Oxide Dispersion Strengthened Research Articles

Experimental study on corrosion compatibility of laser-welded ODS steel with liquid lead lithium

The liquid lead lithium blanket is considered to be one of the most promising blanket concepts. Oxide Dispersion Strengthened (ODS) steel probably emerge as an important candidate for blanket structure material for its substantial enhancement of neutron radiation resistance and mechanical strength at high temperature. However, during the long-term operation of fusion reactor, it’s inevitable for structural material and its welded joints to experience adverse corrosion from liquid lead lithium. In this paper, the self-developed rotating corrosion apparatus was used to conduct a corrosion test of laser-welded ODS steel exposed to liquid lead lithium for thousands of hours. The influence of corrosion time on the corrosion of ODS steel laser welding was analyzed. The results show that the corrosion rates of ODS steel and its welded joints are 76 and 88.6 μm⋅year -1, respectively. After 5000 h of exposure, various pits and gullies are observed on the surface of weld zone, while penetrating cracks are clearly visible in the cross-section. Corrosion occurs preferentially from austenite grain boundaries, and the corrosion mechanism is mainly the dissolution of Fe and Cr alloying elements. Meanwhile, a decrease in the hardness of the welded joints with the increase of corrosion time is observed.

Excellent strength-ductility synergy in oxide dispersion strengthened AlCrFeNi high-entropy composites by heterostructure strategy

Oxide dispersion strengthened (ODS) AlCrFeNi high-entropy composites were produced by different heterostructure strategies to achieve strength and ductility synergy. The effects of different reinforcement types, reinforcement contents and oxide-forming elements in the matrix on the microstructure and mechanical properties of ODS-AlCrFeNi composites were investigated. The results showed that in the composites with different reinforcement types (ternary ODS-CrFeNi, quaternary ODS-CoCrFeNi and quinary ODS-CoCrFeNiMn), spinodal decomposition is observed in the all reinforcements, resulting in the formation of ellipsoidal/cuboidal B2-structured NiAl-rich phase and BCC-structured FeCr phases. As the number of the principal elements in the reinforcing phase decreases, the spinodal decomposition size gradually decreases. In the composites with varying ODS-CrFeNi reinforcement contents (5%, 10%, 15%, 20%), the occurrence of spinodal decomposition is also observed in the reinforcement. The spinodal decomposition size in the composite with 15% and 20% reinforcement content is smaller than that with 5% and 10% content. It is noteworthy that the incorporation of oxide-forming elements of Zr and Ti or only Zr together with Y2O3 in to the matrix result in different reinforcement structures. The former is typical spinodal decomposition, whereas latter displays a gradient network structure comprising a FCC-structured FeNi phase and a BCC-structured Cr-rich phase. The superior strength-ductility synergy, a compressive strength and strain of 7,690 MPa and 15.5%, which are 2.7 and 2 times higher than those of the unreinforced reference alloy, respectively, is achieved. This is mainly contributed by the novel gradient network structure in the reinforcement.

Multiscale Modeling of Nanoparticle Precipitation in Oxide Dispersion-Strengthened Steels Produced by Laser Powder Bed Fusion.

Laser Powder Bed Fusion (LPBF) enables the efficient production of near-net-shape oxide dispersion-strengthened (ODS) alloys, which possess superior mechanical properties due to oxide nanoparticles (e.g., yttrium oxide, Y-O, and yttrium-titanium oxide, Y-Ti-O) embedded in the alloy matrix. To better understand the precipitation mechanisms of the oxide nanoparticles and predict their size distribution under LPBF conditions, we developed an innovative physics-based multiscale modeling strategy that incorporates multiple computational approaches. These include a finite volume method model (Flow3D) to analyze the temperature field and cooling rate of the melt pool during the LPBF process, a density functional theory model to calculate the binding energy of Y-O particles and the temperature-dependent diffusivities of Y and O in molten 316L stainless steel (SS), and a cluster dynamics model to evaluate the kinetic evolution and size distribution of Y-O nanoparticles in as-fabricated 316L SS ODS alloys. The model-predicted particle sizes exhibit good agreement with experimental measurements across various LPBF process parameters, i.e., laser power (110-220 W) and scanning speed (150-900 mm/s), demonstrating the reliability and predictive power of the modeling approach. The multiscale approach can be used to guide the future design of experimental process parameters to control oxide nanoparticle characteristics in LPBF-manufactured ODS alloys. Additionally, our approach introduces a novel strategy for understanding and modeling the thermodynamics and kinetics of precipitation in high-temperature systems, particularly molten alloys.

Prediction of formation energy for oxides in ODS steels by machine learning

The phase and proportion of nano-oxides in oxide dispersion strengthened (ODS) steels are determined by the formation energy and the content of oxide-forming elements, particularly minor reactive elements, which in turn influence the macroscopic properties of the ODS steels. To study the phase, morphology, and interfaces of oxides in six ODS steels, transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and high-resolution transmission electron microscopy (HRTEM) techniques were employed. Subsequently, machine learning (ML) methods were used to predict the formation energies of the oxides in the ODS steels. Among six ML models employed, the light gradient boosting machine (LGBM) model shows the highest accuracy in predicting the formation energy of the oxides, with root mean square error (RMSE) values of 0.18 eV/atom and 0.27 eV/atom, mean absolute error (MAE) values of 0.11 eV/atom and 0.18 eV/atom, and coefficient of determination (R2) values of 0.96 and 0.92 for the training and testing sets, respectively. For Magpie descriptors, the most essential feature is “dev_Electronegativity” with an importance score of 444. The method developed in this study can establish an accurate prediction model for the formation energy of oxides. The knowledge obtained in this work guides the future development of high-performance ODS alloys.

In-situ synthesis of Yttria-based precipitates and their effects on Fe12Cr6Al in laser powder bed fusion

To advance the suitability of FeCrAl alloys for nuclear applications, radiation damage resistance, creep resistance, corrosion resistance, and mechanical properties enhancement are essential. A promising approach is the introduction of a high density of nitride and oxide nanoprecipitates within the alloy matrix. This study aimed to explore the impact of in-situ formed nitride and oxide precipitates on the properties of Fe12Cr6Al alloy by using a nitrogen-rich atmosphere and incorporating nano-sized Y2O3 (0.45 wt %) during laser powder bed fusion (LPBF). As a result of adding Y2O3 nanoparticles to the Fe12Cr6Al alloy, the grain size decreased from 70 μm to 40 μm, and the Al–O, Al–N, and Al–N–O precipitates were replaced by Y–O, Y–Al–O, and Y–N precipitates. Additionally, the nitrogen contents increased from 220 ppm to 470 ppm. The sample with Y2O3 exhibited higher mechanical properties, with yield strength, tensile strength, and hardness (HV) increasing by 80 MPa, 112 MPa, and 23 HV, respectively. This study deviates from previous research on Y2O3-containing FeCrAl alloys by investigating the impact of in-situ synthesized Y–N nitrides on Fe12Cr6Al. The objective is to elucidate the combined effects of these precipitates on the properties of Fe12Cr6Al alloy in comparison to literature-reported oxide dispersion-strengthened (ODS) FeCrAl alloys, which were fabricated in an argon (Ar) atmosphere.

The Use of TiH2 to Refine Y2Ti2O7 in a Nano Mo-ODS Alloy

Mo-ODS alloys have excellent mechanical properties, including an improved recrystallization temperature, greater strength due to dispersed oxides, and the ability to suppress grain growth at high temperatures. In ODS alloys, the dispersed Y2O3 and added Ti form Y-Ti-O complex oxides, producing finer particles than those in the initial Y2O3. The complex oxides increase high-temperature stability and improve the mechanical properties of the alloy. In particular, the use of TiH2 powder, which is more brittle than conventional Ti, can enable the distribution of finer oxides than is possible with conventional Ti powder during milling. Moreover, dehydrogenation leads to a more refined powder size in the reduction process. This study investigated the refinement of Y2Ti2O7 in a nano Mo-ODS alloy using TiH2. The alloy compositions were determined to be Mo-0.5Ti-0.5Y2O3 and Mo-1.0Ti-0.5Y2O3. The nano Mo-ODS alloys were fabricated using Ti and TiH2 to explore the effects of adding different forms of Ti. The sintered specimens were analyzed through X-ray diffraction for phase analysis, and the microstructure of the alloys was analyzed using scanning electron microscopy and transmission electron microscopy. Vickers hardness tests were conducted to determine the effect of the form of Ti added on the mechanical properties, and it was found that using TiH2 effectively improved the mechanical properties.

Interfacial microstructure and corrosion behavior of ODS FeCrAl alloy in oxygen-saturated lead-bismuth eutectic at 450 °C

The interfacial microstructure and corrosion behavior of oxide dispersion strengthening (ODS) FeCrAl alloy with 20 wt% Cr (e.g. 20Cr ODS FeCrAl alloy) exposed to static oxygen-saturated lead-bismuth eutectic (LBE) at 450 °C for 20 h, 40 h, 60 h, 80 h and 100 h as exposure time respectively have been investigated. The results show that multilayer corrosion films of 20Cr ODS FeCrAl alloy can exist at the corrosion interface. Due to formation of protective scales, the primary corrosion mechanism of 20Cr ODS FeCrAl alloy in LBE is oxidation rather than dissolution and penetration of liquid LBE. Specially, the spinel-shaped nano-sized dense external layer without any microcracks is obviously observed. Moreover, the dense and compact scale with micron-meters, i.e. Al-rich Fe-Cr spinel, generates at interface as the middle layer to effectively defend inward diffusion of oxygen and penetration of LBE. Meanwhile, a reinforced and continuous internal layer mixed with Cr2O3 scale and the ferrite inner oxidation zone (IOZ) existing in the form of the interlocking interface between the layers and the substrate can further promote the adhesion between corrosion layers and the substrate of 20Cr ODS FeCrAl.

A Comparative Study of the Impact of La2O3 and La2Zr2O7 Dispersions on Molybdenum Microstructure, Mechanical Properties, and Fracture

We report, for the first time, the effect of lanthanum zirconate (La2Zr2O7) particles on the microstructure and mechanical behavior of an experimental molybdenum oxide dispersion-strengthened alloy. The focus was on the preparation of the novel Mo–La2Zr2O7 composite using high-energy ball milling and spark plasma sintering and on the comparison of its microstructural and mechanical properties with pure Mo and Mo–La2O3 ODS alloy counterparts. Mechanical properties were assessed using a Vickers hardness test at room temperature and a three-point flexural test in the temperature range from − 150 to 150 °C. The microstructure of the studied materials and their fracture behavior were evaluated using x-ray diffraction, energy-dispersive x-ray spectroscopy, and scanning electron and transmission electron microscopy. The strengthening effect of La2Zr2O7 particles was found to be lower than that of La2O3 particles, resulting in a 30-35% lower yield stress and flexural strength of the Mo–La2Zr2O7 alloy compared to the Mo–La2O3 alloy. The experimental Mo–La2Zr2O7 alloy exhibited low plasticity and no distinct ductile-to-brittle transition temperature (DBTT) in the tested temperature range, unlike pure Mo and the Mo–La2O3 alloy, which had the DBTT of 63 and 1 °C, respectively. Fracture occurred mainly in a brittle intergranular manner in the entire testing temperature range, while the counterpart materials showed localized plastic stretching at grain boundaries and within grains at and above the transition region. The observed behavior was primarily related to lower strengthening and brittleness as well as less effective grain boundary purification.

Inhomogeneous distribution of oxides in various 9Cr ODS alloys and their mechanical performance at room temperature and 650 °C

Oxide dispersion strengthened (ODS) steels have been considered as promising structural materials for fusion reactors. In fabricating ODS steels, homogenous distribution of nano-sized oxides is always expected. In this work, Ti and C were respectively added into the 9Cr alloy with 2 wt% Mn, and the effects of individual Ti addition on microstructure and mechanical performance of ODS alloys are studied. Regularly distributed oxides were revealed in the 9Cr alloys fabricated by hot isostatic pressing (HIP), irrespective of alloying compositions, which indicates the strong interaction between oxides and phase interface. Alloying elements within the matrix can be incorporated into these regularly distributed oxides. Oxide nanoparticles will affect the phase transformation behaviors and the microstructure of as-HIPed 9Cr alloys without oxides is different from that of 9Cr ODS alloys. By performing post-HIP heat treatments, the matrix conditions of 9Cr ODS alloys can be adjusted, and ferritic and martensitic 9Cr ODS alloys are respectively obtained. With tensile tests at room temperature and 650 °C, effects of oxides and matrix conditions on 9Cr alloys were evaluated. It is found that individual Ti addition has a softening effect on the 9Cr alloys, despite that whether oxides are added. Simultaneous addition of Ti and C can improve the strength of 9Cr alloys evidently. Poor ductility at 650 °C is a critical issue for 9Cr ODS alloys, while the 9Cr alloys without oxides have favorable combination of strength and ductility at both room temperature and 650 °C.

Maillard Type Reaction for Electroless Copper Plating onto Ceramic Nanoparticles

A high demand for oxide dispersion strengthened (ODS) materials arose in wide fields of application. The electroless plating method has been presented as an elegant way to overcome different surface energies and obtain metal‐plated ceramics. However, copper electroless plating is still performed under harsh conditions with toxic and expensive reagents, including noble metals and harsh reducing agents like formaldehyde and hydrazine. To create a pure copper metal matrix in an environmentally friendly and efficient way a previously reported method is advanced in a way that naturally occurring amino acids are utilized. A screening of several amino acids leads to an improvement in phase purity and atomic efficiency. Therefore, a Maillard type reaction with lysine as amino compound is reported to show the best results in the particle coating for all ceramic nanoparticles evaluated. The metal plating results in uniform round micro particles showing a homogeneous coating. In order to obtain copper matrix composites, the prepared particles are successfully implemented in a PBF‐LB/M process after mixing with a highly conductive copper alloy. The resulting products were investigated by transmission electron microscopy, scanning electron microscopy, powder X‐ray diffraction, and optical density measurements.

Achieving uniform distribution of nanoparticles in oxide dispersion strengthened (ODS) SS316L through laser powder bed fusion (L-PBF)

Oxide dispersion strengthened (ODS) SS316L is a promising candidate for the nuclear industry for its enhanced irradiation resistance and high temperature strength. Additionally, additive manufacturing enables the design flexibility of components. Achieving a uniform distribution of oxides in ODS SS316L is one of the remaining challenges in additive manufacturing. In this paper, we investigated the effects of printing parameters on the microstructure and mechanical properties of ODS SS316L (SS316L + 0.5 wt% Y2O3) through laser powder bed fusion (L-PBF) additive manufacturing. The results showed that plate-like Y-Si-rich oxides (∼50 μm) were observed along the molten pool boundary in the ODS SS316L printed with nominal parameters (48.5 J/mm3) for pure SS 316L, resulting from inadequate heat input in molten pool due to the reduced laser absorption rate of powder feedstock. Through higher volumetric energy density (76.2 J/mm3) and remelting, a bimodal distribution of oxides, including nanoparticles and fine spherical oxide (∼2.5 μm), was achieved. Consequently, this contributed to increased ultimate tensile strength (UTS) and strain of ODS SS316L from 685.2.6 ± 31.4 MPa and 27.8 ± 6.2 % to 706.6 ± 36.2 MPa and 33.0 ± 6.1 %, respectively. The exploration of parameters optimization provides valuable insights into the additive manufacturing of ODS alloys with uniformly distributed nanoparticles.

Experimental and simulation studies to optimize the mechanical alloying process of ODS-FeCrAl alloy powder

Oxide dispersion strengthened (ODS) FeCrAl alloy exhibits superior mechanical qualities at high temperatures and resistance to neutron irradiation, making it a highly promising material for accident-tolerant fuel cladding. The preparation of ODS-FeCrAl alloy powder by mechanical alloying can mix Y2O3 and FeCrAl alloy powder uniformly and promote the uniform dispersion of nanoscale oxide particles in the alloy matrix. The preparation of ODS-FeCrAl alloy powder by the mechanical alloying method is greatly influenced by the rotational speed. Therefore, this study uses a combination of experiments and numerical simulation to investigate the effects of different rotational speeds on the microscopic morphology, particle size and force of alloy powder during the ball milling process. The mechanically alloyed powder was subjected to XRD inspection to determine the optimum ball milling time at each rotational speed by means of the maximum lattice distortion, and then the powder micromorphology and particle size were analyzed. Then the forces on the powder and grinding ball during the ball milling process were analyzed by numerical simulation, and it was found that the collision force and normal force played a dominant role in the ball milling process. The combined experimental and simulation results determined that the ODS-FeCrAl alloy powder obtained by ball milling at 300 rpm for 35 h had the largest lattice distortion, fine grain size and uniform particle size, which achieved the best mechanical alloying effect.

High Temperature Oxidation Behavior of ODS Ferritic Stainless Steel Fe-16Cr-4Al-1Ni-0.4Y<sub>2</sub>O<sub>3</sub>

This study investigates the isothermic oxidation behavior of the new ODS alloy Fe-16Cr-4Al-1Ni-0.4Y2O3 (% by weight) at 700, 800 and 900 °C, with exposure times of 5, 20, 50, and 100 h at each temperature. The purpose is to obtain new data on its high-temperature parabolic oxidation constant for assessing oxidation resistance. The methods used include isothermal oxidation testing, XRD, SEM-EDS characterization, and analysis of oxidation kinetics by monitoring changes in oxide thickness using microscopy and SEM-EDS. The oxide products formed on the sample surface are Fe2O3, Fe3O4, AlFe2O4, and (Fe,Cr)2O3. Al and Cr oxides are located under the dominant Fe oxide layer on the surface of the sample. The oxidation test results showed that the most protective sample was obtained at a temperature of 700 °C for 100 h with an oxide thickness of 263.99 μm. The kinetics analysis correlates strongly with the parabolic equation (R2 ≈ 1). The oxidation rate constants at temperatures of 700, 800, and 900 °C were 681.76, 2957.5, and 12300 μm2 h−1, respectively. The activation energy required by the oxidation reaction in this alloy is 136.5 kJ mol−1. This research enhances understanding and potential applications of the Fe-16Cr-4Al-1Ni-0.4Y2O3 alloy in high-temperature environments.

Comparative First-Principles Study of the Y2Ti2O7/Matrix Interface in ODS Alloys

Oxide-dispersion-strengthened (ODS) alloys generally exhibit extraordinary service performance under severe conditions through the formation of ultrafine nano oxides. Y2Ti2O7 has been characterized as the major strengthening oxide in Fe-based ODS alloys. First-principles energetic analyses were performed to investigate the structural, elastic and interface properties of Y2Ti2O7 in either Fe-based or Ni-based ODS alloys. Y2Ti2O7 has comparable elastic constants to bcc-Fe and fcc-Ni and similar elastic deformation compatibility in Y2Ti2O7-strengthened Fe-based and Ni-based ODS alloys is therefore expected. The Ni/oxide interface has generally better thermostability than Fe/oxide across the whole range of the concerned oxygen chemical potential. Further interface bonding and adhesion calculations revealed that Y2Ti2O7 can enhance the bonding strength of Ni/Y2Ti2O7 through d-d orbital interaction between the interfacial YTi layer and Ni layer, while the interface bonding between the Fe layer and YTi layer is weakened compared to the metal matrix. First-principles calculations suggest that Y2Ti2O7 can be a candidate for strengthening nano-oxides in either Fe-based or Ni-based ODS alloys with well-behaved mechanical properties for fourth-generation fission reactors and further experimental validations are encouraged.

Cost-Effective Thermomechanical Processing of Nanostructured Ferritic Alloys: Microstructure and Mechanical Properties Investigation.

Nanostructured ferritic alloys (NFAs), such as oxide-dispersion strengthened (ODS) alloys, play a vital role in advanced fission and fusion reactors, offering superior properties when incorporating nanoparticles under irradiation. Despite their importance, the high cost of mass-producing NFAs through mechanical milling presents a challenge. This study delves into the microstructure-mechanical property correlations of three NFAs produced using a novel, cost-effective approach combining severe plastic deformation (SPD) with the continuous thermomechanical processing (CTMP) method. Analysis using scanning electron microscopy (SEM)-electron backscatter diffraction (EBSD) revealed nano-grain structures and phases, while scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy (EDS) quantified the size and density of Ti-N, Y-O, and Cr-O fine particles. Atom probe tomography (APT) further confirmed the absence of finer Y-O particles and characterized the chemical composition of the particles, suggesting possible nitride dispersion strengthening. Correlation of microstructure and mechanical testing results revealed that CTMP alloys, despite having lower nanoparticle densities, exhibit strength and ductility comparable to mechanically milled ODS alloys, likely due to their fine grain structure. However, higher nanoparticle densities may be necessary to prevent cavity swelling under high-temperature irradiation and helium gas production. Further enhancements in uniform nanoparticle distribution and increased sink strength are recommended to mitigate cavity swelling, advancing their suitability for nuclear applications.

Influence of injected ions on α’ formation under ion irradiation in Oxide Dispersion Strengthened Steels

Oxide Dispersion Strengthened (ODS) steels hold great promise for applications in next generation reactors. Under irradiation, a phase separation α/ α’ can occur within the Fe-Cr matrix of ODS steels that can alter their mechanical properties. This work presents, for the first time, the characteristics of α’ precipitates enhanced by ion irradiation at 400 °C and examines the influence of the implanted ions. Far from the implanted region, α’ is reported in significant density while at the implanted peak, the α’ density is considerably reduced. This suggests that ion implantation either reduces the fraction of α’ phase formed after irradiation or delays considerably its formation. Through atom probe tomography analysis and comparison with existing literature, the low impact of the damage rate and fluence on the α’ formation in ODS steels is highlighted. Interestingly, the efficiency of ballistic mixing of α’ appears to be less pronounced in ODS steels than in Fe-Cr systems.

Simultaneously improving the strength and ductility of an oxide dispersion-strengthened high-entropy alloy by employing innovative precursors for oxide formation

The engineering of a composite microstructure, achieved by coupling heterogeneous grain distribution with oxide dispersion, has been demonstrated as an effective strategy for enhancing the strength-ductility synergy of alloys at both room and elevated temperatures. The effectiveness of this approach is strongly correlated with the micro-features of dispersed oxides. In this study, we selected the Ni38Co20Cr20Fe18Ti4 high-entropy alloy (HEA) as the base material, which could achieve the desired composite structure through preparation using mechanical alloying and spark plasma sintering methods. Our primary objective was to investigate the effects of incorporating Y2O3 or utilizing hydrides (TiH2 and YH3) as alternative precursors on the modification of oxide precipitates, evolution of a bimodal grain microstructure, and alteration in mechanical properties for the oxide dispersion strengthened (ODS) HEA. The results show that the incorporation of Y2O3 promotes the formation of ultrafine and semi-coherent Y2Ti2O7 ternary oxide particles, in addition to the pre-existing coarse binary TiO in the HEA matrix. Moreover, this leads to a significant increase in the fraction and a decrease in the average size of ultrafine grains (UFGs) within the bimodal microstructure. Notably, utilizing Ti- and Y-hydrides instead of Y2O3 and Ti as oxide-forming precursors within an equivalent composition remarkably amplifies the precipitation proportion of Y2Ti2O7 among dispersoids while further refining UFGs. The ODS-HEA synthesized using hydrides exhibits a simultaneous enhancement of 12 % in yield strength and 13 % in elongation to fracture, compared to that prepared from Ti and Y2O3. This intriguing phenomenon primarily arises from the heightened strengthening and toughening contributions, facilitated by the increased precipitation of advantageous Y2Ti2O7 nanoparticles.

Ferritic–Martensitic Steels in Power Industry: Microstructure, Degradation Mechanism, and Strengthening Methods

Ferritic–martensitic (F–M) steels are widely used for high‐temperature pressure vessels and reactor cladding structures in power plants. The high operating temperatures and pressures, as well as the radiation environment, significantly challenge the mechanical stability of these steels. Here, the degradation mechanisms in F–M steels during creep and thermal aging under these harsh environments are reviewed. The exceptional mechanical properties of F–M steels are mainly attributed to their well‐constructed microstructures and chemical compositions. Microstructural barriers such as dislocations, solid solution atoms, and precipitates play key roles in resisting degradation. During the long‐term service, the microstructures undergo gradual evolution, resulting in a deterioration of mechanical properties at the macrolevel. In addition to the degradation mechanisms, some recent advancements in strengthening methods, including microalloying strengthening, thermomechanical treatment (TMT), and oxide dispersion strengthening, are summarized, aimed at the development of next‐generation F–M steels. The strengthening of the F–M steels is mainly achieved by enhancing the thermal stability of their microstructures. Insight into both the deterioration mechanisms and strengthening methods of F–M steels may pave the way for new approaches in developing high‐performance steels for applications in next‐generation power plants operating at ultrahigh operating temperatures and pressures.

Effect of cryogenic milling on the mechanical and corrosion properties of ODS Hastelloy-N

This study entails the fabrication of two oxide-dispersion strengthened (ODS) Hastelloy-N (HN) alloys utilizing divergent methods. The first alloy was synthesized using cryogenic attritor milling coupled with spark plasma sintering (SPS), while the second was produced via room temperature attritor milling and SPS. The ODS HN alloy derived from cryogenic milling demonstrated superior strength relative to its commercial-grade counterpart. Conversely, the alloy produced through room temperature milling exhibited lower ultimate tensile strength (UTS), attributed to manufacturing defects and the precipitation of Zr at grain boundaries. Corrosion resistance in molten FLiNaK for both ODS samples was found to be inferior compared to commercial HN. Particularly, in the room temperature-milled ODS HN, Zr present at grain boundaries appeared to dissolve more readily than in cryogenic or commercial samples, facilitating enhanced penetration by molten salt. The cryogenically-milled ODS HN contained Zr, yet it was not segregated to grain boundaries. Although the homogeneously dispersed Mo-based compound in the cryogenically-milled ODS HN augmented mechanical properties, it also accelerated corrosion propagation beyond that of the commercial-grade alloy.

Nano-dispersion strengthened and twinning-mediated CoCrNi medium entropy alloy with excellent strength and ductility prepared by laser powder bed fusion

Laser powder bed fusion (LPBF) was explored to produce CoCrNi medium entropy alloys (MEAs) from pre-alloyed powder with a relatively high oxygen concentration (870 ppm). Microstructure, strength, and toughness were investigated, revealing the effect of oxide dispersion strengthening (ODS). LPBF CoCrNi MEAs had a porosity of <0.10 % and superior mechanical properties compared to CoCrNi-based MEAs reported in the literature. Notably, the CoCrNi MEAs denoted as S1 and S2, exhibited an excellent combination of yield strength (>735 MPa), a UTS > 1 GPa, and a total elongation of ∼ 50 %. High strength results from the high number density of nano chromium oxides (average particle size ∼20 nm), which were in situ formed during LPBF, contributing to a calculated dispersion strengthening of ∼220 MPa. The deformation mechanism was investigated by TEM characterization of the specimen after tensile testing, revealing a large number of nano-twins for S1, cellular structure and nano-twins formed for S3. Extremely low porosity and deformation twinning ensure steady plastic deformation in LPBF CoCrNi, enabling high ductility. The prospect of ODS via LPBF provides a pathway for the strengthening and toughening of medium to high entropy alloys (HEAs).