Structure of oxide nanoparticles in Fe-16Cr MA/ODS ferritic steel

Oxide nanoparticles in Fe-16Cr ODS ferritic steel fabricated by mechanical alloying (MA) method have been examined using high-resolution transmission electron microscopy (HRTEM) techniques. A partial crystallization of oxide nanoparticles was frequently observed in as-fabricated ODS steel. The crystal structure of crystalline oxide particles is identified to be mainly Y₄Al₂O₉ (YAM) with a monoclinic structure. Large nanoparticles with a diameter larger than 20 nm tend to be incoherent and have a nearly spherical shape, whereas small nanoparticles with a diameter smaller than 10 nm tend to be coherent or semi-coherent and have faceted boundaries. The oxide nanoparticles become fully crystallized after prolonged annealing at 900 °C. These results lead us to propose a three-stage formation mechanism of oxide nanoparticles in MA/ODS steels.     

Tailoring the secondary phases and mechanical properties of ODS steel by heat treatment

The oxide dispersion strengthened (ODS) steel with the nominal composition of Fe–14Cr–2W–0.3Ti–0.2V–0.07Ta–0.3Y₂O₃ (wt%) was fabricated by mechanical alloying and hot isostatic pressing (HIP). In order to optimize the relative volume fraction of secondary phases, the as-HIPed ODS steel was annealed at 800 °C, 1000 °C, 1200 °C for 5 h, respectively. The microstructures and different secondary phases of the as-HIPed and annealed ODS samples were identified by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The tensile properties of all the ODS steels at room temperature were also investigated. The results indicate that annealing is an effective way to control the microstructure and the integral secondary phases. The annealing process promotes the dissolution of M₂₃C₆ particles, thus promoting the precipitation of TiC. No obvious coarsening of Y₂Ti₂O₇ nanoparticles can be observed during annealing. The tensile results indicate that the annealed ODS sample with the optimized secondary phases and high density possesses the best mechanical properties.     

Microstructural evolution of oxide-dispersion-strengthened Fe–Cr model steels during mechanical milling and subsequent hot pressing

Oxide-dispersion-strengthened (ODS) ferritic steels of Fe–9Cr–0.3Y₂O₃ and Fe–9Cr–0.2Ti–0.3Y₂O₃ (in mass) incorporating nanoscale oxide particles, were produced by mechanical milling (MM) followed by hot pressing (HP). Microstructural evolution of these two types of ODS steels were structurally characterized at each step of the elaboration processes by means of scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), and optical microscope. The observations of structure of the mixed powders and the nanoscale oxide particles in both ODS steels after MM indicate 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 refinement of bcc grains, Y₂O₃ nanocrystals and the formation of amorphous phase of Y₂O₃ during MM. TEM observations of these two Oxide-dispersion-strengthened (ODS) steels exhibit a very fine structure of micrometer-scale grains in which large number of nanoscale oxide particles are distributed after HP process. The observation of some unreinforced domains without the nanoscale oxide particles indicates that there still exist inhomogeneous areas, although the size of those oxide particles reaches nanoscale. Threshold stress of the HPped Fe–9Cr–0.2Ti–0.3Y₂O₃ steel with the relatively homogeneous dispersion 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.     

Microstructure evolution of in-situ nanoparticles and its comprehensive effect on high strength steel

A novel steel strengthened by nanoparticles was investigated in this study. A Fe-based high-strength steel was developed by the trace-element regional supply method during deoxidization to generate in situ nanoparticles with a high number density in the matrix. The results show that the endogenous nanoparticles are aluminum oxide (Al₂O₃) and titanium oxide (Ti₃O₅) formed in the liquid melt. Al₂O₃ functioned as a heterogeneous nucleation site for MnS during solidification; the size of the resultant complex inclusions was approximately 1–2 μm. Furthermore, 13 nm Nb(C,N) precipitates grew with the Ti₃O₅ during the tempering process. These in situ nanoparticles strongly affected refining of the grain and inclusions. The investigated steel was strengthened more than 200 MPa by precipitation strengthening and more than 265 MPa by grain refinement strengthening according to the Ashby–Orowan mechanism and the Hall–Petch relationship, respectively.     

Effect of test atmosphere composition on high-temperature oxidation behaviour of CoNiCrAlY coatings produced from conventional and ODS powders

Abstract

The oxidation behaviour of free-standing CoNiCrAlY coatings produced by low-pressure plasma spraying using conventional powder and oxide dispersion strengthened (ODS) powder containing 2 wt. % Al-oxide dispersion was investigated. Thermogravimetric experiments at 1100 °C in Ar-20%O₂ and Ar-4%H₂-2%H₂O showed lower oxidation rates of the ODS than the conventional coating. In the latter material the scale growth was enhanced by extensive Y-incorporation of Y/Al-mixed oxide precipitates in the scale and apparently by Y-segregation to oxide grain boundaries. In the ODS coating the alumina dispersion bonded Y in the form of Y-aluminate thereby effectively suppressing scale ‘overdoping’. SEM/EBSD studies of all alumina scales revealed a columnar grain structure with the lateral grain size increasing approximately linearly with depth from the oxide/gas interface. For both coatings the alumina scale growth was slower in Ar–H₂–H₂O than in Ar–O₂. The result is believed to be related to a lower oxygen potential gradient and to slower grain boundary diffusion in the scale forming in H₂/H₂O containing gas.     

Microstructure characterisation and tensile properties of 18Cr-4.5Al-0.3Zr- oxide dispersion strengthened steel

The 18Cr–4.5Al–0.3Zr–oxide dispersion strengthened (ODS) steel was fabricated by mechanical alloying (MA) and spark plasma sintering (SPS) technique. A microstructural characterisation was performed on an 18Cr–4.5Al–0.3Zr–ODS steel using high angle annual dark field (HAADF) and synchrotron small angle X-ray scattering (SAXS). HAADF and SAXS results showed that high-density nanoscale oxides are formed in 18Cr–4.5Al–0.3Zr–ODS steel. The oxides in the specimen can be roughly divided into two categories according to their compositions: (1) core/shell structure oxides with Al–O oxide cores and Y shells; (2) nm-scale trigonal-phase Y₄Zr₃O₁₂ oxides. In addition, tensile testing results revealed that the specimen exhibited better tensile strength and ductility as compared with another commercial ODS steels with similar composition.     

A new route for the synthesis of submicron-sized LaB6

Submicron crystalline LaB₆ has been successfully synthesized by a solid-state reaction of La₂O₃ with NaBH₄ at 1200 °C. The effects of reaction temperature on the crystal structure, grain size and morphology were investigated by X-ray diffraction, scanning electron microscope and transmission electron microscope. It is found that when the reaction temperature is in the range of 1000–1100 °C, there are ultrafine nanoparticles and nanocrystals that coexist. When the reaction temperature elevated to 1200 °C, the grain morphology transformed from ultrafine nanoparticle to submicron crystals completely. High resolution transmission electron microscope images fully confirm the formation of LaB₆ cubic structure.     

Relation between microstructure and Charpy impact properties of an elemental and pre-alloyed 14Cr ODS ferritic steel powder after hot isostatic pressing

This article describes the microstructure and Charpy impact properties of an Fe–14Cr–2W–0.3Ti–0.3Y₂O₃ oxide dispersion strengthened (ODS)-reduced activation ferritic (RAF) steel, manufactured either from elemental powders or from an Fe–14Cr–2W–0.3Ti pre-alloyed powder. ODS RAF steels have been produced by mechanical alloying of powders with 0.3 wt% Y₂O₃ nanoparticles in either a planetary ball mill or an attritor ball mill, for 45 and 20 h, respectively, followed by hot isostatic pressing (HIPping) at 1,150 °C under a pressure of 200 MPa for 4 h and heat treatment at 850 °C for 1 h. It was found that the elemental ODS steel powder contains smaller particles with a higher specific surface area and a two times higher oxygen amount than the pre-alloyed ODS steel powder. After HIPping both materials exhibit a density higher than 99%. However, the pre-alloyed ODS steel exhibits a slightly better density than the elemental ODS steel, due to the reduced oxygen content in the former material. Charpy impact experiment revealed that the pre-alloyed ODS steel has a much larger ductile-to-brittle transition temperature (DBTT) (about 140 °C) than the elemental ODS steel (about 25 °C). However, no significant difference in the upper shelf energy (about 3.0 J) was measured. TEM and SEM–EBSD analyses revealed that the microstructure of the elemental ODS steel is composed of smaller grains with a larger fraction of high-angle grains (>15°) and a lower dislocation density than the pre-alloyed ODS steel, which explains the lower DBTT value obtained for the elemental ODS steel.     

Microstructure and mechanical properties of ODS Fe14Cr model alloy processed by equal channel angular pressing

Abstract

A nanograin sized model oxide dispersion strengthened (ODS) ferritic steel with nominal composition Fe–14Cr–0·3Y₂O₃ (wt-%) was produced by mechanical alloying and consolidated by hot isostatic pressing. The alloy was submitted to severe plastic deformation by equal channel angular pressing (ECAP). Microstructural and mechanical characterisation was performed before and after ECAP. It was found that ECAP decreases and homogenises grain size without altering the nanoparticle dispersion, in addition to enhancing ductility and shifting the strength drop at high temperatures.     

Feasibility of using Y2Ti2O7 nanoparticles to fabricate high strength oxide dispersion strengthened Fe–Cr–Al steels

Addition of Al can improve the corrosion resistance of oxide dispersion strengthened (ODS) steels. However, Al reacts with Y₂O₃ to form large Y–Al–O particles in the steels and deteriorates their mechanical properties. Herein, we successfully prepared Y₂Ti₂O₇ nanoparticles (NPs) by the combination of hydrogen plasma-metal reaction (HPMR) and annealing. Y₂Ti₂O₇ NPs with contents of 0.2 or 0.6 wt.% were then added into the Fe–14Cr–3Al–2W–0.35Ti (wt.%) steel to substitute the conventional Y₂O₃ NPs by mechanical alloying (MA). The Y₂Ti₂O₇ NPs transformed into amorphous-like structure after 96 h MA. They crystallized with a fine size of 7.4 ± 3.7 nm and shared a semi-coherent interface with the matrix after hot isostatic pressing (HIP) of the ODS steel with 0.6 wt.% Y₂Ti₂O₇. With the increasing Y₂Ti₂O₇ content from 0.2 to 0.6 wt.%, the tensile strength of the ODS steel increased from 1238 to 1296 MPa, which was much higher than that (949 MPa) of the ODS steel added with Y₂O₃. The remarkably improved mechanical properties of the Al-containing ODS steels were attributed to the increasing number density of Y₂Ti₂O₇ nanoprecipitates. Our work demonstrates a novel route to fabricate high performance ODS steels with both high mechanical strength and good corrosion resistance.     

Effect of hydrothermal growth temperature on structural and optical properties of TiO2 nanoparticles

TiO₂ nanoparticles have been prepared by hydrothermal method at different temperatures. The X-ray diffraction results showed that anatase TiO₂ nanoparticles with grain size in the range of 7–27 nm has been obtained. HRTEM images show the formation of TiO₂ nanoparticles with grain size ranging from 7 to 26 nm. The Raman spectra exhibited peaks corresponding to the anatase phase of TiO₂. Optical absorption studies reveal that the absorption edge shifts towards longer wavelength (red shift) with increasing hydrothermal temperature.     

Au–ZnO hybrid nanostructures prepared by electro-exploding wire technique: Raman signal enhancement and photoluminescence emission quenching

Electro-exploding wire (EEW) technique was employed to prepare ZnO and Au–ZnO hybrid nanoparticles. Average size of the prepared ZnO nanoparticles is found to be 3.8 nm and uniform throughout. These ultrafine ZnO nanoparticles are found to agglomerate around the highly surface active Au nanoparticles. It also acts as a stabilizer for the Au nanoparticles by avoiding self agglomeration. The hybrid nanocrystals show strong crystallinity of face-centered cubic and hexagonal wurtzite structure of gold (Au) and zinc oxide (ZnO), respectively. Presence of Au₃Zn in pristine sample is a clear indication of a strong interaction between ZnO and Au systems. The hybrid system shows strong enhancement in the ZnO Raman signals and quenching in the visible Photoluminescence (PL) emission. Energy-dependent PL analysis shows the dominance of the surface defects over the bulk contribution in these ultrafine ZnO and Au–ZnO hybrid nanostructures.     

Microstructures and properties of Cu-10Sn oil bearings reinforced by Al2O3 nanoparticles

Using Sn and Cu-Al powders as raw materials, three oxide dispersion-strengthened (ODS) Cu-10Sn alloy powders with different Al₂O₃ mass content (0.42%, 0.85% and 1.68%, respectively) were prepared by mechanochemical synthesis combined with diffusion alloying method. The oil bearings were then fabricated by pressing and sintering. The effects of Al₂O₃ on microstructures and properties of the powders and the oil bearings were investigated. The results show that Al₂O₃ nanoparticles with sizes of about 5 nm are uniformly distributed in the ODS Cu powders. The content of Al₂O₃ nanoparticles has no effect on the distribution uniformity of tin during the diffusion process. And three ODS Cu-10Sn alloy powders with homogeneous tin distribution are prepared. However, the temperatures of forming liquid phases in the ODS Cu-10Sn alloys decrease with increasing Al₂O₃ content. This affects the sintering behaviors and mechanical properties of oil bearings. Increasing Al₂O₃ content also has a significant promoting effect on the precipitation of Sn-rich particles. In order to synergize the solid solution strengthening of tin and the dispersion strengthening of Al₂O₃ nanoparticles, the content of Al₂O₃ in the ODS Cu-10Sn alloys will not exceed 0.85%. The ODS Cu-10Sn oil bearings with 0.42% Al₂O₃ sintered at 900 °C have the best comprehensive properties, with uniform radial and axial shrinkage, oil content of 19.1%, radial crushing strength of 284 MPa and micro-hardness of 142 HV.     

(Cu,Fe, Co,or Ni)-doped tin dioxide films deposited by spray pyrolysis: Doping influence on thermal stability of the film structure

The results of doping influence on thermal stability of the SnO₂ film morphology are presented in this article. The SnO₂ films doped by Fe, Cu, Ni, Co (16 at.%) were deposited by spray pyrolysis from 0.2 M SnCl₄–water solution at T_pyr 350–450 °C. The annealing at 850–1030 °C was carried out in the atmosphere of the air. The change of such parameters as film morphology, the grain size, texture and the intensity of X-ray diffraction (XRD) peaks have been controlled. For structural analysis of tested films we have been using X-ray diffraction, Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM) techniques. It was established that the doping does not improve thermal stability of both film morphology and the grain size. It was made a conclusion that the increased contents of the fine dispersion phase of tin oxide in the doped metal oxide films, and the coalescence of this phase during thermal treatment are the main factors, responsible for observed changes in the morphology of the doped SnO₂ films.     

A synergistically enhanced photothermal transition effect from mesoporous silica nanoparticles with gold nanorods wrapped in reduced graphene oxide

In this work, near-infrared (NIR)-responsive core–shell gold nanorods/mesoporous silica/reduced graphene oxide (Au/SiO₂/rGO) nanoparticles with synergistically enhanced photothermal stability and transition effect had been prepared via electrostatic interaction. Gold nanorods (AuNRs) and rGO were employed as the NIR-responsive components. UV–Vis–NIR extinction spectra revealed that the surface plasmon resonance peak of AuNRs from Au/SiO₂/rGO nanohybrids remained unchanged after 9 h NIR exposure. UV–Vis–NIR extinction results also showed that thin silica shell was superior to the thick ones in the photothermal stability improvement of Au/SiO₂/rGO nanoparticles. Moreover, the doxorubicin release of Au/SiO₂/rGO was more rapid than that of Au/SiO₂ upon NIR irradiation, indicating that synergistically enhanced photothermal effect between rGO and AuNRs endowed Au/SiO₂/rGO nanoparticles with excellent photothermal transition efficiency. Such novel NIR-responsive core–shell hybrid nanoparticles with enhanced photothermal stability and transition effect are well suited for further biological applications, such as photothermal therapy, bioimaging and drug delivery.     

Effect of ultrafine grain structure on strain hardening of C–Mn steel warm rolled and subjected to intercritical annealing

Abstract

The behaviour during the work hardening of low carbon–manganese (0·15%C–1·39%Mn) steel with an ultrafine ferritic grain structure was investigated using Jaoul–Crussard analysis. This microstructure was produced through out quenching, warm rolling and intercritical annealing at 800°C. The steel exhibited a high strain hardening exponent and tensile strength.     

Transient liquid phase diffusion bonding of oxide dispersion strengthened nickel alloy MA758

Abstract

Transient liquid phase diffusion bonding has been used to join an oxide dispersion strengthened (ODS) nickel alloy (MA758) using an amorphous metal interlayer with a Ni–Cr–B–Si composition. A microstructural study was undertaken to investigate the effect of parent metal grain size on the joint microstructure after isothermal solidification. The ODS alloy was bonded both in fine grain and recrystallised conditions at 1100°C for various hold times. The work shows that the final joint grain size is independent of the parent alloy grain structure and the bonding time. However, when the alloy is bonded in the recrystallised condition and given a post-bond heat treatment at 1360°C, the joint grain size increases and a continuous parent alloy microstructure across the joint region is achieved. If MA758 is bonded in the fine grain condition and then subjected to a recrystallising heat treatment at 1360°C, the grains at the joint appear to increase in size with increasing bonding time. The joint grains are generally larger than those produced when the alloy is bonded in the recrystallised condition. The differences in microstructural developments across the joint are discussed in terms of stored strain energy of the parent metal grains.     

Achieving superplasticity at high strain rates using equal channel angular pressing

Abstract

Equal channel angular pressing (ECAP) is a processing procedure in which a sample is pressed through a die containing a channel bent into an L shaped configuration. This procedure introduces a high strain into the sample without any change in the cross-sectional area and it may be used to attain an ultrafine grain size with dimensions lying typically within the submicrometer range. This paper describes a series of experiments where ECAP was applied to a commercial Al–Mg–Li–Zr alloy having an initial grain size of ～400 µm. The results demonstrate a refinement in the grain size of this alloy to a size of ～1 µm and it is shown that these small grains are stable up to temperatures >600 K because of the presence of β′-Al₃Zr particles. The stability of these ultrafine grains at elevated temperatures provides an opportunity to achieve superplastic ductilities in this alloy at very high strain rates: for example, the measured elongations to failure under optimum pressing conditions exceed 1000% at a strain rate of 10^-1 s^-1 when testing at temperatures above 600 K.     

Capability of mechanical alloying and SPS technique to develop nanostructured high Cr,Al alloyed ODS steels

Abstract

Oxide dispersion strengthened (ODS) Fe alloys were produced by mechanical alloying (MA) with the aim of developing a nanostructured powder. The milled powders were consolidated by spark plasma sintering (SPS). Two prealloyed high chromium stainless steels (Fe–14Cr–5Al–3W) and (Fe–20Cr–5Al+3W) with additions of Y₂O₃ and Ti powders are densified to evaluate the influence of the powder composition on mechanical properties. The microstructure was characterised by scanning electron microscope (SEM) and electron backscattering diffraction (EBSD) was used to analyse grain orientation, grain boundary geometries and distribution grain size. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) equipped with energy dispersive X-ray spectrometer (EDX) were used to observe the nanostructure of ODS alloys and especially to observe and analyse the nanoprecipitates. Vickers microhardness and tensile tests (in situ and ex situ) have been performed on the ODS alloys developed in this work.     

Effect of Carbon Shell on the Structural and Magnetic Properties of Fe3O4 Superparamagnetic Nanoparticles

Carbon-encapsulated iron oxides (Fe₃O₄/C) with a core/shell structure have been successfully synthesized by using a simple two-step hydrothermal method at 180 ^°C. Fe₃O₄ core nanoparticles were prepared by coprecipitation under two conditions. Synthesized nanoparticles were characterized by transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. TEM images and FTIR results prove that carbon coated iron oxide is formed and the estimated size for most of them is below 11 nm, which was consistent with the XRD result. The Williamson–Hall (W–H) method has been used to calculate crystallite sizes and lattice strain based on the peak broadening of the Fe₃O₄ and Fe₃O₄/C nanoparticles. The results of VSM imply that the Fe₃O₄ core and core–shell nanoparticles are superparamagnetic. The saturation magnetization of Fe₃O₄ and Fe₃O₄/C are 49 emu/gr and 40 emu/gr, respectively. The magnetic behaviors reveal that the amorphous carbon shell can decrease the saturation magnetization of Fe₃O₄ nanoparticles due to core–shell interface effects and shielding.