Three-dimensional Atom Probe Analysis Research Articles

Design of a delay-line position-sensitive detector with improved performance

A delay-line position-sensitive detector with improved performance is presented. In this device, timing is carried out by means of fast digitizer boards. The use of dedicated signal processing procedures leads to a timing accuracy of 70 ps and a dead-time below 1.5 ns. As a result, the spatial resolving power of this detector is close to 1 mm leading to a high multihit capability. A temporary detector has been designed in which the delay-line anode is combined with a phosphor screen allowing additional positioning to be made via a charge-coupled device camera. This additional positioning is used to unambiguously quantify performances in terms of spatial resolution and multihit capabilities. A three-dimensional atom probe analysis of a material containing low evaporation field phases is used to demonstrate the capabilities of this detector.

Characterization of nanocrystalline ferrite produced by mechanical milling of pearlitic steel

Three-dimensional atom probe analysis of the nanocrystalline ferrite produced by mechanically milling pearlitic steel indicated that carbon atoms are strongly segregated at the grain boundaries, and approximately 1.0 at.% carbon are dissolved in the nanocrystalline ferrite. Annealing at 400 °C led to the rejection of carbon from the grains and the precipitation of cementite at the grain boundaries, while the nanocrystalline microstructure was retained.

Behavior of Nb Atoms in Nb Substituted Nd<tex>$_2$</tex>Fe<tex>$_14$</tex>B Nanocrystalline Alloys Investigated by Atom Probe Tomography

The behavior of Nb atoms in Nd/sub 12/Fe/sub 82-x/B/sub 6/Nb/sub x/ (x=0 to 3) nanocrystalline alloys has been investigated by using atom probe tomography (APT) technique on near-atomic scale in the present paper. Three-dimensional atom probe (3DAP) analyses on the Nb substituted alloys clearly reveal that Nb atoms are enriched at grain boundaries with a peak concentration about 4 times higher than the average for Nd/sub 2/Fe/sub 14/B grains. A grain boundary region with a chemical composition near the Nb : Fe : B stoichiometry of 3 : 3 : 5 is also measured for the x=3 alloy. The results provide direct evidence of microstructural refinement due to solute drag of Nb atoms during solidification, resulting in Nb enrichment at grain boundaries and possibly the formation of a Nb-rich interfacial phase which give rise to enhanced magnetic properties.

Nanoscale precipitation in a maraging steel studied by APFIM.

This article summarizes findings from our previous investigations and recent studies concerning precipitation in a maraging steel of type 13Cr-9Ni-2Mo-2Cu (at.%) with small additions of Ti (1 at.%) and Al (0.7 at.%). The material was investigated after aging at 475 degrees C up to 400 h using both conventional and three-dimensional atom-probe analyses. The process of phase decomposition in the steel proved to be complicated. It consisted of precipitation of several phases with different chemistry. A Cu-rich phase was first to precipitate and Mo was last in the precipitation sequence. The influence of the complex precipitation path on the material properties is discussed. The investigation clearly demonstrated the usefulness of the applied techniques for investigation of nanoscale precipitation. It is also shown that, complementary methods (such as TEM and EFTEM) giving structural and chemical information on a larger scale must be applied to explain the good properties of the steel after prolonged aging.

Three-dimensional atom probe analysis of carbon distribution in low-temperature bainite

A bainitic microstructure with fine ferrite plates (20–40 nm) in a matrix of high-carbon retained austenite, obtained by isothermal transformation at 200 °C, was characterized with an energy-compensated optical position-sensitive three-dimensional atom probe. The average carbon concentration in the austenite was found to be 8.0 ± 1.6 at.% and in the bainitic ferrite 1.1 ± 0.7 at.%. The latter concentration is much higher than expected from paraequilibrium between austenite and ferrite, and the austenite carbon concentration is found to be slightly higher than the T 0 limit.

Coincidence Doppler broadening and 3DAP study of the pre-precipitation stage of an Al–Li–Cu–Mg–Ag alloy

Pre-precipitation solute clustering in Al–Li–Cu–Mg–Ag and Al–Cu–Mg–Ag alloys has been investigated by coincidence Doppler broadening (CDB) spectroscopy of positron annihilation and three-dimensional atom probe (3DAP) analysis. Although Ag–Mg co-clusters form in the Al–Cu–Mg–Ag alloy in the early stage of aging, no evidence for the co-cluster formation was obtained from the Li containing alloy using 3DAP. While CDB spectra indicated that vacancies are associated with Ag after aging for 15 s in the Al–Cu–Mg–Ag alloy, vacancy–Ag association is suppressed in the Li containing alloy. Based on the 3DAP and CDB results, the reasons for the completely different clustering behaviors observed in these two similar alloys are discussed.

Mechanisms by which oxygen acts as a surfactant in giant magnetoresistance film growth

The mechanisms by which oxygen acts as a surfactant in giant magnetoresistance multilayers have been elucidated for the first time. Three-dimensional atom probe analysis of Cu/CoFe multilayers reveals the elemental distributions at the atomic level. Interfacial intermixing and oxygen impurity levels have been quantified for the first time. Both with and without oxygen the intermixing is greater at the CoFe-on-Cu interface than at the Cu-on-CoFe one and for both interfaces, oxygen reduced the intermixing. The oxygen largely floats to the growing surface and is incorporated at grain boundaries. The oxygen also reduces conformal roughness and grain boundary grooving, indicating a reduction in long-range surface diffusion.

Combined atomic-scale modelling and experimental studies of nucleation in the solid state.

The process of solid-state nucleation in highly supersaturated solid solutions has been investigated on the atomic scale by a combination of three-dimensional atom probe analysis and atomistic modelling using dynamical Ising models. In binary Cu-Co alloys, a simple atom-exchange model with a single thermodynamic parameter derived from phase-diagram data was able to reproduce the atomic-scale microstructures observed in the atom probe, and also match the measured peak precipitate density. Modelling solute effects in complex copper-bearing steels required a more sophisticated model based on a vacancy-hopping mechanism and a larger number of thermodynamic and kinetic parameters derived from independent experimental data and theoretical calculations. The model gave an excellent match to the experimentally observed microstructures, and it reproduced features such as the clustering of Ni and Mn before the precipitation of Cu. The model also allowed time-dependent behaviour to be investigated, and it showed that solute clustering of Ni and Mn occurs during the cooling of the alloy. These clusters then act as heterogeneous nucleation sites for the formation of copper precipitates. Understanding such complex solute interaction effects through combined experiment and modelling is an essential step to controlling nucleation and hence the fine-scale microstructures in advanced engineering alloys.

Three-dimensional Investigation of Ceramic/Metal Heterophase Interfaces by Atom-probe Microscopy.

The results of a three-dimensional atom probe (3DAP) analysis, on a subnanometer scale, of a ceramic/metal heterophase interface, MgO/Cu, are presented. Segregation of Ag, from the Cu (Ag) matrix, at MgO/Cu interfaces is investigated and the Gibbsian interfacial excess of silver is determined; the range is 2.33 x 10(18) to 5.81 x 10(18) m(-2). Also, silver segregation at the same MgO/Cu interfaces is analyzed employing a new approach that utilizes a proximity histogram or proxigram.

THERMAL STABILITY OF ELECTRODEPOSITED NANOCRYSTALLINE NICKEL

The effect of annealing electrodeposited nanocrystalline nickel at 250°C was studied by microhardness testing, XRD, TEM and three-dimensional atom probe (3DAP) analysis. TEM micrographs revealed that the type of growth is abnormal, i.e. a composite structure is formed with some large (>1 μm) grains embedded in a matrix of nanometre sized grains. 3DAP studies showed no grain boundary segregation in as deposited materials and only low levels in material annealed for 1 h. Upon annealing, a 10% increase in hardness was observed within the first five minutes, after which the hardness was maintained for long times, despite the high volume fraction of large grains. Only after extensive annealing, when the majority of the material was made up of large grains, did the hardness decrease rapidly. The results demonstrate that the hardness is not determined by a law of mixtures but by percolation of the large grains.

Nanoquasicrystalline phase formation in binary Zr–Pd and Zr–Pt alloys

This paper reports nanoquasicrystalline phase formation in Zr100−xPdx (x=30 and 35) and Zr80Pt20 binary alloys and the kinetics of the nanoquasicrystallization process. While the icosahedral phase (i-phase) forms as a metastable phase in the transient stage during the crystallization of Zr–Pd amorphous alloy, it forms directly from the liquid during melt-spinning of Zr–Pt alloy. The isothermal kinetics studies show that i-phase forms from the Zr70Pd30 amorphous alloy by the primary crystallization process with the Avrami exponent in the range of 1.5–2.5. Three-dimensional atom probe analysis results suggest that the i-phase is slightly enriched with Zr with respect to the matrix and its composition is close to Zr75Pd25. The tendency of quasicrystallization of Zr-based alloys appears to have correlation with the enthalpy of mixing of the system.

Atom probe analysis of roughness and chemical intermixing in CoFe/Cu films (invited)

Three-dimensional atom probe analyses of the interfaces between CoFe and Cu layers has shown that both roughness and chemical intermixing can occur independently. Interfaces formed by the deposition of Cu onto CoFe mimic the roughness present in previously deposited interfaces, but have a very small amount of interfacial mixing. In contrast, interfaces formed by the deposition of CoFe onto Cu are less rough, but more chemically intermixed. The region of chemical intermixing formed when CoFe is deposited onto Cu (0.7–1.0 nm) is approximately two times larger than that when Cu is deposited onto CoFe (0.3–0.5 nm).

Three-dimensional Investigation of Ceramic/Metal Heterophase Interfaces by Atom-probe Microscopy

AbstractThe results of a three-dimensional atom probe (3DAP) analysis, on a subnanometer scale, of a ceramic/metal heterophase interface, MgO/Cu, are presented. Segregation of Ag, from the Cu (Ag) matrix, at MgO/Cu interfaces is investigated and the Gibbsian interfacial excess of silver is determined; the range is 2.33 × 1018 to 5.81 × 1018 m−2. Also, silver segregation at the same MgO/Cu interfaces is analyzed employing a new approach that utilizes a proximity histogram or proxigram.

Microstructural investigation of Cu/Co multilayer films

Abstract Multilayer film structures, with applications based on the giant-magnetoresistance principle, comprise an exciting area of research and have the capability to improve the storage density and sensitivity of devices used by the magnetic recording industry. In order to optimise the performance of these devices, it is imperative to characterise the atomic-level structural and compositional variations as a function of processing and operating conditions. A new technique, based on focused ion-beam milling, has been developed to fabricate field-ion specimens from nanometer-scale multilayer films deposited directly onto a planar substrate. This method has allowed field-ion microscopy and three-dimensional atom-probe analysis of this type of structure for the first time. A multilayer thin film structure containing 100 repetitions of a (Cu 2nm /Co 2nm ) bilayer showed 5–20% Cu alloying in the cobalt layers, a high degree of layer curvature (particularly at grain boundaries) and regions where several cobalt layers appear to be in contact. These observations suggest a degree of ferromagnetic coupling, which explains the relatively high coercivity measurements obtained from the same Cu/Co films. Films which were annealed for 1 h at 360°C showed an increase in grain size, but little change in other microstructural properties.

Field-ion specimen preparation using focused ion-beam milling

Preparation of field-ion specimens from various materials has been accomplished using focused ion-beam milling in either a simple cutting mode or by application of an annular-shaped ion-milling pattern. These specimens have been investigated using field-ion microscopy and three-dimensional atom probe analysis. In the cutting mode, gallium implantation is minimised when using a lower beam energy. However, with annular milling, using 30 keV ions opposed to 10 keV ions results in less gallium implantation and produces a smaller shank angle and a sharper apex radius. High-dose ion imaging at 30 keV ion energy, even with relatively low beam currents, results in excessive implantation during field-ion specimen fabrication. Focused ion-beam milling provides not only an alternative method of field-ion sample preparation, but also, in conjunction with atom probe analysis, allows the quantitative investigation of the gallium implantation and damage which occurs during the milling.

Atomic Scale Characterisation of Electrodeposited Nanocrystalline Ni-P Alloys

ABSTRACTNanocrystalline Ni-P alloys produced by electrodeposition have been characterised by three-dimensional atom probe (3DAP) analysis. In the as-deposited materials, there are indications of some variation in P concentration between grains and segregation to grain boundaries. After heat treatment however, strong grain boundary segregation and the formation of Ni3P precipitates have been observed.

Microalloying effect of Cu and Nb on the microstructure and magnetic properties of Fe/sub 3/B/Nd/sub 2/Fe/sub 14/B nanocomposite permanent magnets

This paper reports the microalloying effect of Cu and Nb on the microstructure and magnetic properties of a Fe/sub 3/B/Nd/sub 2/Fe/sub 14/B nanocomposite permanent magnet. Optimum magnetic properties with B/sub r/=1.25 T, H/sub cJ/=273 kA/m and (BH)/sub max/=125 kJ/m/sup 3/ were obtained by annealing a melt-spun Nd/sub 4.5/Fe/sub 75.8/B/sub 18.5/Cu/sub 0.2/Nb/sub 1/ amorphous ribbon at 660/spl deg/C for 6 min. Compared with a ternary Nd/sub 4.5/Fe/sub 77/B/sub 18.5/ alloy, the grain size is much finer in the optimum microstructure of the Cu and Nb containing alloys, and the temperature range of the heat-treatment for obtaining optimum hard magnetic properties was significantly extended. The soft magnetic Fe/sub 23/B/sub 6/ phase coexists with Fe/sub 3/B and Nd/sub 2/Fe/sub 14/B phases in the optimum microstructure. Three-dimensional atom probe (3DAP) analysis results have revealed that the finer microstructure is due to the formation of a high number density of Cu clusters, which promote the nucleation of the Fe/sub 3/B phase. The Nb atoms appear to induce the formation of the Fe/sub 23/B/sub 6/ phase and stabilize it by partitioning into this phase.

Characterisation of a rapidly solidified Al–V–Fe alloy

This paper outlines preliminary results from a combined XRD, TEM and APFIM investigation of a rapidly solidified Al94V4Fe2 alloy that had been produced by melt spinning. The microstructure produced consists of a high volume fraction (≈50%) of 20 nm icosahedral particles in an f.c.c. matrix. Three-dimensional atom probe analysis shows that this second phase is enriched in iron and vanadium compared with the matrix. The thermal stability of the melt-spun ribbon has been investigated using DSC. It was found to be stable up to a temperature of 520°C, after which there were two solid state transformations.

Three-dimensional atom probe analysis of Co–Cr–Ta thin film

Three-dimensional elemental mappings of Co–Cr–Ta thin film sputter-deposited at 200 °C were obtained using a three-dimensional atom probe. We have confirmed that Ta atoms are dissolved in the film homogeneously, and no segregation of Ta was found at the grain boundaries. On the other hand, Cr atoms are inhomogenously dissolved within grains, and it is strongly enriched at the grain boundaries forming a Cr-enriched grain boundary phase with a thickness of approximately 4 nm.

Summary of discussion at the first workshop on three-dimensional atom-probe analysis — its present and future

Summary of discussion at the first workshop on three-dimensional atom-probe analysis — its present and future