Site-specific Atom Probe Tomography Research Articles

Solute segregation, clustering and interphase precipitation in Ti-Mo-Nb microalloyed steel studied by correlated electron backscattering diffraction and atom probe tomography

Site-specific atom probe tomography coupled with electron back scattering diffraction studies were used for the first time to elucidate the multi-element solute segregation and clustering at austenite-ferrite interface during early stages of phase transformation in Fe-Mn-Si-C steel alloyed with Ti, Mo and Nb. The results have shown the existence of negligible partitioning local equilibrium at interfaces with segregation of C, Mn, Ti and Mo to interfaces having the angle deviation in the range from 2.9 to 12° from Kurdjumov-Sacks orientation relationships (K-S ORs), but either irregular incident planes or the ones not following the matching according to K-S ORs. The segregation of Nb was less common, which was linked to the earlier occupancy of interfaces by Ti and Mo. Variation in solute segregation along the same interfaces is highlighted. Presence of clusters within and in close proximity to the interface is detected. The links between interface orientation, solute segregation, clustering and interphase precipitation are discussed.

Austenite grain boundary segregation and precipitation of boron in low-C steels and their role on the heterogeneous nucleation of ferrite

The addition of boron (B) to steels suppresses the austenite to ferrite phase transformation dramatically, thus increasing their hardenability. It achieves this through grain boundary (GB) segregation at the austenitic GBs that delays the ferrite nucleation. Though the effects of B segregation on hardenability have long been known, the mechanisms of B segregation and how exactly B suppresses the ferrite nucleation remain elusive. We designed a B-containing low-C steel to study the B segregation and precipitation behavior. We conducted heat treatments with different austenitization temperatures and cooling rates. Site-specific atom probe tomography, complemented by high-resolution secondary ion mass spectrometry reveals B segregation at prior austenite grain boundaries (PAGBs) along with carbo-boride precipitation. The B segregation mechanisms are discussed in detail based on these observations considering their dependence on austenitization temperature and cooling rate. Furthermore, we analyzed the impact of GB energy reduction through nucleation kinetics calculations. From our analysis, we conclude that the reduction in GB energy affects the grain corner and grain edge nucleation substantially. However, the retarding effects of carbo-boride precipitation on ferrite nucleation cannot be completely excluded. We provide a detailed account of the possibilities of how B-containing precipitates may be suppressing ferrite nucleation.

Hydrogen-assisted decohesion associated with nanosized grain boundary κ-carbides in a high-Mn lightweight steel

While age-hardened austenitic high-Mn and high-Al lightweight steels exhibit excellent strength-ductility combinations, their properties are strongly degraded when mechanically loaded under harsh environments, e.g. with the presence of hydrogen (H). The H embrittlement in this type of materials, especially pertaining to the effect of κ-carbide precipitation, has been scarcely studied. Here we focus on this subject, using a Fe-28.4Mn-8.3Al-1.3C (wt%) steel in different microstructure conditions, namely, solute solution treated and age-hardened. Contrary to the reports that grain boundary (GB) κ-carbides precipitate only during overaging, site-specific atom probe tomography and scanning transmission electron microscopy (STEM) reveal the existence of nanosized GB κ-carbides at early stages of aging. We correlate this observation with the deterioration of H embrittlement resistance in aged samples. While H pre-charged solution-treated samples fail by intergranular fracture at depths consistent with the H ingress depth (∼20 µm), age-hardened samples show intergranular fracture features at a much larger depth of above 500 µm, despite similar amount of H introduced into the material. This difference is explained in terms of the facile H-induced decohesion of GB κ-carbides/matrix interfaces where H can be continuously supplied through internal short-distance diffusion to the propagating crack tips. The H-associated decohesion mechanisms are supported by a comparison with the fracture behavior in samples loaded under the cryogenic temperature and can be explained based on dislocation pileups and elastic misfit at the GB κ-carbide/matrix interfaces. The roles of other plasticity-associated H embrittlement mechanisms are also discussed in this work based on careful investigations of the dislocation activities near the H-induced cracks. Possible alloying and microstructure design strategies for the enhancement of the H embrittlement resistance in this alloy family are also suggested.

How Sn addition influences texture development in single-phase Fe alloys: Correlation between local chemical information, microstructure and recrystallisation

Sn addition is known to affect the recrystallisation texture development of α -Fe. To date, even though the texture effect of Sn has been experimentally verified, although indirectly, the underlying mechanism has not been studied. In the present study, we focus on the static recrystallisation behaviour of Fe-Si-Sn alloys to elucidate the texture preferences assessing for the effect of Sn solute addition. The experimental approach is based on sequential electron back-scatter diffraction (EBSD) texture analysis complemented with a site-specific chemical analysis of grain boundaries by atom probe tomography (APT). It is directly observed through our experiments that boundary segregation phenomena, which occur at the onset and later stages of recrystallisation, provide crucial understanding of texture development. Especially in the presence of solute, the orientation dependence of the energy stored by deformation largely determines the course of the subsequent recrystallisation. In the Fe-Si-Sn alloy, recrystallisation at the highest stored energy grains is hindered. As a consequence of segregation, {001}〈110〉 orientations recrystallise at later stages of the process, forming predominantly grains with orientations of the { h 11}〈1/ h 12〉 fibre. These grains are suggested to form following an oriented nucleation mechanism, while their growth could be interpreted with Ibe and Lücke's theory of selective growth of 26.5 〈110〉 boundaries in bcc metals. • Sn affecting recrystallization texture evolution; during short annealing times. • Local correlative study with in-situ EBSD and site-specific atom probe tomography. • Key mechanisms discussed; segregation effect on recrystallization nucleation-growth. • Gamma-fibre ({111} <uvw>) recrystallization greatly reduced due to Sn segregation. • Segregation promoting the intensity of {411}<148> recrystallization texture.

New approach for FIB-preparation of atom probe specimens for aluminum alloys.

Site-specific atom probe tomography (APT) from aluminum alloys has been limited by sample preparation issues. Indeed, Ga, which is conventionally used in focused-ion beam (FIB) preparations, has a high affinity for Al grain boundaries and causes their embrittlement. This leads to high concentrations of Ga at grain boundaries after specimen preparation, unreliable compositional analyses and low specimen yield. Here, to tackle this problem, we propose to use cryo-FIB for APT specimen preparation specifically from grain boundaries in a commercial Al-alloy. We demonstrate how this setup, easily implementable on conventional Ga-FIB instruments, is efficient to prevent Ga diffusion to grain boundaries. Specimens were prepared at room temperature and at cryogenic temperature (below approx. 90K) are compared, and we confirm that at room temperature, a compositional enrichment above 15 at.% of Ga is found at the grain boundary, whereas no enrichment could be detected for the cryo-prepared sample. We propose that this is due to the decrease of the diffusion rate of Ga at low temperature. The present results could have a high impact on the understanding of aluminum and Al-alloys.

Cross-shaped markers for the preparation of site-specific transmission electron microscopy lamellae using focused ion beam techniques

We describe the use of a cross-shaped platinum marker deposited using electron-beam-induced deposition (EBID) in a focused ion beam – scanning electron microscope (FIB-SEM) system to facilitate site-specific preparation of a TEM foil containing a trench defect in an InGaN/GaN multiple quantum well structure. The defect feature is less than 100 nm wide at the surface. The marker is deposited prior to the deposition of a protective platinum strap (also by EBID) with the centre of the cross indicating the location of the feature of interest, while the arms of the square cross make an acute angle of 45° with the strap's long axis. During the ion-beam thinning process, the marker may be viewed in cross-section from both sides of the sample alternately, and the coming together of the features relating to the arms of the cross indicates increasing proximity to the feature of interest. Although this approach does allow increased precision in locating the region of interest during thinning, it also increases the time required to complete the sample preparation. Hence, this method is particularly well suited to directly correlated multi-microscopy investigations in previously characterised material where high yield and the precise location are more important than preparation time. In addition to TEM lamella preparation, this method could equally be useful for preparing site-specific atom probe tomography (APT) samples.

Fast Diffusion and Segregation along Threading Dislocations in Semiconductor Heterostructures

Heterogeneous integration of semiconductors combines the functionality of different materials,enabling technologies such as III-V lasers and solar cells on silicon and GaN LEDs on sapphire. However, threading dislocations generated during theepitaxy of these dissimilar materialsremain a key obstacle to the success of this approach due to reduced device efficiencies and reliability. Strategies to alleviate this and understand charge carrier recombination at threading dislocations now needan accurate description of the structure of threading dislocationsin semiconductor heterostructures. We show that the composition around threading dislocations intechnologically important InGaAs/GaAs/Ge/Si heterostructures areindeed different from that of thematrix. Site-specific atom probe tomography enabled by electron channeling contrast imaging reveals this at individual dislocations.We present evidence for the simultaneousfast diffusion of germanium and indium up and down adislocation, respectively, leading to unique compositional profiles. We also detect the formation of clusters of metastable composition at the interface between Ge and GaAs, driven by intermixing in these two nearly immiscible materials.Together,our results have important implications for the properties of dislocations and interfaces in semiconductors andprovide new tools for their study.

Quantification of precipitate hardening of twin nucleation and growth in Mg and Mg-5Zn using micro-pillar compression

In polycrystalline materials, the stress corresponding to twin nucleation is difficult to separate from twin growth because these events occur concurrently during deformation. In this work, we separate the nucleation stress and growth stress of {101¯2} twinning by compression of micro-pillars containing pre-existing twins through their centre. Micro-pillar compression results showed a strong size effect on both twin nucleation and twin growth stresses for pure Mg and Mg-5Zn alloys. Taking this into account, the critical stress for growth of twins in pure magnesium is found to be ∼7 MPa which is consistent with previously published measurements on macroscopic single crystals. These experiments have been used to deduce the precipitate hardening of twin growth, and for the present precipitate dispersion this has been measured to be ∼30 MPa. Back-stress calculations based on elastic bending of the precipitates showed close agreement to the measured precipitate hardening, and this model therefore accounts well for the observed strengthening. Site-specific atom probe tomography of the twin boundaries showed that room temperature ageing is sufficient to produce segregation of zinc to the twin boundary. This was found to immobilise the twin, and is believed to be the first report of solute locking of twins from room temperature exposure.

Tetragonality of bcc Phases in a Transformation-Induced Plasticity Steel

In a low-alloyed multi-phase transformation-induced plasticity steel, solute carbon content in polygonal ferrite, bainitic ferrite, and martensite was characterized using site-specific atom probe tomography. Selected area diffraction patterns were obtained using transmission electron microscopy, and the geometric distortion thereof was determined. The results showed that the lattice distortion increased in a sequence of polygonal ferrite, lath-like bainitic ferrite, and martensite. This increasing distortion corresponded to an increase in carbon content of the phase.

Preventing damage and redeposition during focused ion beam milling: The “umbrella” method

Focused ion beam (FIB) milling has enabled the development of key microstructure characterization techniques (e.g. 3D electron backscatter diffraction (EBSD), 3D scanning electron microscopy imaging, site-specific sample preparation for transmission electron microscopy, site-specific atom probe tomography), and micro-mechanical testing techniques (e.g. micro-pillar compression, micro-beam bending, in-situ TEM nanoindentation). Yet, in most milling conditions, some degree of FIB damage is introduced via material redeposition, Ga+ ion implantation or another mechanism. The level of damage and its influence vary strongly with milling conditions and materials characteristics, and cannot always be minimized. Here, a masking technique is introduced, that employs standard FIB-SEM equipment to protect specific surfaces from redeposition and ion implantation. To investigate the efficiency of this technique, high angular resolution EBSD (HR-EBSD) has been used to monitor the quality of the top surface of several micro-pillars, as they were created by milling a ringcore hole in a stress-free silicon wafer, with or without protection due to an “umbrella”. HR-EBSD provides a high-sensitivity estimation of the amount of FIB damage on the surface. Without the umbrella, EBSD patterns are severely influenced, especially within 5 µm of the milled region. With an optimized umbrella, sharp diffraction patterns are obtained near the hole, as revealed by average cross correlation factors greater than 0.9 and equivalent phantom strains of the order 2 × 10−4. Thus, the umbrella method is an efficient and versatile tool to support a variety of FIB based techniques.

Atom probe tomography of intermetallic phases and interfaces formed in dissimilar joining between Al alloys and steel

While Si additions to Al are widely used to reduce the thickness of the brittle intermetallic seam formed at the interface during joining of Al alloys to steel, the underlying mechanisms are not clarified yet. The developed approach for the site specific atom probe tomography analysis revealed Si enrichments at grain and phase boundaries between the θ (Fe4Al13) and η (Fe2Al5) phase, up to about ten times that of the concentration in Al. The increase in Si concentration could play an important role for the growth kinetics of the intermetallic phases formed for example in hot-dip aluminizing of steel.

Atomic-scale characterization of prior austenite grain boundaries in Fe–Mn-based maraging steel using site-specific atom probe tomography

The embrittlement and de-embrittlement behavior of an Fe–10Mn–1Pd (wt.%) maraging steel upon isothermal aging at 500°C is related to microstructural changes at prior austenite grain boundaries (PAGBs). Site-specific atom probe tomography measurements were performed to analyze the local chemistry of the PAGBs. Tensile tests and hardness measurements were conducted of the ternary alloy and of a binary non-hardenable Fe–10Mn alloy for comparison. Isothermal aging of the binary steel led to a decrease in strength along with a considerable increase in uniform elongation. The Pd-containing alloy, on the other hand, showed significant age-hardening, and an embrittlement and de-embrittlement transition was revealed. Ductile behavior was observed in the initial as-quenched and over-aged states, but there was zero tensile elongation in the intermediate under- and peak-aged regions, where intergranular fracture along the PAGBs occurred. In the brittle peak-aged state a large number of small nanometer-sized particles rich in Mn and Pd formed inside the grains and decorated the PAGBs. The precipitates grew in size on prolonged aging. Mn segregation to the PAGBs was revealed; the Mn concentration level at the boundaries varied with aging time and was highest in the peak-aged condition. Embrittlement and de-embrittlement mechanisms are discussed and compared to these observations.

Examinations of Oxidation and Sulfidation of Grain Boundaries in Alloy 600 Exposed to Simulated Pressurized Water Reactor Primary Water

High-resolution characterizations of intergranular attack in alloy 600 (Ni-17Cr-9Fe) exposed to 325°C simulated pressurized water reactor primary water have been conducted using a combination of scanning electron microscopy, NanoSIMS, analytical transmission electron microscopy, and atom probe tomography. The intergranular attack exhibited a two-stage microstructure that consisted of continuous corrosion/oxidation to a depth of ~200 nm from the surface followed by discrete Cr-rich sulfides to a further depth of ~500 nm. The continuous oxidation region contained primarily nanocrystalline MO-structure oxide particles and ended at Ni-rich, Cr-depleted grain boundaries with spaced CrS precipitates. Three-dimensional characterization of the sulfidized region using site-specific atom probe tomography revealed extraordinary grain boundary composition changes, including total depletion of Cr across a several nm wide dealloyed zone as a result of grain boundary migration.

Atom-probe tomography of surface oxides and oxidized grain boundaries in alloys from nuclear reactors

ABSTRACTThe preparation of site-specific atom-probe tomography (APT) samples containing localized features has become possible with the use of focused ion beams (FIBs). This technique was used to achieve the analysis of surface oxides and oxidized grain boundaries in this paper. Transmission electron microscopy (TEM), providing microstructural and chemical characterization of the same features, has also been used, revealing crucial additional information.The study of grain boundary oxidation in stainless steels and nickel-based alloys is required in order to understand the mechanisms controlling stress corrosion cracking in nuclear reactors. Samples oxidized under simulated pressurized water reactor primary water conditions were used, and FIB lift-out TEM and APT specimens containing the same oxidized grain boundary were prepared and fully characterized. The results from both techniques were found fully consistent and complementary.Chromium-rich spinel oxides grew at the surface and into the bulk material, along grain boundaries. Nickel was rejected from the oxides and accumulated ahead of the oxidation front. Lithium, which was present in small quantities in the aqueous environment during oxidation, was incorporated in the oxide. All phases were accurately quantified and the effect of different experimental parameters were analysed.

A quantitative atom probe study of the Nb excess at prior austenite grain boundaries in a Nb microalloyed strip-cast steel

Most modern HSLA steels rely on the effect of Nb in steels to achieve the properties desired for a specific application. While the role of Nb in forming precipitates has been well characterized, its role in a solid solution is less well understood due to the difficulty of obtaining quantitative experimental data. In the current work, site-specific atom probe tomography was used to quantify the amount of Nb present at prior austenite grain boundaries in a commercial strip-cast steel, produced via the Castrip® process. This was compared to the amount of Nb found at ferrite–ferrite grain boundaries that had formed during the transformation from austenite to ferrite. With the interfacial excess Nb measured, thermodynamic calculations were carried out and compared to the change in transformation temperature obtained by dilatometry, with reference to a comparable Nb free, strip-cast steel.

Segregation of B, P, and C in the Ni-Based Superalloy, Inconel 718

Small additions of B, P, and C can result in dramatic changes in the mechanical performance and welding behavior of Inconel 718. Although additions of B and P improve the mechanical properties, they have a detrimental effect on weldability. Adding C mitigates the negative effect on welding while retaining the improvements in mechanical performance. In this study, precise observations of the segregation of B, P, and C to the grain boundaries are made using site-specific atom probe tomography and NanoSIMS. Changes in the segregation behavior provide a quantitative explanation for the welding response, where hot cracking is attributed to the formation of a eutectic film. Calculations of the relative positions of the B, P, and C revealed that these atoms were less likely to cluster together in the presence of C, providing insight into the likely mechanisms behind the segregation behavior in this complex, multicomponent alloy.