Nanostructured Ferritic Alloys Research Articles

Anisotropic Tensile Properties of a 14YWT Nanostructured Ferritic Alloy: On the Role of Cleavage Fracture

Two plates of nanostructured ferritic alloy NFA-1 were processed by ball milling atomized Fe-14Cr-3W-0.4Ti-0.2Y (wt.%) with FeO powders, canning, and hot-extrusion at 850 °C, followed by annealing and multipass cross-rolling at 1000 °C. This produces a severe (001) brittle cleavage texture on planes running parallel to the plate faces. In the first plate (P1), pre-existing microcracks (MCs) formed on the cleavage planes during cross-rolling. The second plate (P2) contained far fewer, if any, MCs. Here, we compare the tensile data for out-of-plane (S) and in-plane (L) tensile axis orientations, at temperatures from −196 °C to 800 °C. We also assess the tensile property differences between P1 and P2, and the effect of specimen size. The L-orientation strength and ductility were excellent; for example, the room temperature (RT) yield stress, σy ≈ 1042 ± 102 MPa, and the total elongation, εt ≈ 12.9 ± 1.5%. In contrast, the S-orientation RT σy ≈ 708 ± 57 MPa, and εt ≤ 0.2%. These differences were due to cleavage on the brittle (001) planes. Cleavage leads to beneficial delamination toughening, but is deleterious to deformation processing and through-wall heat transfer. Therefore, it is important to quantitatively characterize the pronounced NFA-1 strength anisotropy due to severe crystallographic texturing and cleavage fracture.

Characteristics of oxide-dispersion strengthened alloys produced by high-temperature severe deformation

This study is to explore an economically attractive and technically feasible processing method for oxide-nanoparticle strengthened alloys for fusion reactor application. Despite many scientific merits of the advanced oxide-dispersion strengthened (ODS) alloys, such as the nanostructured ferritic alloy (NFA) 14YWT, the only viable production path for a high-quality NFA is the high-power mechanical alloying process. This process is often a multi-day high-speed ball milling of alloy powder with a small quantity of yttria (Y2O3) powder, followed by the milled-powder consolidation using extrusion or other methods and additional thermomechanical processing (TMP) for property control. This complex production path, including the low-temperature mechanical alloying in particular, has limited technical advancement toward the cost-effective and scale-up production of ODS alloy components. To overcome such a practical limitation, we proposed to explore alternative low-cost processing routes using traditional thermomechanical processing (TMP) method only. A series of continuous TMP cycles, which were designed to impose high-temperature severe plastic deformation (HT-SPD) conditions to the consolidated powder mixtures, were applied to achieve the effective distribution of oxide particles in nanograin structure and thus desirable mechanical properties. Since the reduced-activation ferritic-martensitic (RAFM) alloy powders (Fe-10Cr and Fe-14Cr alloys with various Y contents) are available in our inventory, we focused to utilize the new solid-state synthesis approach for controlling oxide (oxygen source) dissolution and nanoscale clustering in nanograin structure in those alloys. A combination of powder consolidation at 900 °C and continuous thermomechanical activation at 600 °C yielded two essential ODS alloy microstructure contents–nanograin structure and nanoparticle distribution–and thus demonstrated a good combination of strength and ductility.

Heterostructural nanolamellar oxide-dispersion-strengthened ferritic alloy with exceptional strength and ductility

Nanostructured ferritic alloys (NFAs) have demonstrated exceptional mechanical properties (strength and creep resistance) and radiation tolerance. However, the high strength (typically above 1.5 GPa) always comes with a significantly limited ductility. Here, we report a novel heterostructural nanolamellar ferritic alloy (NLFA) with microcrystalline (MC) domains (∼ 35 % of volume fraction) dispersed in an ultrafine-grained (UFG) matrix (∼ 500 nm grain size), enabling the alloy to achieve ultrahigh yield strength of 2 GPa and an excellent tensile elongation of ∼ 13 %. The nanolamellar-shaped grains remarkably strengthen the alloy via grain-boundary strengthening. The soft MC domains can effectively suppress the microcrack initiation and propagation along the oxide-rich preliminary powder boundaries (PPBs) via local plastic deformation. Moreover, linear oxide bands can induce numerous interlayer cracking and trigger delamination toughening, collaboratively ensuring the alloy a large ductility. The present study opens a new frontier toward high tensile ductility for the most cutting-edge ultra-strong NFAs.

Microstructure dependent burst behavior of oxide dispersion–strengthened FeCrAl cladding

Oxide dispersion–strengthened (ODS) FeCrAl alloys are an advanced cladding material that combines the high-temperature steam oxidation resistance of conventional wrought FeCrAl with the irradiation resistance and high-temperature strength of nanostructured ferritic alloys. In the present study, the effect of microstructure on the performance of the ODS FeCrAl alloy system in light-water reactor–accident scenarios is evaluated using a combination of experimentation and modeling. ODS FeCrAl alloy with nominal composition Fe-12Cr-6Al-0.3Zr-0.3Y2O3 was produced into a thin-walled tube via high-precision tube rolling. A portion of the as-received (AR) tube was subjected to a full recrystallization heat treatment. Both variants were subjected to burst testing at Oak Ridge National Laboratory’s Severe Accident Test Station. The recrystallized tube exhibited burst temperature margins 200 °C higher than that of C26M, whereas the AR tube only achieved burst properties comparable to the wrought FeCrAl alloy. The burst experiments were simulated in the BISON finite element code through the development of new viscoplastic material databases for each heat treatment condition, informed by high temperature tensile tests of tube specimens and existing literature data on creep mechanisms. Finally, an ODS FeCrAl burst criterion was obtained using simulated burst experiments across the entire range of test conditions.

Preferential nucleation of (O, Y/La/Ce/Hf)-core nano-clusters in multi-element micro-alloyed ferritic alloys: A first-principles study

The high creep strength and high radiation resistance of nanostructured ferritic alloys (NFAs) have been generally attributed to the high-density precipitation of (O, Y)-core nano-clusters and nano-oxides. In this work, the early-stage nucleation of (O, X)-core (X = La, Ce, and Hf) nano-clusters (NCs) with respect to (O, Y)-core NCs in multi-element micro-alloyed NFAs was investigated using first-principles calculations. Their competitive precipitation preference and possible growth sequence were predicted. The (O–O) pair was suggested as the most fundamental core of all types of NCs. Yttrium always has the strongest affinity with the (O–O) pairs to initiate the nucleation of NCs during the early-stage nucleation. Under the absence or depletion of yttrium, other types of (O, X)-core (X = La, Ce, Hf) structures could form, leading to different nucleation paths with the preference ordering of (O, La)-core > (O, Ce)-core > (O, Hf)-core. NC nucleation and composition in NFAs would, thus, strongly depend on the number availabilities of micro-alloying solute species in the neighborhood of the (O–O) pairs.

Cold spray manufacturing of oxide-dispersion strengthened (ODS) steels using gas-atomized and ball-milled 14YWT powders

The cold spray deposition process has been investigated for the manufacture of 14YWT oxide-dispersion strengthened (ODS) steel, a nanostructured ferritic alloy (NFA), using gas-atomized and ball-milled feedstock powders. Cold spraying of the gas-atomized powder resulted in a thick dense deposit, but annealing them above 900 °C induced grain growth and heterogeneous precipitation of nanoparticles, thus resulting in lower hardness (∼ 170 HV) compared to 14YWT ODS steel manufactured by the conventional method. The ball-milled powder (Fe-14Cr-3W-0.4Ti-0.2Y-0.125O) was cryomilled in liquid N2 to achieve small particle sizes followed by annealing in a H2/Ar environment at 1000 °C and 1100 °C to reduce the hardness from ∼ 8 GPa to ∼ 5 GPa. Both smaller particle sizes and lower hardness of annealed powders made them amenable to the cold spray deposition process, resulting in thick and dense deposits, while retaining the favorable fully solutionized microstructure of the ball-milled powders. The microstructure of the as-deposited ODS exhibited very fine grain sizes (50–250 nm) with a uniform dispersion of Y-Ti-O nanoprecipitates in the ferritic steel matrix. The hardness values of the cold spray deposits using the powders annealed at 1000 °C and 1100 °C was 770 HV and 496 HV, respectively. Post-heat treatment of the as-deposited material up to 1100 °C showed a high-density microstructure with uniformly dispersed nanoscale oxide particles. The hardness of the annealed deposits up to 1000 °C had a notably higher hardness than bulk 14YWT ODS steel manufactured by conventional methods. The study demonstrates that cold spray deposition of the microstructurally-tailored feedstock powders can be used to successfully manufacture ODS steel with microstructure and properties comparable to those produced by the conventional methods, while providing for an attractive option for the more rapid and cost-effective manufacturing of ODS steel cladding tubes.

Accurate Fe–He machine learning potential for studying He effects in BCC-Fe

Nanostructured Ferritic Alloys (NFAs) containing oxide nanoparticles are prospective advanced structural materials in future fusion reactors. Lacking fusion neutron sources with an adequate flux, modeling is critical to understand radiation damage in the NFAs, including helium effects. Machine learning interatomic potentials (MLPs) have been demonstrated to be capable of describing atomic interactions in chemically complex systems with comparable accuracy to density functional theory (DFT) but with much less computational costs. Hence, MLPs are chosen to study He bubble accumulation in chemically complex NFAs. As a preliminary step, we develop a Fe–He potential, based on ∼10,000 atomic configurations. The developed MLP accurately predicts the bulk properties of BCC-Fe compared to DFT, including the lattice constant (2.832 Å), elastic constants (c11=271 GPa, c12=141, c44=93), and phonon frequencies (maximum error < 3.5%). The MLP predicts He binding energies in HenV bubbles and Hen clusters with a mean absolute error (MAE) of only ∼70 meV, which is ∼3 to ∼5 times smaller than the MAE of binding energies estimated using empirical potentials. The MLP also accurately predicts the migration barrier of interstitial He (64 meV). Furthermore, the MLP correctly predicts the maximum number of He atoms (critical size) at which a HenV bubble (Hen cluster) can kick-out a self-interstitial atom (SIA), forming a bigger HenV2 bubble (HenV bubble). Subsequently, we use the MLP to perform molecular dynamics simulations to investigate the effect of temperature on the critical size of HenV bubbles and Hen clusters. We find that the critical size decreases with increasing temperature — indicating that smaller bubbles and clusters can kick-out an SIA at higher temperatures, thereby escalating the He bubble growth. The current Fe–He MLP can be further developed to include all chemical interactions in NFAs.

Investigation of Nano-Scale Segregation in Nanostructured Ferritic Alloy 14YWT after Heavy Ion Irradiation.

Oxide-dispersion-strengthened (ODS) steels, which contain nano-scale Y-Ti-O particles, are being considered for high-temperature radiation environments of nuclear reactors. It is important to accurately characterize the structure of grain boundaries and understand the behavior of segregation at grain boundaries in ODS steels during irradiation. The effect of heavy ion irradiation at 700 °C on Nanostructured Ferritic Alloy 14YWT was investigated using Atom Probe Tomography. Enrichment of Cr occurs at the grain boundaries as well as at nano oxide particle surfaces in the unirradiated sample. The enrichment of Ti and Y at a grain boundary corresponds with Y-Ti-O nano oxide particles with larger size compared to those in the grain, and the Cr enrichment is particularly accentuated at these larger nano oxide particles. The segregation of W occurs at the grain boundaries that are without nano oxide particles. O is segregated at grain boundaries without oxide particles after irradiation. The segregation behavior of Cr, W, Ti, and Y at the grain boundary in the irradiated samples is similar to that in the unirradiated sample. The nano oxide particles embedded in the grain boundary are a primary reason for the increase in Cr segregation at the grain boundary.

Helium interaction with solutes and impurities in neutron-irradiated nanostructured ferritic alloys: A first principles study

Density functional theory (DFT) calculations are performed to explore the binding between He and alloying solutes, impurities, and transmutation products expected in neutron irradiated nanostructured ferritic alloys (NFAs), here 14YWT is taken as an example. Elements that exhibit significant binding (attraction) with an interstitial He are: Y (binding energy = 0.46 eV), Mg (0.32), O (0.33), Ti (0.16), and C (0.15). Those that provide significant binding to a substitutional He are: O (1.44), Y (1.24), N (0.73), H (0.56), Mg (0.52), Ti (0.34), Si (0.34), C (0.33), Al (0.32), Ni (0.26), Ta (0.23), and Mn (0.16). The presence of these elements in Fe matrix could reduce the transport of He towards oxide particles, dislocations, and internal boundaries, and could promote He bubble nucleation in the matrix. For convenience, we compile existing binding energy data of He with Hen and HenV (He-vacancy) clusters. Dissociation pathway analysis reveals that, in general, the most likely dissociation of a HenV cluster is by a sequential emission of individual He atoms. Furthermore, larger bubbles are more prone to dissociation than smaller ones. In addition, we estimate the binding energy (segregation energy) of He in bulk Y2Ti2O7 (YTO) single crystal, YTO/Fe interface, and YTO particle embedded in Fe, with respect to interstitial He in Fe, from existing formation energies of He in these structures. We also compile available data of He binding with Fe self-interstitial atom (SIA), SIA clusters, and edge and screw dislocations. Note that given the absence of DFT data, the binding with SIA clusters and dislocations are gathered from simulations with empirical potentials. The data presented in this paper is important to inform multiscale simulations of He bubble accumulation.

The effect of sink strength on helium bubble formation at elevated temperatures

In present study, the model Fe–9Cr alloy and two nanostructured ferritic alloys (NFAs) CNA1 and 9YWTV with high application potential in advanced nuclear power systems were selected to systematically investigate the effect of sink strength on helium bubble behavior at elevated temperatures. Helium bubble formation for a constant implanted He concentration of ∼7500 atomic parts per million (appm) was characterized using transmission electron microscopy (TEM) at 500 and 700 °C. A quantitative research and comparison of the influence for a specific sink strength on bubble formation was illustrated. Results showed that the introduction of nanoparticles into NFAs can effectively reduce bubble coarsening by sequestering the implanted helium into dispersed small bubbles due to the sufficient He entrapment by interfaces between the nanoprecipitates and the surrounding matrix. In addition, the NFAs with higher sink strength (9YWTV) revealed a higher suppression effects on bubble growth compared to general particle-free Fe–9Cr alloy and CNA1 with lower sink strength. This can be attributed to the comparable mean free path of He diffusion with the inter-particle distance for high particle density in 9YWTV. Meanwhile, the suppression effects of sinks on bubble growth decreased with increasing temperature from 500 to 700 °C. The sink strength of the order of 1015 m−2 and 1016 m−2 showed favorable suppression effect on bubble coarsening at 500 °C and 700 °C, respectively.

Solid-State Welding of the Nanostructured Ferritic Alloy 14YWT Using a Capacitive Discharge Resistance Welding Technique

Joining nanostructured ferritic alloys (NFAs) has proved challenging, as the nano-oxides that provide superior strength, creep resistance, and radiation tolerance at high temperatures tend to agglomerate, redistribute, and coarsen during conventional fusion welding. In this study, capacitive discharge resistance welding (CDRW)—a solid-state variant of resistance welding—was used to join end caps and thin-walled cladding tubes of the NFA 14YWT. The resulting solid-state joints were found to be hermetically sealed and were characterized across the weld region using electron microscopy (macroscopic, microscopic, and nanometer scales) and nanoindentation. Microstructural evolution near the weld line was limited to narrow (~50–200 μm) thermo-mechanically affected zones (TMAZs) and to a reduction in pre-existing component textures. Dispersoid populations (i.e., nano-oxides and larger oxide particles) appeared unchanged by all but the highest energy and power CDRW condition, with this extreme producing only minor nano-oxide coarsening (~2 nm → ~5 nm Ø). Despite a minimal microstructural change, the TMAZs were found to be ~10% softer than the surrounding base material. These findings are considered in terms of past solid-state welding (SSW) efforts—cladding applications and NFA-like materials in particular—and in terms of strengthening mechanisms in NFAs and the potential impacts of localized temperature–strain conditions during SSW.

Highly stable nanocrystalline oxide dispersion strengthened alloys with outstanding helium bubble suppression

Oxide dispersion strengthened (ODS) alloys containing high-density nano-oxides show excellent mechanical properties and irradiation tolerance. However, ODS alloys are often ultrafine-grained materials since the grain size of matrix is located between 100 and 1000 nm. Here, nanocrystalline and ultrafine-grained ODS alloys with different grain sizes (50 nm and 300 nm) were prepared by mechanical alloying and high-pressure consolidation techniques, which show exceptional thermal stability. To understand their irradiation response, the two alloys were irradiated with 400 keV helium ions to a fluence of 1 × 1017He+ /cm2 at 350 ℃, 400 ℃ and 450 ℃. The results show that the size and volume fraction of helium bubbles in nanocrystalline ODS alloys increase when the irradiation temperature increases from 350 ℃ to 450 ℃. Grain boundaries (GBs) play a key role in bubble formation and evolution. We identified the bubble denuded zone (BDZ) near the GBs, but the width of the denuded zone narrowed and finally vanished with increasing helium concentration. Nanocrystalline ODS alloy with 50 nm grains (Y50) can disperse helium into much finer bubbles than ultrafine-grained ODS alloy with 300 nm grains (Y300) because of the extremely high density of GBs and oxide-matrix boundaries. To our best knowledge, Y50 alloy presents the best helium bubble suppression ability among all studied ODS and nanostructured ferritic alloys (NFA), which makes it a promising candidate material for using in fission and fusion nuclear reactors.

Bubble formation in helium-implanted nanostructured ferritic alloys at elevated temperatures

Helium bubble formation was examined by scanning/transmission electron microscopy (S/TEM) in Fe-9/10Cr binary alloys and two dispersion strengthened nanostructured alloys (CNA3 and 14YWT containing 5–10 nm diameter carbide and oxide particles, respectively) after ex-situ and in-situ He implantation to ~10,000 appm at 500 to 900 °C. The combination of high-resolution STEM images and electron energy loss spectroscopy (EELS) revealed that the Y-Ti-O nanoparticles in 14YWT were uniformly distributed and exhibited a one-to-one relationship for bubble attachment to the nanoclusters. In the in-situ experiment at 900 °C, grain boundary cracking was severe in the Fe-10Cr model alloy, but not in the nanostructured alloys. From 500 to 900 °C, the bubble size generally increased with increasing irradiation temperature, while the bubble density decreased with increasing temperature. At the same temperatures, the bubble size in the implanted materials was in the order of Fe-9/10Cr > CNA3 > 14YWT, while the bubble density showed the opposite order. The observed bubble number densities for the nanostructured alloys are comparable to the nanoparticle density, suggesting that the nanoparticles in both alloys were effective in trapping He. Our results indicate that very high He concentrations can be managed in nanostructured alloys by sequestering the helium into smaller bubbles (which leads to a lower volume swelling value) and to shield He from the grain boundaries. This can be attributed to the much higher sink strength associated with the nanoclusters or the He trapping ability between different types of nanoclusters.

Enhancement of Nanostructured Ferritic Alloy 14YWT Properties via Heat Treatment for Post-Consolidation Processing

The nanostructured ferritic alloy 14YWT is a promising candidate for in-core use in generation IV nuclear reactors, due to a dense dispersion of insoluble, ultrafine-scale Y-Ti-O nano-oxides, which provide a high degree of irradiation tolerance and thermal stability. This study investigates the effects of heat treatment on the workability of 14YWT, along with the effect of processing history on abnormal grain structures and radial microstructural uniformity. In this study, a 14YWT rod consolidated at 850 °C was heat treated in argon at 1100 °C, 1150 °C, 1200 °C, or 1250 °C for 1 or 8 hours and changes in mechanical properties and microstructure were examined using microhardness and electron backscattered diffraction (EBSD). Two distinct types of large abnormal grains were observed, each with unique processing origins, including one with high and the other with low strain energy. Consolidation via direct extrusion resulted in radial microstructural gradients, where the center of the rod was softer with larger grain sizes and lower strain energies. These gradients persisted and intensified throughout heat treatment. Based upon this work, the recommended heat treatment for increased workability with minimal microstructural change is 1150 °C for 1 hours.

Joining of Nanostructured Ferritic 14YWT Alloys by Spark Plasma Technique

Spark plasma joining of nanostructured ferritic 14YWT alloys has been carried out at 800°C, 1020 °C and 1030 °C in vacuum. The characteristics of joining as a function of temperatures were investigated using bulk density, nanoindentation and electron backscattered diffraction techniques. Joining performed at 1020 °C and 1030 °C shows a complete, uniform bonding of the samples, whereas the samples joined at 800 °C showed a visible crack that runs along the entire sample length. Bulk density measurements indicate that the reduction in density of samples joined at 800 °C is due to the presence of voids at the interface. The presence of columnar grains only close to the interface at 800 °C suggests the possibility of a steep temperature rise locally at the interface that is attributed to high electrical contact resistance at the interface.

On a new Ti-carbooxinitride redistribution driven microcrack healing mechanism in an annealed 14YWT nanostuctured ferritic alloy

FCRD NFA-1 is a larger best practice heat of Fe-14Cr nanostructured ferritic alloy (NFA) dispersion strengthened by a large population of nano-scale Y2Ti2O7 nano-oxides. NFA-1 was produced by ball milling FeO with argon atomized Fe-14Cr-3W-0.35Ti-0.25Y (wt.%) powders, followed by hot extrusion at 850 °C, followed by 1 h annealing and multipass cross-rolling at 1000 °C to form an ≈ 10 mm thick plate. Cross-rolling results in a large population of {001}<110> cleavage system microcracks (MCs) running on planes parallel to the plate faces. A series of high temperature vacuum anneals were carried out from 1100 to 1300 °C for 1 and 5 h to explore the possibility of self-healing, not only of the MCs, but also macroscopic fatigue cracks. Most notably, a new crack self-healing mechanism was discovered, involving the redistribution and coarsening of initially fine matrix Ti, C, O and N precipitates (TiCONs) to bridging sites on the crack faces. The relocated TiCONs grow and coarsen, thereby consuming a volume of the adjoining Fe-Cr matrix. The displaced matrix atoms diffuse to further fill the crack. Extensive microstructural, fractographic and mechanical property characterization measurements show that MC self-healing begins at 1100 °C/1 h, increasing with annealing time and temperature, up to completion at 1300 °C. The NFA-1 grains and texture are relatively stable. However, at 1300 °C the nano-oxides and dislocations coarsen or recover, respectively, leading to significant softening.

Nucleation of Y–Si–O Nano-clusters in Multi-microalloyed Nano-structured Ferritic Alloys: a First-principles Study

First-principles energetics analyses were performed to investigate the nucleation of (Y–Si–O) nano-clusters (NCs) in nano-structured ferritic alloys (NFAs). It was predicted that the nucleation of (Y-Si–O) NCs follows the same general pathway as previously found for (Y–Ti/Al/Zr–O) NCs in NFAs: they all tend to begin with the (O–O) pairs and grow to (O–Y)-cores and further to larger NCs by attracting Y and other solute elements nearby. Nucleation of a hexa-atomic –[2Y–Si–3O]– cluster can reduce the total energy by 4.71 eV. Among various microalloying-induced NCs, the nucleation preference ordering was predicted as (Y–Zr–O) > (Y–Ti–O) > (Y–Al–O) > (Y–Si–O). The number densities and chemical compositions of NCs in multi-microalloyed NFAs would largely depend on the number availability of (O,Y)-cores, as well as the relative abundance and atomic diffusivities of microalloying solute elements nearby.

Equilibrium distribution of point defects in Fe-Y-O as a typical representative of nanocluster-strengthened alloys

The statistical mechanic approach [1, 2] has been used to estimate the equilibrium defect distribution in a multiphase system by minimizing its free energy using microstructural characteristics and a finite set of defects obtained by first-principles calculations. Recently the approach was extended to take in consideration defects at the interface of the precipitate and matrix [3]. Herein we apply the developed approach to the investigation of the stability of bcc Fe containing yttria, Y2O3, nanoclusters as a prototype of nanostructured ferritic alloys. It has been obtained that (100)Fe-O interface is unstable with respect to vacancies production. In a contrast to (100)Fe-O interface, the so called Klim interface is stable, i.e. local vacancy concentration at this interface at 600 K is below 10−12. It has been demonstrated that due to large defect formation energies the ODS particles are extremely stable and the main defect corresponds to Fe atoms substitute Y in Y2O3 precipitate. Moreover, under thermodynamic equilibrium condition, the preexist vacancies in bulk Fe do not accumulate oxygen atom. The later observation not necessarily forbid the existence of a large amount of preexisting Fe vacancy - oxygen atom clusters at the initial stages of alloy formation far from equilibrium.

Microstructural and micro-mechanical analysis of 14YWT nanostructured Ferritic alloy after varying thermo-mechanical processing paths into tubing

Microstructural analyses and micro-pillar compression were conducted on 14YWT nanostructured ferritic alloy (NFA) to compare different processing pathways: hydrostatic extrusion and Pilger processing with varying annealing temperatures into thin walled tubing, and after hot extrusion and cross-rolling into a plate. Hydrostatic extrusion at 815 °C resulted in the smallest grain sizes and highest yield strength of 1.20 GPa. Pilger processing with annealing at 800 °C had fine grained regions and bands of coarse grains, leading to a large variation in yield strength of 0.9–1.40 GPa. Higher annealing temperatures of 1200 °C after pilger processing significantly increased the grain size and lowered the yield strength to 1.01 GPa. These tubes showed a stronger 〈111〉 crystallographic texture in the normal direction and elongated grains in the extrusion direction. Characterization of the nano-oxides using TEM reveals more numerous, smaller oxides present in tubing processed at lower temperatures. This work shows NFA tubing after hydrostatic extrusion and pilger processing can lead to fine grained microstructures and texturing leading to higher yield strengths at lower annealing temperatures (e.g. 800 °C).

In-situ observation of nano-oxide and defect evolution in 14YWT alloys

Nanostructured ferritic alloys (NFAs) are considered as candidates for structural components in advanced nuclear reactors due to their excellent radiation resistance as a result of a high density of nano-oxides (NOs) in the microstructure. Therefore, gaining an understanding on the stability of NOs under irradiation is crucial. In this study, we have investigated the evolution of defects and NOs in 14YWT NFAs under in-situ Kr ion irradiation at room temperature (RT) and 450 °C up to 10 dpa. It has been found that irradiations at 450 °C do not create any changes in the NOs, similar to the bulk irradiations. On the other hand, elemental mapping indicates that NOs dissolve mostly after 10 dpa irradiations at RT. Thus, while defects are both annihilated and pinned by NOs at low doses (before the dissolution of NOs), glissile loops start to escape to the foil surface at high doses (after the dissolution of NOs), justifying the significantly low fraction of <111> loops compared to the literature values. High resolution transmission electron microscopy analysis has shown that the NOs are mostly coherent Y2Ti2O7 particles with pyrochlore crystal structure after both RT and 450 °C irradiations, similar to those observed before irradiation.