Carbide precipitates are pivotal for strengthening steels and improving their resistance to hydrogen-embrittlement. Combining aberration-corrected high-resolution transmission electron microscopy and atom probe tomography, this work provides an atomic-scale evidence for the precipitation mechanism from embryo cluster to nano-precipitate in ferritic steels. Carbide precipitation begins with disc-shaped embryo clusters of several atom layers thick, which are fully coherent with the ferrite matrix. As the diameter increases, the embryo clusters grow into clusters that are analogous to the Guinier-Preston zone in Aluminium alloys. With further increase in size, particularly in thickness, clusters transform into a new phase with NaCl-structure.

Hot-deformation can refine grain size and change the dynamics of austenite-ferrite phase transformation during thermo-mechanical processing of microalloyed steels. Here, atom probe tomography has been used to characterize nanoscale precipitates and segregation in Ti-Mo microalloyed steels processed with and without hot-deformation at 890°C. It provides a comprehensive understanding of solute redistribution in both the grain interior and grain boundary regions. The results show that coarse (Ti, Mo)C precipitates are formed at grain boundaries, whereas fine precipitates are densely distributed in grain interiors, regardless of deformation conditions. Precipitate-free zones are developed near grain boundaries in the undeformed Ti-Mo steel, but absent in the hot-deformed steel. The elimination of precipitate-free zone in the hot-deformed steel is attributed to the high dislocation density and accelerated γ→α transformation caused by hot-deformation. The majority of Ti, Mo, and C atoms partition into (Ti, Mo)C precipitates, but Mn, Si, and Al atoms are mainly in solute state in ferrite matrix and segregation at grain boundaries. The hot-deformation significantly changes the C segregation at grain boundaries, but has little effect on other solute elements.

Site-specific transmission electron microscopy (TEM) and atom probe tomography (APT) have been used to study the Ti-Mo-Fe-C atomic clustering during interphase precipitation at the early stages of austenite to ferrite transformation in a Fe–0.03C–1.5Mn–0.2Si–0.29Cr–0.22Mo–0.05Ti–0.025Al (in wt.%) steel. We observed: (i) the formation of only clusters at the beginning of austenite-to-ferrite transformation and co-existence of numerous clusters along with interphase nano-scale precipitates during development of the transformation, (ii) the nano-precipitates were formed via transient metastable precipitates, and (iii) a cluster-hardening mechanism was proposed based on hardness test and APT and TEM characterization. It was suggested that the rapid hardening reactions, due to the formation of solute clusters, can accompany the early stages of solid solution decomposition, which precede the formation of interphase precipitates.

The presence of nanovoids in the vicinity of oxide particles in FeCrAl oxide dispersion strengthened (ODS) alloy has been identified. These nanovoids are inherent to the manufacturing route and remain quite resistant during heat treatments. Positron annihilation spectroscopy (PAS) experiments demonstrate that these nanovoids trap Cr inside thereby reducing the Cr-content in the matrix. This might lead to a delay in the alpha-alpha' phase separation process as observed by atom probe tomography (APT). (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

An Fe-0.26C-1.96Si-2Mn with 0.31Mo (wt%) steel was subjected to a novel thermomechanical processing route to produce fine ferrite with different volume fractions, bainite, and retained austenite. Two types of fine ferrites were found to be: (i) formed along prior austenite grain boundaries, and (ii) formed intragranularly in the interior of austenite grains. An increase in the volume fraction of fine ferrite led to the preferential formation of blocky retained austenite with low stability, and to a decrease in the volume fraction of bainite with stable layers of retained austenite. The difference in the morphology of the bainitic ferrite and the retained austenite after different isothermal ferrite times was found to be responsible for the deterioration of the mechanical properties. The segregation of Mn, Mo, and C at distances of 2–2.5 nm from the ferrite and retained austenite/martensite interface on the retained austenite/martensite site was observed after 2700 s of isothermal hold. It was suggested that the segregation occurred during the austenite-to-ferrite transformation, and that this would decrease the interface mobility, which affects the austenite-to-ferrite transformation and ferrite grain size.

The precipitation of nanoparticles plays a key role in determining the properties of many structural materials, and the understanding of their formation and stabilization mechanisms has been a long standing interest in the material field. However, the critical issues involving the group precipitation of various nanoparticles and their cooperative hardening mechanism remain elusive in the newly discovered Fe-based alloys with nanostructures. Here we quantitatively elucidate the nucleation mechanism, evolution kinetics and hardening effects of the group-precipitated nanoparticles in the Fe-Cu-Ni-Al-based alloys by atom probe tomography together with both first-principles and thermodynamic calculations. Our results provide the compelling evidence for two interesting but complex group precipitation pathways of nanoparticles, i.e., the Cu-rich and NiAl-based precipitations. The co-existence of the two precipitation pathways plays a key role in age hardening kinetics and ultimately enhances the hardening response, as compared to the single particle type of strengthening, therefore providing an effective new approach for strengthening materials for structural applications.

The precipitate transformation from NiAl-type to Ni2AlMn-type and its influence on the mechanical properties of the Fe-5Ni-1Al-xMn (x = 0, 1, 3, and 5 wt.%) alloys were studied thoroughly through a combination of three-dimensional atom probe tomography (APT), first-principles calculations and mechanical tests. APT reveals the precipitation of NiAl-type nanoparticles in the 0-3Mn alloys and the co-precipitation of fine NiAl-type and coarse Ni2AlMn-type nanoparticles in the 5Mn alloy, in which the Ni2AlMn-type nanoparticles are formed as a result of the coarsening of the NiAl-type ones. The first principles calculations indicate that the Ni2AlMn-type nanoparticles are energetically more favorable than the NiAl-type ones, but their nucleation and growth kinetics are relatively slow, resulting in the initial precipitation of the NiAl-type nanoparticles and the later precipitate transformation from NiAl-type to Ni2AlMn-type. A quantitative analysis of the strengthening mechanism reveals that although the Ni2AlMn-type nanoparticles have a higher strengthening efficiency than the NiAl-type ones, they make a minor contribution to the total strengthening effects due mainly to their small number density. In addition, the Ni2AlMn-type nanoparticles with a composition consistent with that of the Heusler phase are likely to promote the brittle cleavage and intergranular fracture of the 5Mn alloy, resulting in a significant decrease of the alloy ductility. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Surveillance samples of a low copper (nominally 0.05 wt.% Cu) forging and a higher copper (0.23 wt.% Cu) submerged arc weld from the R. E. Ginna reactor pressure vessel have been characterized by atom probe tomography (APT) after exposure to three levels of neutron irradiation, i.e., fluences of 1.7, 3.6 and 5.8 x 10(23) n.m(-2) (E > 1 MeV), and inlet temperatures of similar to 289 degrees C (similar to 552 degrees F). As no copper-enriched precipitates were observed in the low copper forging, and the measured copper content in the ferrite matrix was 0.04 +/- < 0.01 at.% Cu, after neutron irradiation to a fluence of 1.7 x 10(23) n.m(-3), this copper level was below the solubility limit. A number density of 2 x 10(22) m(-3) of Nie, Mne Si-enriched precipitates with an equivalent radius of gyration of 1.7 +/- 0.4 nm were detected in the sample. However, Cu-, Ni-, Mn-enriched precipitates were observed in specimens cut from different surveillance specimens from the same forging material in which the overall measured copper level was 0.08 +/- < 0.01 at.% (fluence of 3.6 x 10(23) n.m(-3)) and 0.09 +/- < 0.01 at.% Cu (fluence of 5.8 x 10(23) n.m(-3)). Therefore, these slightly higher copper contents were above the solubility limit of Cu under these irradiation conditions. A best fit of all the composition data indicated that the size and number density of the Cu-enriched precipitates increased slightly in both size and number density by additional exposure to neutron irradiation. High number densities of Cu-enriched precipitates were observed in the higher Cu submerged arc weld for all irradiated conditions. The size and number density of the precipitates in the welds were higher than in the same fluence forgings. Some Cu-enriched precipitates were found to have Ni-, Mn-Si-, and P-enriched regions on their surfaces suggesting a preferential nucleation site. Atom maps revealed P, Ni, and Mn segregation to, and preferential precipitation of, Cu-enriched precipitates over the surface of a grain boundary in the low fluence weld. Published by Elsevier B.V.

Evolution of microstructure and magnetic properties during annealing of a Cu-doped Fe75.3C7.0Si3.3B5.0P8.7Cu0.7 metallic glass were investigated via atom probe tomography. Cu segregation, which contains a rapid nucleation stage and a sluggish growth stage, was found to precede and may trigger the segregation of other elements and promote the following nanocrystallization. The nano-sized α-Fe precipitates have a composition close to that of the grain-orientated high-silicon steel, giving rise to the enhancement in the magnetization.

Here, a coordinated investigation of the elemental composition and morphology of ultra-fine-scale nanoparticles as a function of size within a variety of austenitic oxide dispersion-strengthened (ODS) steels is reported. Atom probe tomography was utilized to evaluate the elemental composition of these nanoparticles. Meanwhile, the crystal structures and orientation relationships were determined by high resolution transmission electron microscopy. The nanoparticles with sufficient size (>4 nm) to maintain a Y2Ti2-xO7-2x stoichiometry were found to have a pyrochlore structure, whereas smaller YxTiyOz nanoparticles lacked a well-defined structure. The size-dependent characteristics of the nanoparticles in austenitic ODS steels differ from those in ferritic/martensitic ODS steels. (c) 2016 Elsevier B.V. All rights reserved.

The microstructural evolution of DAFA26, an alumina-forming austenitic (AFA) stainless steel, was investigated during aging. The effect of aging at 750°C and 800°C on the growth of spherical γ’-Ni3(Al, Ti) particles present in the as-processed state was studied extensively using X-ray diffraction, microhardness testing, scanning electron microscopy, transmission electron microscopy, and atom probe tomography. The γ’ particles had a cube-on-cube orientation relationship with the matrix (i.e. ((010)m//(010)p, [100]m//[100]p)). The coarsening kinetics of γ’-Ni3Al particles were in agreement with the Lifshitz, Slyozof-Wagner theory. Coarse Laves phase particles were also present in the as-processed state, and during the aging process both smaller Laves phase precipitates and B2-NiAl precipitates formed on both the grain boundaries and in the matrix. The γ’ precipitates were determined to have the most impact on the room temperature hardness.

Advanced ultra-high strength steels are highly desirable for a wide range of engineering applications. Nanoscale co-precipitation strengthening in steels has received increasing attention in recent years and has become a new cornerstone for the development of advanced steels with superior combination of mechanical, welding, and irradiation properties for a new era. In this review, we highlight recent advances in computation-aided alloy design, nanostructural characterization, and unique properties of newly developed nanoscale co-precipitation-strengthened steels. In particular, our emphasis is on elucidating alloy design strategies, the co-precipitation mechanism, and cooperative evolution of multiple types of nanoparticles, and the correlation between nanostructures and bulk steel properties. Finally, future research areas for this class of nanostructured steels are critically discussed.

Nanostructured ferritic alloys (NFAs) are new generation materials for use in high temperature energy systems, such as nuclear fission or fusion reactors. However, joining these materials is a concern, as their unique microstructure is destroyed by traditional liquid-state welding methods. The microstructural evolution of a friction stir welded 14YWT NFA was investigated by atom probe tomography, before and after a post-weld heat treatment (PWHT) at 1123K. The particle size, number density, elemental composition, and morphology of the titanium-yttrium-oxygen-enriched nanoclusters (NCs) in the stir and thermally-affected zones were studied and compared with the base metal. No statistical difference in the size of the NCs was observed in any of these conditions. After the PWHT, increases in the number density and the oxygen enrichment in the NCs were observed. Therefore, these new results provide additional supporting evidence that friction stir welding appears to be a viable joining technique for NFAs, as the microstructural parameters of the NCs are not strongly affected, in contrast to traditional welding techniques.

The microstructural changes of a tempered F82H ferritic–martensitic steel following He implantation at 60 and 500°C have been examined by transmission electron microscopy (TEM) and atom probe tomography (APT). After irradiation at 500°C, numerous He bubbles were formed throughout the matrix, whereas after irradiation at 60°C, no bubbles were seen to form in the matrix. In both irradiations, He bubbles were observed to have formed within large carbide precipitates, determined by APT compositional analysis to be M23C6. The observed preferential He bubble formation in carbides during low temperature He irradiation occurs as a result of the diffusing He being trapped in the carbide due to the strong He–C bond. As the He concentration increases in the carbide due to trapping, He bubbles are formed.

In order to achieve energy conversion efficiencies of >50 pct for steam turbines/boilers in power generation systems, materials are required that are both strong and corrosion-resistant at >973 K (700 °C), and economically viable. Austenitic steels strengthened with Laves phase, NiAl and Ni 3 Al precipitates, and alloyed with aluminum to improve oxidation resistance, are potential candidate materials for these applications. The microstructure and microchemistry of recently developed alumina-forming austenitic stainless steels have been characterized by scanning electron microscopy, transmission electron microscopy, and synchrotron X-ray diffraction. Different thermo-mechanical treatments were performed on these steels to improve their mechanical performance. These reduced the grain size significantly to the nanoscale (~100 nm) and the room temperature yield strength to above 1000 MPa. A solutionizing anneal at 1473 K (1200 °C) was found to be effective for uniformly redistributing the Laves phase precipitates that form upon casting.

Permanent magnets (PM) with rear-earth (RE) elements, such as dysprosium (Dy) and neodymium (Nd), have been widely used in motors and generators for hybrid electronic vehicles and wind turbines.[1] Concern over supply and price of the RE alloys has stimulated the search for alternative PMs.[1,2] One of the attractive non-RE PMs is alnico, a family of magnetic alloys composed primarily of Al, Ni, Co and Fe, with excellent magnetic stability at high temperature. The magnetic properties of alnico alloys are closely related to the control of the spinodal decomposition (SD) into an FeCo-rich (α1 phase) hard magnetic phase and a non-magnetic NiAlrich phase (α2 phase). Improving alnico will require subtle changes in chemistry and processing to reduce the diameter of the magnetic phase while maintaining its volume fraction. Alnico 9 is the current available commercial alnico alloy with the highest energy product (BH)max, which is both grain aligned and spinodally decomposed with an applied magnetic field.[1] Needed improvements can only be achieved through a better understanding of nanostructuring during SD. This study focuses on structural characterization of alnico 9 alloy from Arnold Magnetic Technologies. Atom-probe tomography (APT) and a combination of TEM techniques, including diffraction contrast TEM, high resolution transmission electron microscopy (HREM), high-angle annular-dark-field (HAADF) scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy, and Lorentz microscopy, were used.

A new class of advanced structural materials, based on the Fe-O-vacancy system, has exceptional resistance to high-temperature creep and excellent tolerance to extremely high-dose radiation. Although these remarkable improvements in properties compared to steels are known to be associated with the Y-Ti-O-enriched nanoclusters, the roles of vacancies in facilitating the nucleation of nanoclusters are a long-standing puzzle, due to the experimental difficulties in characterizing vacancies, particularly in-situ while the nanoclusters are forming. Here we report an experiment study that provides the compelling evidence for the presence of significant concentrations of vacancies in Y-Ti-O-enriched nanoclusters in a nanostructured ferritic alloy using a combination of state-of-the-art atom-probe tomography and in situ small angle neutron scattering. The nucleation of nanoclusters starts from the O-enriched solute clustering with vacancy mediation. The nanoclusters grow with an extremely low growth rate through attraction of vacancies and O:vacancy pairs, leading to the unusual stability of the nanoclusters.

An oxide dispersion-strengthened (ODS) 316 steel was developed to simultaneously provide the advantages of ODS steels in mechanical strength and radiation tolerance as well as the excellence of austenitic steels in creep performance and corrosion resistance. The precipitate phases within the austenite matrix were identified by the combined techniques of atom probe tomography (APT), scanning transmission electron microscopy equipped with electron dispersive X-ray spectroscopy (STEM-EDS), and synchrotron wide-angle and small-angle X-ray scattering (WAXS and SAXS). Coarse TiN, hexagonal YAlO3 and orthorhombic YAlO3 precipitates were found along with fine Y–Ti–O nanoparticles. In situ WAXS experiments were performed at room and elevated temperatures to examine the size effect on the load partitioning phenomenon for TiN, hexagonal YAlO3 and Y2Ti2O7 phases. In addition, the dislocation density evolution throughout the tensile tests was analyzed by the modified Williamson–Hall method and confirmed by transmission electron microscopy (TEM) observations, revealing the difference in plasticity at various temperatures.

Neutron irradiation of structural materials introduces He through (n, α) transmutation, which degrades the mechanical properties through swelling, blistering, and He embrittlement [1]. He bubbles are trapped on grain boundaries and dislocations, degrading properties; recent studies show that trapping on the surfaces of nanoclusters and larger precipitates [2] may mitigate materials degradation. Hence, it is important to investigate He bubble distributions in materials used in current reactors, such as reduced activated ferritic-martensitic (RAFM) steels. (S)TEM and APT are powerful tools to study microstructural features and provide complementary information. Correlative APT-TEM studies [3] have illustrated the direct comparison of the same features visible via each technique, which can provide more reliable spatial and chemical information. Here, preliminary results correlating TEM and APT of helium bubbles are shown.

The solid-solution hardening potential of the refractory elements rhenium, tungsten and molybdenum in the matrix of single-crystal nickel-based superalloys was experimentally quantified. Single-phase alloys with the composition of the nickel solid-solution matrix of superalloys were cast as single crystals, and tested in creep at 980°C and 30–75MPa. The use of single-phase single-crystalline material ensures very clean data because no grain boundary or particle strengthening effects interfere with the solid-solution hardening. This makes it possible to quantify the amount of rhenium, tungsten and molybdenum necessary to reduce the creep rate by a factor of 10. Rhenium is more than two times more effective for matrix strengthening than either tungsten or molybdenum. The existence of rhenium clusters as a possible reason for the strong strengthening effect is excluded as a result of atom probe tomography measurements. If the partitioning coefficient of rhenium, tungsten and molybdenum between the γ matrix and the γ′ precipitates is taken into account, the effectiveness of the alloying elements in two-phase superalloys can be calculated and the rhenium effect can be explained.