical identity and spatial information are obtained for individual ions field-evaporated from the specimen surface. This allows the variations in composition originally present in the sample to be reconstructed in 3-D with sub-nanometre resolution. The prototype position-sensitive atom probe is being used to study phase chemistry in a number of metallurgical alloys, including accurate composition determination of 1–2 nm Cu-rich precipitates formed in Fe–1.3% Cu–1.4%Ni aged to peak hardness. Other applications of the position-sensitive atom probe (POSAP) include the analysis of surface layers on superconductors and atom probe studies of semiconductor multiple quantum wells. These initial applications of the instrument are reported, and the limitations and intended improvements to the instrument are discussed.

Materials analysis with a position-sensitive atom probe

Improvements in three-dimensional atom probe design

Fe–Cr alloys and the ferritic phase of duplex stainless steels may undergo embrittlement during aging in the temperature range 300–500°C, owing to a spinodal reaction process. The atom probe at the University of Oxford. has been used to investigate the microstructural changes accompanying the spinodal in a low carbon, low molybdenum steel (ASME SA351 CF3) in the temperature range 300–400°C for aging times of up to 20 000 h. The amplitude and wavelength of the spinodal were determined. In addition, some other steels, which in previous studies by Trautwein and Gysel had exhibited an extremely low value of activation energy for embrittlement, were examined using both the atom probe and the position sensitive atom probe. Some of this material was re-heat treated to attempt to explain its anomalous behaviour. Steel from the primary cooling circuit of the Shippingport reactor was also examined and carbon and phosphorus were found to have segregated to a phase boundary.MST/1202

T.J. Godfrey

Papers

Application of a position-sensitive detector to atom probe microanalysis

Performance of an energy-compensated three-dimensional atom probe

Materials analysis with a position-sensitive atom probe

Improvements in three-dimensional atom probe design

Quantitative atom probe analysis of spinodal reaction in ferrite phase of duplex stainless steel