Distribution Of Oxide Particles Research Articles

Microstructural Characterization of AlCrCuFeMnNi Complex Concentrated Alloy Prepared by Pressureless Sintering.

A significant and increasing number of studies have been dedicated to complex concentrated alloys (CCAs) due to the improved properties that these metallic materials can exhibit. However, while most of these studies employ melting techniques, only a few explore powder metallurgy and pressureless sintering as production methods. In this work, a microstructural characterization of AlCrCuFeMnNi CCA samples obtained by powder metallurgy and pressureless sintering using mixtures of powders with different compositions was carried out. One batch of samples (B1) was prepared using commercial powders of Al, Cr, Cu, Fe, Mn, and Ni. Another batch (B2) used mixtures of CrFeMn, AlNi, and Cu powders. A third set of samples (B3) was obtained by adding 1% at. of Mg to the B2 powder. The samples were characterized by X-ray diffraction, scanning and transmission electron microscopy, energy dispersive spectroscopy, density measurements, and hardness tests. Thermodynamic calculations were also used to complement the microstructural characterization. All the obtained samples exhibited high relative density and hardness values. However, B3 samples showed a higher hardness, attributed to the finer distribution of oxide particles, which was promoted by the presence of Mg during sintering. These last samples presented a hardness/density ratio of 62 HV/(g cm-3), surpassing that of some martensitic stainless steels and nickel-titanium alloys.

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.

Uncertainty and statistics of dislocation-oxide interactions on strength and creep failure of oxide dispersion strengthened steels

Oxide dispersion strengthening (ODS) steels exhibit excellent mechanical and creep properties due to the nanoscale oxide particles as barriers to dislocation motion, for the great application potentials to structural components of advanced nuclear power plants. However, the classical ODS model described by the Orowan mechanism or cutting mechanism only depends upon a single average size, ignoring uncertainty and statistics of dislocation-oxide interactions induced by the spatial and dimensional random distribution of oxide particles. When the oxide particle size is larger than the critical size to determine the interaction mechanism transformation from cutting to looping, the Orowan mechanism and cutting mechanism would exist simultaneously. Here we propose a modified ODS model for strength and creep rate considering the spatial distribution and size distribution of oxide particles, which is more consistent with the experimental result compared to the classical ODS model. In addition, the contribution of oxide particles to the yield strength and creep strength is evaluated quantitatively for various precipitate sizes and volume fractions. It is believed that this model provides an analytical framework to accurately predict the strength and creep rate of ODS steels.

Mechanical properties and microstructural analysis of ultra-fine grained Ni-based ODS alloy processed by powder forging

In the present study powder forging was employed to consolidate mechanically alloyed Ni-20Cr-20Fe-0.6Y2O3 Ni-based ODS powder. A mild steel can was filled with the mechanically alloyed powder and heated to 1200 °C in a Ar+H2 (90:10) atmosphere and hot forged in a channel die. The forged slab was characterized in terms of residual porosity, microstructure, oxide particle size and mechanical properties at room as well as elevated temperatures. The forged slab exhibited close to full density (> 99%), ultra-fined grain size of ∼503 nm and an average oxide particle size of 25.6 nm. Enhanced mechanical properties at room and elevated temperatures were obtained due to a combination of ultra-fine grains and uniform distribution of oxide particles. The high strain rates involved during powder forging cause the breaking or shearing out the oxide layer, oxide particles, and other prior particle boundary (PPB’s) forming compounds present on the surface of the particles and dispersing them into the grains interior and along the grain boundaries. The exposed material and sliding due to shear stress also enhances strong metallurgical bonding across collapsed interfaces and consequently improves mechanical properties. The absence of prior particles boundaries (PPB) network, nearly isotropic properties and almost full density indicated that the powder forging route can emerge as an alternative consolidation technique to HIP and hot extrusion for the consolidation of powder metallurgy ODS superalloys.

Biphasic solid electrolytes with homogeneous Li-ion transport pathway enabled by metal–organic frameworks

Solid-state lithium batteries (SSLBs) based on non-flammable inorganic solid electrolytes have been proposed as promising technical solutions to resolve safety issues caused by flammable organic liquid electrolytes of current Li-ion batteries. Biphasic solid electrolytes (BSEs) comprising Li+-conducting oxides and polymers have garnered significant interest for SSLBs because of their mechanical robustness and high Li+ conductivity. However, the non-uniform distribution of oxide particles and polymer species in BSEs may cause inhomogeneous Li+ conduction, thereby resulting in poor interfacial stability with electrodes during repeated charge–discharge cycles. Herein, we report a Li7La3Zr2O12-based BSE with homogeneous Li+ transport pathways achieved by a metal–organic framework (MOF) layer. To regulate and homogenize the Li+ flux across the interface between the BSE and electrode, a free-standing BSE is integrated with the MOF layer. The MOF-integrated BSE forms smooth and uniform interfaces with nanoporous channels in contact with the electrodes, effectively enhancing the interfacial solid–solid contact and facilitating homogeneous Li+ transport. An SSLB with the MOF-BSE membrane shows enhanced cycling stability and rate-capability compared to the battery with bare BSE. This study demonstrates that the proposed electrolyte design provides an effective approach for improving the conducting properties and interfacial stability of BSEs for high-performance and long-cycling SSLBs.

How fluid-mediated rock transformations can mimic hydro-fracturing patterns in hydrothermal dolomite

Numerous studies of hydrothermal dolomite bodies describe brecciated rocks characterized by dark angular fragments of dolomicrite bounded together by saddle dolomite crystals forming veins. Often associated with ore deposits, these hydrothermal dolomite breccia (HDB) patterns are usually interpreted as related to hydro-fracturing and so are considered as univocal markers of fluid overpressure. In this contribution we report an occurrence of typical HDB pattern in a dolomitized sedimentary breccia related to Mesozoic hyper-extension located in the Northern Pyrenees (France). We propose a model that accounts for the textural and crystallographic patterns of a HDB, without any fluid overpressure. The following observations lead us to propose that this HDB results from texturally controlled, hydrothermal fluid-initiated recrystallization of the initial sedimentary breccia by grain boundary migration: (1) the occurrence of rough contacts between the dolomite crystals and an initial dolomitized sedimentary breccia structure; (2) the crystallographic pattern of the hydrothermal dolomite; (3) the different grain size distribution in HDB texture and initial breccia structure; (4) the spatial distribution of oxide particles. Our alternate interpretation was further tested by 2D numerical simulations of the replacement process using the ‘ELLE’ modelling toolbox. Simulations confirm that advection of hot fluids along porous zones can locally trigger recrystallization by grain boundary migration, a replacement of the initial breccia texture and a preservation of larger, less permeable and cold fragments. We propose key features in textural evidence to assess whether HDB texture is pinpointing towards episodes of fluid overpressure and fracturing in carbonates, or if it rather is related to fluid-mediated rock recrystallization.

Combination of enhanced strength and sufficient tensile ductility in a sintered ultrafine-grained CoFeMnNi medium-entropy alloy

A CoFeMnNi medium-entropy alloy (MEA) was prepared by mechanical alloying (MA) and spark plasma sintering (SPS). The mechanically alloyed powders were sintered to near-full density and the as-sintered material consists of FCC matrix comprising a minor distribution of oxide particles. A high twin density is observed in MEA bulk samples. The twins are not generated during high-energy ball-milling but formed in the recrystallized grains during the sintering process. Compressive and tensile tests were performed on the sintered specimen at room temperature. The tension-compression yield asymmetry is weakened with increasing sintering temperature. An ultrafine-grained microstructure with an average grain size of 535 nm is obtained at a sintering temperature of 1000 °C. The sintered MEA exhibits an enhanced yield strength of 740 MPa, which is by 244% higher than that of its coarse-grained counterpart, meanwhile maintaining a sufficient tensile ductility of 14.4%. These excellent mechanical properties mainly stem from the high relative density, fully recrystallized ultrafine grains, and precipitation of fine particles that significantly strengthen the material while simultaneously providing high strain-hardening capability.

Phonon Scattering and Suppression of Bipolar Effect in MgO/VO2 Nanoparticle Dispersed p-Type Bi0.5Sb1.5Te3 Composites.

Bismuth-Telluride-based compounds are unique materials for thermoelectric cooling applications. Because Bi2Te3 is a narrow gap semiconductor, the bipolar diffusion effect is a critical issue to enhance thermoelectric performance. Here, we report the significant reduction of thermal conductivity by decreasing lattice and bipolar thermal conductivity in extrinsic phase mixing of MgO and VO2 nanoparticles in Bi0.5Sb1.5Te3 (BST) bulk matrix. When we separate the thermal conductivity by electronic , lattice , and bipolar thermal conductivities, all the contributions in thermal conductivities are decreased with increasing the concentration of oxide particle distribution, indicating the effective phonon scattering with an asymmetric scattering of carriers. The reduction of thermal conductivity affects the improvement of the ZT values. Even though significant carrier filtering effect is not observed in the oxide bulk composites due to micro-meter size agglomeration of particles, the interface between oxide and bulk matrix scatters carriers giving rise to the increase of the Seebeck coefficient and electrical resistivity. Therefore, we suggest the extrinsic phase mixing of nanoparticles decreases lattice and bipolar thermal conductivity, resulting in the enhancement of thermoelectric performance over a wide temperature range.

Effect of Re2O3–MgO additives on the microstructure evolution and properties of β-Si3N4 ceramics

β-Si3N4 ceramics were prepared by gas pressure sintering using α-Si3N4 powders, β-Si3N4 whiskers and Re2O3–MgO (Re = Y, La, Gd, Ho, Yb) additives. The phase transformation from α-Si3N4 to β-Si3N4 was completed after sintering. The distribution of oxide particles gets more homogeneous with the increase of the atomic number of the Re element, resulting in an increase of the grain size and the formation of elongated β-Si3N4 grains, which significantly influence the fracture mode and the mechanical properties of β-Si3N4 ceramics. The thermal conductivity of β-Si3N4 ceramics is determined by both the grain size and the distribution homogeneity of oxide particles, thus showing no regular trend between different specimens.

Microstructural evolution of two-way cold rolled 12Cr ODS steel under 1073–1373 K annealing

Abstract Microstructural evolution and hardness variations of cold rolled 12Cr ODS steel with 60%–90% thickness reductions after 1073–1373 K annealing were systematically studied by electron backscatter diffraction technique, transmission electron microscopy, and Vickers hardness measurements. Recovery took place with heating up to 1273 K in specimens with 60% thickness reduction accompanying with gradual hardness decrease, and followed by recrystallization at 1373 K with an evident hardness reduction. By contrast, regarding specimens with 75% and 90% thickness reductions, recrystallization was observed at 1273 K which is 100 K lower than that of 60% thickness reduction, and 400 K lower than that of as-fabricated specimen without further cold rolling, indicating that a higher thickness reduction stimulated the onset of recrystallization at a lower temperature. Furthermore, partial recrystallization was characterized at 1073 K and 1173 K in the specimen with 90% thickness reduction, which produced equiaxed grain bands along the final rolling direction with none/lower density of oxide particles, and full recrystallization occurred at 1273 K and above, forming coarse elongated grains with high density of oxide particles and a smaller fraction of equiaxed grains. These bimodal grains were probably ascribed to heterogeneous distribution of oxide particles in the matrix, which were partially dissolved during cold rolling process, and re-precipitated after the thermal annealing.

The role of Co/Fe ratio and in-situ Y dispersoids on microstructure and mechanical properties of CoFeNiMnCr high entropy alloys

Abstract In this study, non-equiatomic CoFeNiMnCr high entropy alloys (HEAs) with the different Co/Fe ratios and the variation of Y contents were synthesized by mechanical alloying. The effects of the in-situ Y dispersoids and Co/Fe ratios on microstructure evolution and mechanical properties of the model HEAs were investigated. The results demonstrated that the model HEA with a low Co/Fe ratio can cause the formation of pores and agglomeration of large oxide particles. A number of Cr-rich oxides and carbides were found in all the HEA samples after high temperature sintering. Additionally, the twinned structures are frequently observed in the FCC solid solution grains of HEAs. The presence of Y in HEAs can promote the formation of in-situ Y dispersoids and precipitation of the complex Cr-Y-O oxides, which have a strong influence on microstructure homogeneity, oxide particle distribution, densification and mechanical properties.

Oxide dispersion strengthened FeCoNi concentrated solid-solution alloys synthesized by mechanical alloying

FeCoNi concentrated solid-solution alloys (CSAs) and its modified version strengthened by in-situ formed nano-sized particles are successfully manufactured through mechanical alloying and spark plasma sintering method. Both the as-milled powders and the bulk materials are of single face-centered cubic structure. After sintering and heat treatment, grains are refined by 68% with the addition of oxides. In oxide dispersion strengthened (ODS) FeCoNi CSAs, the majority of nano-sized particles with diameter ranging from a few to ~ 40 nm are uniformly distributed in the matrix, and most of them are identified as Y2Ti2O7. Electron backscattered diffraction results reveal that the recrystallization process is suppressed due to the existence of the nano-sized particles. The size and distribution of oxide particles in the matrix can be controlled by adjusting experiment parameters of the mechanical alloying process and annealing treatment. Refined grains and high-density nano-sized particles result in enhanced compressive yield strength for ODS-FeCoNi CSAs at room temperature and 700 °C, which is 1295 MPa and 127 MPa, respectively.

Microstructure and texture evolution and ring-tensile properties of recrystallized FeCrAl ODS cladding tubes

Oxide dispersion strengthened (ODS) FeCrAl ferritic steels are being developed as potential accident tolerance fuel cladding materials for the light water reactors (LWRs) due to significant improvement in steam oxidation by alumina forming scale and good mechanical properties up to high temperatures. In this study, the microstructural characteristics and tensile properties of the two FeCrAl ODS cladding tubes with different extrusion temperatures of 1100 °C and 1150 °C were investigated during processing conditions. While the hot extruded sample showed micron sized elongated grains with strong α-fiber in <110> texture, cold pilger rolling process change the microstructure to submicron/micron size grain structure along with texture evolution to both α-fiber (<110> texture) and γ-fiber ({111} texture) via crystalline rotations. Subsequently, final annealing resulted in evolution of microstructure to large grain recrystallized structure starting at recrystallization temperature of ~810–850 °C. Two distinct texture development happened in recrystallized cladding tubes, i.e., only large elongated grains of (110) <211> texture following extrusion temperature of 1100 °C; and two texture components of (110) <211> and {111} <112> following higher extrusion temperature of 1150 °C. The different texture development and retarding of recrystallization progress in 1100°C-extruded cladding tubes were attributed to higher distribution of oxide particles. Furthermore, the ring-tensile test properties of recrystallized FeCrAl ODS cladding tubes were evaluated in the temperature range of 25–800 °C and showed a very good combination of strength-ductility with a lower strength and higher ductility for 1150 °C extruded tube up to ~500 °C and opposite properties at higher temperatures.

Improvement of high temperature mechanical properties of Ni-based oxide dispersion strengthened alloys by preferential formation of Y-Ti-O complex oxide

Y2O3 is mainly applied as a dispersoid in Ni-based oxide dispersion strengthened (ODS) alloys. The added Y2O3 reacts with Ti and Al at a temperature above 1150 ℃ to form a complex oxide such as Y-Ti-O or Y-Al-O complex oxide. However, previous studies have shown that Y-Al-O complex oxide has relatively coarser particles than Y-Ti-O complex oxide.In this study, Ti was first added to form Y-Ti-O complex oxide, which has relatively fine particles. Subsequently, Al was added to inhibit the formation of Y-Al-O complex oxide, thus improving the high-temperature mechanical properties of ODS alloys. Ni-based ODS alloy powders with composition Ni-15Cr-xTi-1.1Y2O3 (without aluminum) were mechanically alloyed using a planetary mill. Thereafter, the mechanically alloyed powders were heat treated to form a complex oxide of Y-Ti-O. A second mechanical alloying was performed by adding 4.5 wt% Al to the heat treated powders. The products obtained after the second alloying were sintered by spark plasma sintering. The phase of the sintered specimens were analyzed using X-ray diffraction, their microstructures were analyzed by transmission electron microscopy and the size distribution of oxide particles was confirmed by image analysis. Moreover, the high-temperature mechanical properties of each composition were analyzed using a hot hardness tester.

Ab initio modelling of the Y, O, and Ti solute interaction in fcc-Fe matrix

Abstract Strengthening of the ODS steels by Y2O3 precipitates permits to increase their operation temperature and radiation resistance, which is important in construction materials for future fusion and advanced fission reactors. Both size and spatial distribution of oxide particles significantly affect mechanical properties and radiation resistance of ODS steels. Addition of the Ti species (present also as a natural impurity atoms in iron lattice) in the particles of Y2O3 powder before their mechanical alloying leads to the formation of YTiO3, Y2TiO5, and Y2Ti2O7 nanoparticles in ODS steels. Modelling of these nanoparticle formation needs detailed knowledge of the energetic interactions between ions. We have performed detailed first principles calculations of the interactions between two Y and O atoms, two Ti and O atoms as well as between Y, Ti, and O atoms which will be used in further kinetic Monte Carlo modelling of the nanoparticle formation.

The effect of the initial microstructure in terms of sink strength on the ion-irradiation-induced hardening of ODS alloys studied by nanoindentation

Oxide dispersion strengthened (ODS) Fe-Cr alloys are promising candidates for structural components in nuclear energy production. The small grain size, high dislocation density and the presence of particle matrix interfaces may contribute to the improved irradiation resistance of this class of alloys by providing sinks and/or traps for irradiation-induced point defects. The extent to which these effects impede hardening is still a matter of debate. To address this problem, a set of alloys of different grain size, dislocation density and oxide particle distribution were selected. In this study, three-step Fe-ion irradiation at both 300 °C and 500 °C up to 10 dpa was used to introduce damage in five different materials including three 9Cr-ODS alloys, one 14Cr-ODS alloy and one 14Cr-non-ODS alloy. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), small angle neutron scattering (SANS), and nanoindentation testing were applied, the latter before and after irradiation. Significant hardening occurred for all materials and temperatures, but it is distinctly lower in the 14Cr alloys and also tends to be lower at the higher temperature. The possible contribution of Cr-rich α′-phase particles is addressed. The impact of grain size, dislocation density and particle distribution is demonstrated in terms of an empirical trend between total sink strength and hardening.

Mechanical Characterization of a Nano-ODS Steel Prepared by Low-Energy Mechanical Alloying

An oxide dispersion strengthened (ODS) ferritic steel with nanometric grain size has been produced by low-energy mechanical alloying (MA) of steel powder (Fe-14Cr-1W-0.4Ti) mixed with Y2O3 particles (0.3 wt %) and successive hot extrusion (HE). The material exhibits superior mechanical properties with respect to the unreinforced steel up to 400 °C; then such differences tend to progressively decrease and at 700 °C yield stress (YS) and ultimate tensile strength (UTS) values are very close. The microstructure and mechanical behaviour have been compared with those of ODS steels prepared by the most common process, high-energy MA, consolidation through hot isostatic pressing (HIP) or hot extrusion (HE), annealing around 1100 °C for 1–2 h. The main strengthening mechanisms have been examined and discussed to explain the different behaviour. In addition, heat treatments in the range 1050–1150 °C were carried out and a microstructural evolution with a relevant hardness decrease has been observed. TEM observations evidenced defect recovery and partial grain coarsening owing to the not perfectly homogeneous distribution of oxide particles.

Nano-CeO 2 and -LaCrO 3 dispersed ferritic stainless steels as potential interconnect materials for solid oxide fuel cells

Abstract Novel nano-oxide dispersed ferritic stainless steel alloys have been developed as potential interconnect materials for solid oxide fuel cells. Nanopowders of CeO 2 and LaCrO 3 were mixed with ferritic stainless steel powder (SUS430) using conventional ball milling. Rectangular pellets were prepared and sintered at 1400 °C in pure H 2 . The microstructural analysis of the sintered pellets showed a uniform distribution of oxide particles. Long-term area specific resistance (ASR) of the pellets was monitored using the four-probe DC method at 800 °C for 1000 h in air, revealing that an increase in the CeO 2 amount from 0.5 to 3 and 5 wt % increased the oxidation resistance of the steel. ASR values as low 32.2 and 5.5 mΩ cm 2 were observed, respectively, for 3 and 5 wt % nano-CeO 2 dispersed SUS430 steel. However, in the case of LaCrO 3 addition, no such relationship between the oxidation resistance and LaCrO 3 concentration was found. This study showed that the dispersed nano-CeO 2 particles effectively reduced the Cr 2 O 3 scale growth, likely by retarding the inward diffusion of oxygen, hence resulting in a high oxidation resistance at the SOFC operating conditions.

Comparison of Ductile-to-Brittle Transition Behavior in Two Similar Ferritic Oxide Dispersion Strengthened Alloys.

The ductile-to-brittle transition (DBT) behavior of two similar Fe-Cr-Al oxide dispersion strengthened (ODS) stainless steels was analyzed following the Cottrell–Petch model. Both alloys were manufactured by mechanical alloying (MA) but by different forming routes. One was manufactured as hot rolled tube, and the other in the form of hot extruded bar. The two hot forming routes considered do not significantly influence the microstructure, but cause differences in the texture and the distribution of oxide particles. These have little influence on tensile properties; however, the DBT temperature and the upper shelf energy (USE) are significantly affected because of delamination orientation with regard to the notch plane. Whereas in hot rolled material the delaminations are parallel to the rolling surface, in the hot extruded material, they are randomly oriented because the material is transversally isotropic.

Nanoscale lamellar γ/γ′ structure and preferred distribution of oxide particles in a new ODS superalloy

Two microstructural phenomena have been studied in a newly designed Ni-based ODS superalloy with a high solvus temperature. For the first time, a nanoscale lamellar γ/γ′ structure was obtained along the elastically soft [100] direction by a eutectoid-like reaction from a severely deformed matrix. In addition, the preferred distribution of semi-coherent Y2Hf2O7 oxide particles was observed only in the γ phase due to the prior partitioning of Y and O to this phase. The microstructure development during the processing route is discussed based on the solvus temperature of this new ODS superalloy in comparison to commercial ODS superalloys.