Three-dimensional Atom Probe Field Ion Research Articles

On the Physics of Anomalies of Boron Nanosegregation at Dislocations in FeAl

We present results of the constructive critical analysis and interpretation of some recent studies (Blavette, Sauvage, Wilde and others) at the atomic scale (using three-dimensional atom-probe field-ion microscopy) of impurity nanosegregation at dislocations, including “Cottrell atmospheres”, and grain boundaries in deformed intermetallics and metallic materials, and their relevance to mechanical properties and diffusion processes.

Simulation of competitive Cu precipitation in steel during non-isothermal aging

A numerical model has been developed to simulate Cu precipitation in dilute bcc Fe–Cu alloys during non-isothermal aging taking into account competitive nucleation at grain boundaries, dislocations and in the matrix, and structural transformation of Cu particles that occurs during growth. The temporal evolution of number density, mean particle size and size distribution during continuous cooling is simulated and is compared with experimental observations under transmission electron microscope and three-dimensional atom probe field ion microscope. With decreasing temperature the growth and coarsening rates diminishes rapidly whereas nucleation continues to occur down to lower temperatures due to the decrease in the activation energy of nucleation and thus, distributions of fine particles can be obtained relatively easily after cooling. Precipitation and dissolution during continuous heating are simulated and are compared with experimental observations in the literature.

SANS response of VVER440-type weld material after neutron irradiation, post-irradiation annealing and reirradiation

It is well accepted that the reirradiation behaviour of reactor pressure vessel (RPV) steel after annealing can be different from the original irradiation behaviour. We present the first small-angle neutron scattering (SANS) study of neutron irradiated, annealed and reirradiated VVER440-type RPV weld material. The SANS results are analyzed both in terms of the size distribution of irradiation-induced defect/solute atom clusters and in terms of the ratio of total and nuclear scattering intensity in a saturation magnetic field (A ratio). The measured A ratio is compared with calculations performed on the basis of the cluster composition reported for a similar weld material investigated by means of three-dimensional atom probe field ion microscopy. The observed deviation between both estimates and possible reasons for the discrepancy are discussed. Special emphasis is placed on the differences between the materials response to the original irradiation and to reirradiation after annealing. The results indicate that reirradiation-induced clusters are slightly different in their average composition and their formation saturates at a lower volume fraction than in the case of the original irradiation.

The effect of Cu on mechanical and precipitation properties of Al–Zn–Mg alloys

The effect of Cu on the mechanical and precipitation properties of a high strength Al-2.4 at.% Zn-2.1 at.% Mg alloy was investigated by compression and indentation tests, as well as by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and three-dimensional atom probe field ion microscopy (3DAPFIM). The addition of 0.5 at.% Cu introduces significant changes in the precipitation process and consequently in the age-hardening behavior of the alloy. Microstructural measurements reveal that the addition of Cu changes the density of GP zones, but it also changes partly the shape and composition of the particles. Mechanical and microstructural results together lead to the conclusion that clustering of solute atoms and vacancies during or immediately after water quenching plays an important role in the nucleation of intermediate phase precipitates in one-step aging and the addition of Cu to ternary Al–Zn–Mg leads to changes also in the initial clustering process.

Cu precipitation in a prestrained Fe-1.5 wt pct Cu alloy during isothermal aging

The control of Cu precipitation at low temperatures, e.g., bake hardening of Cu bearing steels, has recently attracted considerable attention due to the potential of achieving good formability and high strength. An Fe-1.5 wt pct Cu alloy, solution treated and 10 pct prestrained, exhibits a two-step age-hardening behavior, i.e., a smaller, but substantial hardening around 200 °C to 300 °C and a major hardening around 500 °C, while only the latter hardening occurs in undeformed specimens. The precipitation behavior of nanoscale Cu particles or bcc Cu clusters that plays a major role in age hardening was simulated by Cahn-Hilliard nonclassical nucleation theory and the Langer-Schwartz model. Simulation results are compared with the distribution of Cu particles observed under three-dimensional atom probe field ion microscope (3-D APFIM) and transmission electron microscope (TEM), and age hardening behavior as well. The increase in hardness in prestrained specimens at low temperatures (≤400 °C) can be ascribed to Cu particles nucleated preferentially at dislocations or to Cu particles that were formed in the matrix as early as at dislocations presumably due to excess vacancies introduced by prestraining.

Microstructure and microchemistry variation during thermal exposure of low alloy steels

The microstructure of an ex-service (136,000 h) Fe–1Cr–1Mo–0.25V (wt%) low alloy steel turbine rotor was investigated using energy dispersive X-ray spectroscopy (EDXS) techniques in TEM/STEM on extraction replicas. Since different stages of the rotor experience different thermal histories, an examination of microstructural variations can serve as a method to indicate metallurgical factors affecting service-lifetime of the material. The coldest, hottest and most embrittled stages were taken for investigation. The overall fraction of carbide precipitation was measured using image analysis techniques and results indicated that the hottest stage possessed the highest precipitate volume fraction. Large area EDXS analysis using a defocused electron beam was also performed on the replica samples and this was supported by spot analyses using scanning TEM to identify individual carbides so as to allow quantification of the enrichment of solute into carbides as a function of service temperature. Three-dimensional atom probe field ion microscopy was also used to assess the chemistry at the interface between matrix and precipitate.

Microstructure Feature of Bulk Glassy Cu<SUB>60</SUB>Zr<SUB>30</SUB>Ti<SUB>10</SUB> Alloy in As-cast and Annealed States

The microstructures of a bulk glassy Cu 60 Zr 30 Ti 10 alloy in as-cast state and annealed at glass transition temperature (T g ), i.e., 440°C, have been studied using an electron transmission microscope (TEM) and a high-resolution transmission electron microscope (HRTEM). Nanocrystals with a size of 4 nm were observed in the cast rod with a diameter of 4 mm. The nanocrystal was determined to be CuZr, which is primitive cubic structure with a lattice parameter of a = 0.3262 nm by analyzing its nano-beam electron diffraction pattern, suggesting that such nanocrystals have a positive effect on mechanical properties. Obviously distinguished interval dark and bright regions were observed in the glassy Cu 60 Zr 30 Ti 10 alloy annealed for 10 min at T g . Nanocrystalline phase with a size of ∼10 nm was recognized after the annealing treatment. The concentration distribution of Cu, Zr and Ti in the alloy annealed for 10 min at T g was examined with a three-dimensional atom probe field ion microscopy (3DAP-FIM). It was found that the Cu and Zr elements fluctuated along depth direction while the Ti element remained almost unchanged. It is therefore interpreted that the annealing-induced nanocrystalline phase results from nucleation and growth at the sites of the Cu-rich and/or Zr-rich regions as well as from growth of the cast-in nanocrystals.

Investigation of boron-enriched Cottrell atmospheres in FeAl on an atomic scale by three-dimensional atom-probe field-ion microscopy

In 1949, on the basis of theoretical considerations, Cottrell proposed the concept of 'atmospheres' (called later by his name) to explain some specific behaviour of materials during plastic deformation, such as sharp yield-point formation or the Portevin-LeChatelier effect. In this letter, atomic-scale observations and three-dimensional analyses of a Cottrell atmosphere are reported. They have been performed by three-dimensional atom-probe field-ion microscopy techniques. The ability of this new experimental method to provide atomic-resolution images, both structural and chemical, was confirmed; the basic stacking structure of (001) planes in FeAl could be visualized with success. Moreover the presence of a <001> edge dislocation was also detected in the analysed zone. Further, B enrichment was measured in the vicinity of this defect; the B-rich region appeared as a pipe 5 nm in diameter, parallel to the dislocation line. The concentration of B in the core reached 3 at.%; this local enrichment in boron was accompanied by an Al depletion of more than 10 at.%. Boron in FeAl has a well known tendency to segregate to internal interfaces. In this letter, we show experimental evidence of the solute segregation to dislocation lines. The observed effects of this segregation on mechanical properties of FeAl, both at room temperature and high temperatures, are also discussed.

Experimental studies of the phase separation mechanism in Ti-15at%A1

Abstract The phase decomposition mechanism operating in a binary Ti-A1 alloy in the two-phase α+α2 region has been investigated. A Ti-15at%A1 alloy was aged in the range 550–750°C for various times to produce material at different stages of phase separation. The resulting microstructures have been examined experimentally using three-dimensional atom probe field ion microscopy and neutron diffraction. Results indicate that significant ordering occurs in the system prior to the onset of chemical phase separation which occurs via a spinodal reaction. The mechanism is believed to lie in the class of ‘conditional spinodal’ mechanisms in which one stage of the phase separation process is thermodynamically contingent upon another.

Oxygen Distribution in Zr&lt;sub&gt;65&lt;/sub&gt;Cu&lt;sub&gt;15&lt;/sub&gt;Al&lt;sub&gt;10&lt;/sub&gt;Pd&lt;sub&gt;10&lt;/sub&gt; Nanocrystalline Alloys

Oxygen behavior in the nanocrystalline Zr65Cu15Al10Pd10 alloy crystallized from the amorphous precursor has been investigated by three-dimensional atom probe field ion microscopy. Oxygen impurity (about 0.1 at.%) is dissolved uniformly in the as-quenched amorphous alloy even if oxygen is not added intentionally. When the alloy is crystallized, oxygen redistribution occurs and it is partitioned in the crystalline phase such as Zr3(Cu, Pd)2 phase. The oxygen concentration in the crystalline phase is approximately 4 at.% . This demonstrates that oxygen redistribution occurs during the crystallization reaction thereby influence the kinetics of crystallization.

Three-dimensional atom probe field-ion microscopy observation of Cu/Co multilayer film structures

Focused ion-beam milling has been used to fabricate field-ion specimens from a multilayer film structure containing 100 repetitions of a (Cu2 nm/Co2 nm) bilayer deposited directly onto a planar substrate. The as-deposited films showed a magnetoresistance ratio of ∼5% over a 250 Oe range at room temperature, and a coercivity of ∼60 Oe. The magnetic data suggest that the films are coupled ferromagnetically. Successful field-ion specimen preparation has allowed the observation of these layers by field-ion imaging and three-dimensional atom probe compositional analysis. Examination of the multilayer images reveals that, in some regions, the layers are nonparallel, but the interfaces are chemically quite sharp, with a diffuse interface region of ∼3 atomic layers. In addition, in some areas adjacent cobalt layers appear to be in contact. The fact that the layers are wavy suggests that the ferromagnetic coupling may be a result of Néel “orange peel” type magnetostatic coupling between adjacent cobalt layers. The relatively high coercivity may be a result of the poor layer planarity leading to a high number of domain wall pinning sites.