Nanostructured Ferritic Alloys Research Articles

Microstructural stability of ODS Fe–14Cr (–2W–0.3Ti) steels after simultaneous triple irradiation

Simultaneous triple-ion beam irradiation experiments with Fe5+, He+ and H+ ions were performed to simulate fusion damage on two nanostructured ferritic alloys with nominal composition Fe–14Cr–0.3Y2O3 and Fe–14Cr–2W–0.3Ti–0.3Y2O3. Samples were irradiated at 600°C to an estimated dose of ∼ 30dpa, 600appm He, 1500appm H, and the effects on the microstructure of these alloys investigated by analytical transmission electron microscopy. The results reveal the development of nanovoids, or small bubbles, undetected in the unirradiated samples, and a virtual compositional stability of the dispersion. Nevertheless, upon irradiation the measured size distribution indicates a slight growth of those dispersoids having the smaller sizes.

Tensile deformation and fracture properties of a 14YWT nanostructured ferritic alloy

A new larger heat of a 14YWT nanostructured ferritic alloy (NFA), FCRD NFA-1, was synthesized by ball milling FeO and argon atomized Fe-14Cr-3W-0.4Ti-0.2Y (wt%) powders, followed by hot extrusion, annealing and cross rolling to produce an ≈10mm-thick plate. NFA-1 contains a bimodal size distribution of pancake-shaped, mostly very fine scale, grains. The as-processed plate also contains a large population of microcracks running parallel to its broad surfaces. The small grains and large concentration of Y–Ti–O nano-oxides (NOs) result in high strength up to 800°C. The uniform and total elongations range from ≈1–8%, and ≈10–24%, respectively. The strength decreases more rapidly above ≈400°C and deformation transitions to largely viscoplastic creep by ≈600°C. While the local fracture mechanism is generally ductile-dimple microvoid nucleation, growth and coalescence, perhaps the most notable feature of tensile deformation behavior of NFA-1 is the occurrence of periodic delamination, manifested as fissures on the fracture surfaces.

Study of spark plasma sintered nanostructured ferritic steel alloy with silicon carbide addition

Pure nanostructured ferritic steel alloy (NFA) and NFA–silicon carbide (SiC) composites with different compositions (97.5vol% NFA-2.5vol% SiC and 95vol% NFA-5vol% SiC) have been sintered by spark plasma sintering (SPS) and systematically investigated based on XRD, SEM, density, Vickers hardness, and nano-hardness. Minor γ-Fe phase formation from the main α-Fe matrix occurs in pure NFA between the sintering temperature of 950°C and 1000°C. However, this is hindered in the NFA-SiC composite sintering. Densities for both the pure NFA and the NFA-SiC composites increase with the sintering temperature but decrease with the SiC content. The NFA-SiC composites have higher porosity than pure NFA under the same sintering condition. All the samples have the average grain sizes between 6µm and 8µm. Vickers hardness of the pure NFA and NFA-SiC composites is related to density and phase composition. By estimation, the 97.5vol% NFA-2.5vol% SiC composite sample has the maximum yield strength of 3.14±0.18GPa. Nano-hardness of the NFA-SiC composite is degraded by diffusion and reaction between NFA and SiC. The addition of SiC decreases the elastic modulus of the NFA-SiC composites.

Optimization of process parameters for spark plasma sintering of nano-structured ferritic Fe-18Cr-2Si alloy

Spark plasma sintering (SPS) process parameters were optimized in terms of densification, microhardness and corrosion resistance to develop a nano-structured Fe-18Cr-2Si ferritic alloy. The powders were mechanically alloyed (MA) in a planetary ball mill and subsequently SPS was carried out under different processing conditions. Powders and sintered samples were characterized using X-Ray Diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). XRD phase evolution studies on ball milled powders confirmed the formation of a crystallized alloy after 100h milling. XRD of sintered alloy confirmed the development of a nano-structured ferritic alloy with a crystallite size in the range of 20–40nm. Maximum densification of 96.0%, Vickers hardness of 710HV and pitting potential (Epit=−430mVSCE) was achieved at a sintering temperature of 1100°C, for 15min holding time, at 50°C/min of heating rate and at an applied pressure of 60MPa.

Ab initio investigation of helium in Y2Ti2O7: Mobility and effects on mechanical properties

Oxide nanoclusters (NCs) in nanostructured ferritic alloys (NFAs) are known to be efficient trapping sites for the transmutation product helium. In this study, the migration barriers and potential energy surfaces of helium in Y2Ti2O7 are presented to explain the mobility of helium through oxide NCs and shed light on the accumulation of helium and the trapping mechanisms of the oxides. A complex tunnel-shaped potential energy surface is identified and gives rise to relatively large migration barriers. Subsequently, the effect of helium accumulation on the mechanical properties of Y2Ti2O7 oxide nanoclusters is investigated and it is shown that the mechanical properties of the oxide do not significantly degrade as helium accumulates.

The crystal structure, orientation relationships and interfaces of the nanoscale oxides in nanostructured ferritic alloys

The capability of nanostructured ferritic alloys (NFAs) to manage high levels of transmutation product helium will help resolve one of the grand challenges to transforming the promise of C-free fusion energy into a reality. NFAs are dispersion strengthened by an ultrahigh density of Y-Ti-O nano-oxides (NOs), which result in both high strength and temperature limits, as well as unique irradiation tolerance. Here, aberration-corrected high-resolution transmission electron microscopy was used to characterize the NOs in four NFA conditions, including following severe deformation and extreme neutron radiation exposure. Fast Fourier Transform analysis of focal series images revealed the NO crystal structure, including the smallest at < 2 nm in diameter, to be Y2Ti2O7 pyrochlore in all cases, consistent with both exit wave analysis and scanning transmission Z-contrast imaging of the atomic columns in a larger feature. The faceted NOs exhibit a quasi-epitaxial orientation relationship with the ferrite matrix: [110]YTO||[100]Fe and [001]YTO||[010]Fe, forming a 5 × 7 near coincidence site interface. The NOs also exhibit size-dependent strains in both the oxide and matrix ferrite phases.

First-principles calculation of formation energies and electronic structures of hydrogen defects at tetrahedral and octahedral interstitial sites in pyrochlore-type Y2Ti2O7 oxide

The formation energies of hydrogen (H) defects at tetrahedral and octahedral interstitial sites in a pyrochlore-type yttrium–titanium oxide (Y2Ti2O7) were calculated using density functional theory. The results were compared with the corresponding energies of H and helium (He) defects in iron (Fe) with a body-centered cubic (bcc) crystal structure and Y2Ti2O7 interstitial and substitutional sites in the matrix. The formation energy of a H defect at a tetrahedral site in Y2Ti2O7 (2.87 eV), which is lower than that at an octahedral site (3.01 eV), was larger than the corresponding energies (0.11, 0.24, and 0.41 eV) at tetrahedral interstitial, octahedral interstitial, and substitutional sites in Fe, respectively. This result indicates that H atoms in nanostructured ferritic alloys, constructed by only bcc Fe containing nanometer-sized Y2Ti2O7 nanofeatures, prefer to occupy tetrahedral interstitial sites in Fe, whereas He atoms prefer to occupy octahedral interstitial sites in Y2Ti2O7. The electron-density distribution and density of states of interstitial H in Y2Ti2O7 reveal an essentially attractive chemical interaction resulting from charge transfers from neighboring O2− anions and surrounding Y3+ and Ti4+ cations, whereas those of interstitial He reveal a repulsive interaction. Therefore, the doping H and He atoms prefer to occupy smaller tetrahedral and larger octahedral sites, respectively.

Effect of tube processing methods on the texture and grain boundary characteristics of 14YWT nanostructured ferritic alloys

Texture and microstructure of tubes and plates fabricated from a nanostructured ferritic alloy (14YWT), produced either by spray forming followed by hydrostatic extrusion (Process I) or hot extrusion and cross-rolling a plate followed by hydrostatic tube extrusion (Process II) have been characterized in terms of their effects on texture and grain boundary character. Hydrostatic extrusion results in a combination of plane strain and shear deformations which generate low intensity α- and γ-fiber components of {001}<110> and {111}<110> together with a weak ζ-fiber component of {011}<211> and {011}<011>. In contrast, multi-step plane strain deformation by hot extrusion and cross-rolling of the plate leads to a strong texture component of {001}<110> together with a weaker {111}<112> component. Although the total strains are similar, shear dominated deformation leads to much lower texture indexes compared to plane strain deformations. Further, the texture intensity decreases after hydrostatic extrusion of the alloy plate formed by plane strain deformation, due to a lower number of activated slip systems during shear dominated deformation. Notably, hot extruded and cross-rolled plate subjected to plane strain deformation to ~50% engineering strain creates only a modest population of low angle grain boundaries, compared to the much larger population observed following the combination of plane strain and shear deformation of ~44% engineering strain resulting from subsequent hydrostatic extrusion.

Energetic Study of Helium Cluster Nucleation and Growth in 14YWT through First Principles.

First principles calculations have been performed to energetically investigate the helium cluster nucleation, formation and growth behavior in the nano-structured ferritic alloy 14YWT. The helium displays strong affinity to the oxygen:vacancy (O:Vac) pair. By investigating various local environments of the vacancy, we find that the energy cost for He cluster growth increases with the appearance of solutes in the reference unit. He atom tends to join the He cluster in the directions away from the solute atoms. Meanwhile, the He cluster tends to expand in the directions away from the solute atoms. A growth criterion is proposed based on the elastic instability strain of the perfect iron lattice in order to determine the maximum number of He atoms at the vacancy site. We find that up to seven He atoms can be trapped at a single vacancy. However, it is reduced to five if the vacancy is pre-occupied by an oxygen atom. Furthermore, the solute atoms within nanoclusters, such as Ti and Y, will greatly limit the growth of the He cluster. A migration energy barrier study is performed to discuss the reduced mobility of the He atom/He cluster in 14YWT.

Investigation of helium at a Y2Ti2O7 nanocluster embedded in a BCC Fe matrix

Nanostructured ferritic alloys (NFAs) are prime candidates for structural and first wall components of fission and fusion reactors. The main reason for this is their ability to effectively withstand high concentrations of the transmutation product helium. A high number density of oxide nanoclusters dispersed throughout a BCC Fe matrix act as trapping sites for helium and prevent its eventual delivery to high risk nucleation sites. The current study uses density functional theory to investigate the helium trapping mechanisms at the boundary between BCC iron and Y2Ti2O7, a common stoichiometry of the oxide nanoclusters in NFAs. The investigation is carried out on a structure matched oxide nanocluster that is embedded within a BCC Fe supercell. Investigation of the electronic structure and a mapping of the potential energy landscape reveals that the localized iono-covalent bonds present within the oxides create a potential energy-well within the metallically bonded BCC Fe matrix, so that trapping of helium at the oxide nanocluster is thermodynamically and kinetically favorable.

First-principles investigation of helium in Y2O3

Density functional theory has been used to investigate the relative stability, migration barriers and potential energy surfaces of helium in Y2O3 in order to provide insight to the trapping mechanisms of helium at the oxide-Fe interface in nanostructured ferritic alloys. The helium interstitial sites have been located and their relative stability in Y2O3 is determined from the solution energy. The interaction of helium with the oxide has been investigated using the charge density and electron localization function. The migration barriers have been calculated and further investigated using potential energy surfaces computed on various lattice planes. Helium trapping has been attributed to very large diffusion barrier energies, especially close to oxygen atoms.

On nano-oxide coarsening kinetics in the nanostructured ferritic alloy MA957: A mechanism based predictive model

Nanostructured ferritic alloys are dispersion strengthened and imbued with remarkable neutron irradiation damage tolerance by an ultrahigh density of nano-oxide precipitates that must remain stable under prolonged high-temperature service. Here, previously reported long-time and high-temperature thermal aging data is used to derive a simple, but physically based and quantitatively predictive nano-oxide coarsening model using least square fits to equations representing different underlying mechanisms. A dislocation pipe diffusion model adequately fit the data with a dislocation-detachment climb mechanism providing the best fit. This model is consistent with microstructural observations and recently reported thermo-kinetic nano-oxide nucleation, growth and coarsening simulations that rationalize the underlying physics of coarsening. It is predicted that the nano-oxides will remain fully stable up to 900°C. This remarkable coarsening resistance is due to the extremely low matrix solubility and diffusion rate of Y in equilibrium with Y2Ti2O7 that activates lower resistant paths along dislocations.

Effect of friction stir welding and post-weld heat treatment on a nanostructured ferritic alloy

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 ferrite/oxide interface and helium management in nano-structured ferritic alloys from the first principles

Nano-size Y-Ti-oxides are largely responsible for the extraordinary mechanical properties and irradiation tolerance of nano-structured ferritic alloys (NFAs). Here we report a theoretical study to assess the characters and possible roles of the ferrite/oxide interface in managing neutron transmutation product helium in NFAs. Using one observed cube-on-cube orientation relationship, various candidate structures of the ferrite/Y2Ti2O7 interfaces were constructed and the associated energies were carefully evaluated. The interface phase diagram is obtained by expressing the energy as a function of temperature and internal oxygen activity (expressed in terms of oxygen partial pressure). The oxide interfaces are predicted to be Y/Ti-rich at thermodynamic equilibrium for the wide temperature range of interest. Vacancy formation energies are lower and helium segregates to the interfaces, in preference to the iron matrix and grain boundaries. Combined with our previous results on bulk-phase Y2Ti2O7, the profound implications of nano-oxides to helium management in NFAs are discussed.

Small Angle Neutron Scattering Study on the Microstructural Evolution of Nano-Structured Ferritic Alloys with Different Mechanical Alloying Times

Small Angle Neutron Scattering Study on the Microstructural Evolution of Nano-Structured Ferritic Alloys with Different Mechanical Alloying Times

Microstrcutural Stability of Nanostructured Ferritic Alloys (NFA)

Microstrcutural Stability of Nanostructured Ferritic Alloys (NFA)

The role of processing route on the microstructure of 14YWT nanostructured ferritic alloy

Nanostructured ferritic alloys have outstanding high temperature creep properties and enhanced tolerance to radiation damage over conventional ferritic alloys. To achieve these properties, NFAs are fabricated by mechanical alloying of metallic and yttria powders. Atom probe tomography has demonstrated that milling times of at least 40 h are required to produce a uniform distribution of solutes in the flakes. After milling and hot extrusion, the microstructure consists of α-Fe, high number densities of Ti–Y–O-vacancy-enriched nanoclusters, and coarse Y2Ti2O7 and Ti(O,C,N) precipitates on the grain boundaries. In contrast, the as-cast condition consists of α-Fe with 50–100 μm irregularly-shaped Y2Ti2O7 pyrochlore precipitates with smaller embedded precipitates with the Y3Al5O12 (yttrium–aluminum garnet) crystal structure indicating that this traditional processing route is not a viable approach to achieve the desired microstructure. The nano-hardnesses were also substantially different, i.e., 4 and 8 GPa for the as-cast and as-extruded conditions, respectively. These variances can be explained by the microstructural differences and the effects of the high vacancy content introduced by mechanical alloying, and the strong binding energy of vacancies with O, Ti, and Y atoms that retard diffusion.

Vacancy-controlled ultrastable nanoclusters in nanostructured ferritic alloys.

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.

Thermodynamic and kinetic modeling of oxide precipitation in nanostructured ferritic alloys

The mechanical properties and radiation tolerance of nanostructured ferritic alloys rely on a dense population of nanometer-scale Y–Ti oxides. The stability of these nano-oxides during extended service is critical in high temperature applications. Here, a model framework is developed for the thermodynamics and kinetics of Y–Ti oxide nucleation, growth and coarsening. The model, which is based upon available thermodynamic and kinetic data as well as key density functional theory calculations, shows that nano-oxide nucleation and growth are highly driven and that pipe diffusion is the dominant mode of their coarsening, in agreement with previous analyses of experimental high temperature data. The model predicts that the nano-oxides are thermally stable for 80 or more years below 1175K. This analysis also provides insights into the effect of O and Ti on nano-oxide sizes, and on optimization of alloy microstructure.

When will Low-Contrast Features be Visible in a STEM X-Ray Spectrum Image?

When will a small or low-contrast feature, such as an embedded second-phase particle, be visible in a scanning transmission electron microscopy (STEM) X-ray map? This work illustrates a computationally inexpensive method to simulate X-ray maps and spectrum images (SIs), based upon the equations of X-ray generation and detection. To particularize the general procedure, an example of nanostructured ferritic alloy (NFA) containing nm-sized Y2Ti2O7 embedded precipitates in ferritic stainless steel matrix is chosen. The proposed model produces physically appearing simulated SI data sets, which can either be reduced to X-ray dot maps or analyzed via multivariate statistical analysis. Comparison to NFA X-ray maps acquired using three different STEM instruments match the generated simulations quite well, despite the large number of simplifying assumptions used. A figure of merit of electron dose multiplied by X-ray collection solid angle is proposed to compare feature detectability from one data set (simulated or experimental) to another. The proposed method can scope experiments that are feasible under specific analysis conditions on a given microscope. Future applications, such as spallation proton-neutron irradiations, core-shell nanoparticles, or dopants in polycrystalline photovoltaic solar cells, are proposed.