Nanostructured Ferritic Alloys Research Articles

Helium Interaction with Y2Ti2O7: A First Principles Study

ABSTRACTHelium embrittlement poses a great threat to materials used in both fusion and fissionreactor systems due to (n,α) transmutation reactions. Because of this, materials capable of moderating the helium content reaching grain boundaries and voids must be developed and improved to prevent catastrophic failure of reactor materials. Nanostructured ferritic alloys (NFAs) have shown great promise in preventing helium embrittlement due to the large number density of nanoscale precipitates acting as trapping sites for helium clusters and helium bubbles. In this study, we present density functional theory calculations on the interaction of helium with nanoscale precipitates found in NFAs as a preliminary study to furthering our understanding of the energetic mechanisms causing the precipitates to act as trapping sites for helium.

Response of nanostructured ferritic alloys to high-dose heavy ion irradiation

A latest-generation aberration-corrected scanning/transmission electron microscope (STEM) is used to study heavy-ion-irradiated nanostructured ferritic alloys (NFAs). Results are presented for STEM X-ray mapping of NFA 14YWT irradiated with 10MeV Pt to 16 or 160 dpa at −100°C and 750°C, as well as pre-irradiation reference material. Irradiation at −100°C results in ballistic destruction of the beneficial microstructural features present in the pre-irradiated reference material, such as Ti–Y–O nanoclusters (NCs) and grain boundary (GB) segregation. Irradiation at 750°C retains these beneficial features, but indicates some coarsening of the NCs, diffusion of Al to the NCs, and a reduction of the Cr–WGB segregation (or solute excess) content. Ion irradiation combined with the latest-generation STEM hardware allows for rapid screening of fusion candidate materials and improved understanding of irradiation-induced microstructural changes in NFAs.

Helium solubility and bubble formation in a nanostructured ferritic alloy

The response of a nanostructured ferritic alloy to He implantation and post-irradiation annealing (PIA) at 750°C was characterized by atom probe tomography and transmission electron microscopy. The supersaturated He concentration in the ferrite at a dose of ∼2.1 displacements per atom was similar for the as-implanted, 75±7 appm, and a 10h PIA treatment, 71±7 appm, but decreased to 38±2 appm after a 100h PIA treatment. Approximately 91–97% of the He bubbles were present as isolated bubbles in the ferrite and ∼1–5% on the surface of the nanoclusters in the ferrite. The remainder were associated with the grain boundaries with a small fraction on the surface of Ti(N,C,O) precipitates. Their average size and number density were similar for the as-implanted and 10h PIA treatment with a small increase in the size and a significant increase in the number density after the 100h PIA treatment. Swelling in the high dose region increased from ∼1% in the as-implanted and 10h PIA conditions to ∼5% after the 100h PIA treatment but the estimated number of He atoms per unit volume in the He bubbles decreased by an order of magnitude. Number densities increased from ∼8×1023m−3 in the as-implanted to ∼15×1023m−3 in the 10h PIA condition, with little change (to ∼12×1023m−3) in the 100h PIA condition. This trend may indicate nucleation of new bubbles up to 10h, with growth and possible consumption of the smaller bubbles between 10 and 100h.

Evidence for core–shell nanoclusters in oxygen dispersion strengthened steels measured using X-ray absorption spectroscopy

Nanostructured ferritic alloys (NFA) dispersion strengthened by an ultra high density of Y–Ti–O enriched nano-features (NF) exhibit superior creep strength and the potential for high resistance to radiation damage. However, the detailed character of the NF, that precipitate from solid solution during hot consolidation of metallic powders mechanically alloyed with Y2O3, are not well understood. In order to clarify the nature of the NF, X-ray absorption spectroscopy (XAS) technique, including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were used to characterize the local structure of the Ti and Y atoms in both NFA powders and consolidated alloys. The powders were characterized in the as-received, as-milled and after annealing milled powders at 850, 1000 and 1150°C. The consolidated alloys included powders hot isostatic pressed (HIPed) at 1150°C and commercial vendor alloys, MA957 and J12YWT. The NFA XAS data were compared various Ti and Y-oxide standards. The XANES and EXAFS spectra for the annealed and HIPed powders are similar and show high temperature heat treatments shift the Y and Ti to more oxidized states that are consistent with combinations of Y2Ti2O7 and, especially, TiO. However, the MA957 and J12YWT and annealed–consolidated powder data differ. The commercial vendor alloys results more closely resemble the as-milled powder data and all show that a significant fraction of substitutional Ti remains dissolved in the (BCC) ferrite matrix.

The Effect of Zr Incorporation Caused by Ball Abrasion in a Milled Fe-Y2O3 Model Alloy

Y-O nanoparticles which are homogeneously distributed in the matrix can improve the thermal properties of steels. Several studies of mechanically alloyed steels showed that especially Y-Ti-O particles can cause a further improvement of the mechanical properties at elevated temperatures. It is also assumed that an addition of Zr instead of Ti may have a similar or even stronger effect. This study presents a new way of producing nanostructured ferritic alloys as Zr is incorporated by attrition of yttrium-stabilized zirconia balls during milling. Additionally, the effect of Zr incorporation is demonstrated as well as the particle size distribution of the Y-Zr-O nanoparticles analyzed by transmission electron microscopy. This is compared to a specimen milled with common steel balls. Atom probe tomography and transmission electron microscopy show that the incorporated zirconia lowers the minimum particle size and causes a finer particle distribution. This particle refinement causes a higher hardness after hipping.

Process development for 9Cr nanostructured ferritic alloy (NFA) with high fracture toughness

This article is to summarize the process development and key characterization results for the newly-developed Fe–9Cr based nanostructured ferritic alloys (NFAs) with high fracture toughness. One of the major drawbacks from pursuing ultra-high strength in the past development of NFAs is poor fracture toughness at high temperatures although a high fracture toughness is essential to prevent cracking during manufacturing and to mitigate or delay irradiation-induced embrittlement in irradiation environments. A study on fracture mechanism using the NFA 14YWT found that the low-energy grain boundary decohesion in fracture process at a high temperature (>200°C) resulted in low fracture toughness. Lately, efforts have been devoted to explore an integrated process to enhance grain bonding. Two base materials were produced through mechanical milling and hot extrusion and designated as 9YWTV-PM1 and 9YWTV-PM2. Isothermal annealing (IA) and controlled rolling (CR) treatments in two phase region were used to enhance diffusion across the interfaces and boundaries. The PM2 alloy after CR treatments showed high fracture toughness (KJQ) at represented temperatures: 240–280MPa√m at room temperature and 160–220MPa√m at 500°C, which indicates that the goal of 100MPa√m over possible nuclear application temperature range has been well achieved. Furthermore, it is also confirmed by comparison that the CR treatments on 9YWTV-PM2 result in high fracture toughness similar to or higher than those of the conventional ferritic–martensitic steels such as HT9 and NF616.

Advanced oxide dispersion strengthened and nanostructured ferritic alloys

Nanostructured ferritic alloy is a subcategory of oxide dispersion strengthened steels intended for advanced reactor applications. The complex ultrafine grained microstructure of an advanced nanostructured ferritic alloy, as determined by electron microscopy and atom probe tomography, is summarised. Three distinct populations of precipitates were observed: 20–50 nm Ti(N,O,C), 5–10 nm diameter Y2Ti2O7/Y2TiO5 and 1–4 nm diameter Ti,Y,O enriched nanoclusters. The first two populations were predominantly located along grain boundaries together with Cr, W and C segregation. A dense population of nanoclusters was observed both in the grain interior as well as on the grain boundaries. These nanoclusters are highly tolerant to high dose irradiation at elevated temperatures.

Multi-detector STEM-EDS mapping of ion-irradiated nanostructured ferritic alloys

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

A Correlated APT and TEM Approach to Understand Nanostructured Ferritic Alloys

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

The anatomy of grain boundaries: Their structure and atomic-level solute distribution

The full macroscopic parameters of grain boundaries in a nanostructured ferritic alloy have been experimentally measured. A new atom-probe-tomography-based method determines the five degrees of freedom of the orientation relationship of the adjacent grains, and the local variations in the habit plane and solute excesses for tungsten and chromium across the grain boundary with a spatial resolution of up to 1 nm × 1 nm. The method also distinguishes ultrafine precipitates and ferrite–ferrite regions for a full description of the grain boundary.

Fabrication and Characterization of Naturally Selected Epitaxial Fe-{111} Y2Ti2O7 Mesoscopic Interfaces: Some Potential Implications to Nano-Oxide Dispersion-Strengthened Steels

The smallest features of ≈2 to 3 nm in nanostructured ferritic alloys (NFA), a variant of oxide dispersion-strengthened steels, include the Y2Ti2O7 complex oxide cubic pyrochlore phase. The interface between the bcc Fe-Cr ferrite matrix and the fcc nanometer-scale Y2Ti2O7 plays a critical role in the stability, strength, and damage tolerance of NFA. To complement other characterization studies of the actual nanofeatures (NF) themselves, mesoscopic interfaces were created by electron beam deposition of a thin Fe layer on a 5 deg miscut {111} Y2Ti2O7 bulk single crystal surface. While the mesoscopic interfaces may differ from those of the embedded NF, the former facilitate characterization of controlled interfaces, such as interactions with point defects and helium. The Fe-Y2Ti2O7 interfaces were studied using scanning electron microscopy, including electron backscatter diffraction, atomic force microscopy, X-ray diffraction, and transmission electron microscopy (TEM). The polycrystalline Fe layer has two general orientation relationships (OR) that are close to (a) the Nishiyama–Wasserman (NW) OR \( \left\{ {110} \right\}_{\text{Fe}} ||\left\{ {111} \right\}_{{{\text{Y}}_{2} {\text{Ti}}_{2} {\text{O}}_{7} }} \) and \( \left\langle {100} \right\rangle_{\text{Fe}} ||\left\langle {110} \right\rangle_{{{\text{Y}}_{2} {\text{Ti}}_{2} {\text{O}}_{7} }} \) and (b) \( \left\{ {100} \right\}_{\text{Fe}} ||\left\{ {111} \right\}_{{{\text{Y}}_{2} {\text{Ti}}_{2} {\text{O}}_{7} }} \) and \( \left\langle {100} \right\rangle_{\text{Fe}} ||\left\langle {110} \right\rangle_{{{\text{Y}}_{2} {\text{Ti}}_{2} {\text{O}}_{7} }} \). High-resolution TEM shows that the NW interface is near-atomically flat, while the {100}Fe grains are an artifact associated with a thin oxide layer. However, the fact that there is still a Fe-Y2Ti2O7 OR is significant. No OR is observed in the presence of a thicker oxide layer.

High temperature deformation mechanisms of nano-structured ferritic alloys in the context of internal variable theory of inelastic deformation

The stress relaxation behavior of 14YWT and ODS-Eurofer97 was examined within the framework of an internal-variable theory of inelastic deformation. Stress versus strain rate curves obtained by stress relaxation tests for 14YWT were described well by the equations for grain-matrix deformation while those of ODS-Eurofer97 were fitted with the combined curves of grain matrix deformation and grain boundary sliding. The sudden drop of total elongation of 14YWT was discussed in light of fracture surface observations. Grain boundary decohesion at 900°C was identified.

Formation of Nanocale Oxide Particles and Evaluation of Threshold Stress of Fe-9Cr-0.2Ti-0.3Y<sub>2</sub>O<sub>3</sub> Nanostructured Ferritic Alloy

Nanostructured ferritic alloys (NFA) of Fe-9Cr-0.2Ti-0.3Y2O3 (in mass) incorporating nanoscale oxide particles, was produced by mechanical milling (MM) followed by hot pressing (HP). The formation process of the nanoscale oxide particles were structurally characterized at each step of the elaboration processes by means of transmission electron microscope (TEM). The observations of structure of the mixed powders and the nanoscale oxide particles in the NFA after MM indicated that the initial powders, coupled with the original yttria powders, get fractured by severe plastic deformation and ultrafine bcc grains (~20 nm) of the matrix, and Y2O3 nanocrystals with irregular edges are formed during MM. The addition of titanium (Ti) promotes the formation of amorphous phase of [YMamorphous during MM. Microstructure of the NFA observed by TEM exhibited a very fine structure of nanostructured grains in which large number of nanoscale oxide particles were distributed after HP process. HRTEM observations of partially and fully crystallized oxide nanoparticles in the matrix of NFA gain an insight into the formation mechanism of nanoscale oxide particles in HP process. The formation process of nanoscale oxide particles consist of the reaction between Y2O3 fragments and titanium (Ti) and crystallization of [YMamorphous.Threshold stress which can quantitatively shows the effect of dispersion strengthening was carefully evaluated on the basis of higher magnified images of the nanoscale oxide particles. Different values of threshold stress were obtained due to the various dispersions of the nanoscale oxide particles within different areas. That may be the reason why the threshold stress cannot be clearly obtained by the results of creep tests.

Simple pair-wise interactions for hybrid Monte Carlo–molecular dynamics simulations of titania/yttria-doped iron

We present pair-wise, charge-neutral model potentials for an iron–titanium–yttrium–oxygen system. These simple models are designed to provide a tractable method of simulating nanostructured ferritic alloys (NFAs) using off-lattice Monte Carlo and molecular dynamics techniques without deviating significantly from the formalism employed in existing Monte Carlo simulations. The model is fitted to diamagnetic density functional theory (DFT) calculations of the various species over a range of densities and concentrations. The resulting model potentials provide reasonable and in some cases even excellent mechanical and thermodynamic properties for the pure metals. The model replicates the qualitative trends in formation energy predicted by DFT, though the energies of formation do not agree as well for dilute systems as they do for more concentrated systems. We find that on-lattice models will consistently favor tetrahedral oxygen interstitial sites over octahedral interstitial sites, while relaxed systems typically favor octahedral sites. This emphasizes the need for the off-lattice simulations for which this potential was designed.

Helium bubble distributions in a nanostructured ferritic alloy

A 14YWT nanostructured ferritic alloy (NFA) was implanted with He+ ions to fluences of 6.75×1021Hem−2 at 400°C in order to simulate the effects of high He concentrations produced in advanced fission and future fusion reactors at an accelerated timescale. The He bubble size distributions associated with specific microstructural features were characterized by a combination of transmission electron microscopy and atom probe tomography. Helium bubbles were observed on grain boundaries, dislocations, and on the surfaces of nanoclusters and larger Ti(N,C) precipitates. A polydisperse distribution of bubble sizes was observed in the ferrite matrix. With the exception of He bubbles on dislocations, bubbles were observed to increase in size with increasing fluence. The combined TEM and APT data indicates that ∼4.4% of the bubbles are located on coarse precipitates, ∼12.2% at dislocations, ∼14.4% at grain boundaries, and ∼48.6% on nanoclusters, and the remainder as isolated bubbles in the ferrite matrix. The abundances of these different trapping sites, especially the nanoclusters, might reduce the availability and mobility of He, and possibly the susceptibility of these alloys to He embrittlement.

XAFS and TEM studies of the structural evolution of yttrium-enriched oxides in nanostructured ferritic alloys fabricated by a powder metallurgy process

The speciation and structural evolution of nanoscale yttrium-enriched oxides in numerous reduced activation oxide dispersion strengthened (ODS) ferritic steels have been investigated by X-ray absorption fine structure (XAFS) spectroscopy (including X-ray absorption near-edge structure and extended X-ray absorption fine structure) and transmission electron microscopy (TEM). The local structure and speciation of Y-enriched oxides during the fabrication process have been traced by Yttrium (Y) K-edge XAFS in fluorescence mode for both mechanical alloyed (MA) powders and compacted ODS alloys. After 24 h of milling, only 10%–14% of the initially added 0.3 wt. % Y2O3 dissolves into the steel matrix and titanium (Ti) exhibits a minor influence on the Y solid solution during the MA. The EXAFS analysis for compacted ODS alloys indicates the formation of new Y-enriched oxides rather than initial Y2O3. The presence of Y–Cr–O/Y2O3 has been confirmed by EXAFS and energy dispersed X-ray spectroscopy (EDX) elemental mapping in the compacted ODS alloy without Ti, while Y–Ti–O/Y2O3 was observed in Ti-containing (0.2–0.4 wt. %) ODS alloys. The addition of Ti exhibits an evident influence on the consolidation process rather than in the MA.

Temperature dependence of strengthening mechanisms in the nanostructured ferritic alloy 14YWT: Part I—Mechanical and microstructural observations

This paper presents experimental results on the mechanical and microstructural behaviors of the nanostructured ferritic alloy 14YWT and provides discussion relevant to deformation mechanisms over a wide range of temperature. The temperature dependence of strength was investigated over a wide temperature range of −196 to 1000°C. Detailed microstructural characterization before and after deformation was conducted to obtain the key information on deformation mechanisms; microstructural information before and after tensile deformation, including crystallographic texture, dislocation structures, and nanoclusters, was obtained from the focused ion beam lift-out specimens using EBSD, TEM, and APT techniques. Multiple deformation stages could be identified in the yield strength versus temperature curve. The dislocation structures in the specimens deformed at room temperature and at 900°C were found to be significantly different. EBSD results were used to elucidate changes in the crystallographic textures of the specimens before and after deformation at 900°C.

Temperature dependence of strengthening mechanisms in the nanostructured ferritic alloy 14YWT: Part II—Mechanistic models and predictions

The temperature dependence of strengthening mechanisms in the nanocluster-strengthened 14YWT alloy was investigated to elucidate the relative significance of contributing mechanisms in different temperature ranges. This study was also aimed at providing the prediction capability of yield strength for the nanostructured ferritic alloys over a wide range of temperature. The four major strengthening mechanisms: the Peierls stress, grain boundary strengthening, direct nanocluster strengthening, and dislocation forest hardening, were taken into account in the calculation, and their roles and characteristics in different temperature ranges were extensively discussed. The results indicated that the contribution of grain boundary strengthening to total strengthening was the most significant component. Yield strength calculation was made by combining all the strengthening components and the results were compared with the experimental data. Further, the validation of the proposed approach was attempted by applying to the yield strength of other alloys.

On the structure and chemistry of complex oxide nanofeatures in nanostructured ferritic alloy U14YWT

The remarkable radiation damage resistance of nanostructured ferritic alloys (NFAs) is attributed to the large numbers of matrix nanofeatures (NFs) of various types, which can enhance the recombination of displacement defects and trap transmutant helium in fine scale bubbles. Characterizing the chemistry, crystallographic structure and orientation relationships of the NFs is critical to understanding how they enhance the radiation damage resistance of NFAs. Conventional and high-resolution transmission electron microscopy and energy-dispersive spectroscopy were used to characterize the various types of NF and larger oxide phases in a model 14Cr–3 W–0.4Ti–0.25Y2O3 NFA (14YWT) hot isostatic pressed (HIP-ed) at 1150°C. Large CrTiO3 precipitates (50–300 nm) and small diffracting NFs (<5 nm) were found in this alloy. One major new result is the observation of an additional type of nanofeature (10–50 nm), orthorhombic in structure, with a square center cross-section, which constitutes a new kind of Y–Ti-oxide phase with lattice parameters different from those of known Y and Ti complex oxides. The interfaces of these particles seem to be semicoherent, while manifesting a possible orientation relationship with the BCC matrix. The ratio of Y to Ti varies between <1 and 2 for these larger NFs.

Transmission electron microscopy characterization of the nanofeatures in nanostructured ferritic alloy MA957

Bright field transmission electron microscopy (TEM), high-resolution transmission electron microscopy, scanning transmission electron microscopy and energy dispersive X-ray spectroscopy were combined to comprehensively characterize the Ti–Y–O enriched nanoscale features (NF) in a nanostructured ferritic alloy (NFA), MA957. The characterization included the NF number densities, size distributions, volume fractions, crystal structures and compositions. In-foil measurements were complemented by studies of NF on C extraction replica foils. The conditions examined included: two heats of extruded MA957, one of these heats following long-term thermal aging at 1000°C for 19.53kh, and two friction-stir-weld variants. The study revealed the presence of diffracting complex oxide phases that varied somewhat with the feature size. However, the smallest features, with diameters (d)<5nm, were found to be consistent with cubic pyrochlore Y2Ti2O7 complex oxides with average Y/Ti ratios of ∼1. Long-term thermal aging coarsened the NF, while friction stir welding had relatively little effect on their number density and size distribution. The results presented here are briefly compared with those reported in other recent publications along with a short discussion of the reasons why the compositions measured by TEM differing from those found in atom probe tomography studies, and the implications of the comprehensive database developed here to the irradiation tolerance and mechanical behavior of NFA.