Ti Complex Oxides Research Articles

Phase equilibria of Al2O3–TiOx system under various oxygen partial pressure: Emphasis on stability of Al2TiO5–Ti3O5 pseudobrookite solid solution

Phase equilibria of Al2O3–TiOx system were experimentally determined in a wide range of T and PO2: 1300 °C ≤T≤ 1600 °C and −16.6≤logPO2≤−8.0 (in atm). X-Ray diffraction and electron probe micro-analysis were employed to obtain equilibrium phases and compositions. Corundum (Al2O3 with limited Ti2O3), rutile (TiO2 with limited Al2O3), and pseudobrookite (Al2TiO5–Ti3O5) solid solutions were found. The liquid oxide phase was not found up to 1650 °C. At each temperature, the cationic ratio of the pseudobrookite solid solution (RTi = nTi/(nAl + Ti)) increased as PO2, thereby forming a wide range of solid solution from Al2TiO5 (RTi = 1/3) to Ti3O5 (RTi = 1.0). The pseudobrookite solid solution was a complete miscible solution at 1600 °C, while it exhibited a small miscibility gap at 1500 °C. Based on the results obtained in the present study, the behavior of Al2TiO5 was discussed in two aspects: (1) changes in physical properties and (2) evolution of Al–Ti complex oxide inclusions in liquid steel.

Influence of Non-metallic Inclusions on Corrosive Properties of Polar Steel

Polar steel requires excellent toughness and corrosion resistance for breaking icy surfaces in low-temperature seawater environments. In this study, the effect of inclusions on the corrosion resistance of polar steel was examined. In the experiments conducted, the composition and morphology of the inclusions in steel were controlled using different deoxidation methods during steel refining. The morphology and composition of the corrosion-resistant active inclusions were analyzed using scanning electron microscopy and energy dispersive spectroscopy. The corrosion resistance of polar steel was determined by measuring the saturation current density of the anodic dissolution of steel in a corrosive medium via an electrochemical method. The corrosion resistance under simulated seawater was also investigated under laboratory conditions. It was found that as the ratio of Al/Mg approaches the stoichiometric composition of the spinel (2.3–2.5), the inclusions become less active; as the ratio increases further, the corrosion-resistant activity increases due to the formation of MgAl2O4⋅CaO complexes, leading to an increase in local stress around the inclusions. If steel is deoxidized with Zr–Ti, small Zr–Ti complex oxides form in the steel, providing nucleation particles for the precipitation of spheroidized and uniformly distributed MnS. Therefore, steel deoxidized with Zr–Ti has better seawater corrosion resistance than Al-deoxidized steel.

Crystal and metal/oxide interface structures of nanoparticles in 15Cr–2W–0.1Ti–4Al–0.6Hf–0.35Y2O3 ODS steel

FeCrAl oxide dispersion strengthened (ODS) steel is one of the most promising candidate cladding materials of generation IV nuclear fission reactors because of its excellent resistance to not only corrosion but also creep and irradiation due to the ultrahigh density nanometer-scale oxides. Crystal and metal/oxide interface structures of the nanoparticles in FeCrAl-ODS steel with Hf addition, i.e., Fe–15Cr–2W–0.1Ti–4Al–0.6Hf–0.35Y2O3, have been studied by high resolution transmission electron microscopy (HRTEM). The characterization of crystal structure was accomplished on up to 153 particles, which have diameter of 2–10 nm and peak number fraction and, therefore, represent the oxides contributing most significantly to the macroscopic properties of the ODS steel, and, subsequently, the proportions of particles having crystal structures consistent with various types oxides were determined by statistical analyses. Relative to FeCrAl-ODS steel without Hf addition, i.e., Fe–15.5Cr–2W–0.1Ti–4Al–0.35Y2O3 ODS steel, the coherency of the oxide nanoparticles was considerably improved. The crystal structure of about 51% of the nanoparticles were found to be consistent with Y2Hf2O7 oxide with anion-deficient fluorite structure while the crystal structures of only ∼32% of the nanoparticles are consistent with Y–Al complex oxides, which indicates that the addition of 0.6 wt.% Hf into FeCrAl-ODS steel inhibits the formation of Y–Al complex oxides remarkably while prompts the significant occurrence of Y–Hf complex oxides. The crystal structures of about 17% of the nanoparticles are consistent with Y–Ti complex oxides, which indicates even a very small amount of Ti could promote the significant formation of Y–Ti complex oxides. The crystallographic orientation correlations of the oxides and matrix were determined. The formation mechanisms of various kinds of oxides and, moreover, the reasons of the unusual irradiation tolerance and thermal stability of the ODS steel are discussed based on the results.

Effects of contents of Al, Zr and Ti on oxide particles in Fe–15Cr–2W–0.35Y2O3 ODS steels

FeCrAl oxide dispersion strengthened (ODS) steel is one of the most promising candidate cladding materials of generation IV nuclear reactors because of its excellent resistance to not only corrosion but also creep and irradiation due to the ultrahigh density nanometer-scale oxides. Effects of the contents of Al, Zr and Ti on the crystal and metal/oxide interface structures of the particles in Fe–15Cr–3.7Al–2W–0.1Ti–0.6Zr–0.35Y2O3 (3.7Al–0.1Ti–0.6Zr) and Fe–15Cr–3.8Al–2W–0.12Ti–0.5Zr–0.35Y2O3 (3.8Al–0.12Ti–0.5Zr) were studied by high resolution transmission electron microscopy (HRTEM). For 3.7Al–0.1Ti–0.6Zr and 3.8Al–0.12Ti–0.5Zr ODS steels, phase identification was accomplished on 175 and 213 particles, respectively, which have peak number fraction and, therefore, represent the oxides contributing most significantly to the macroscopic properties of the ODS steels. The proportions of various types of oxides were determined. For 3.7Al–0.1Ti–0.6Zr and 3.8Al–0.12Ti–0.5Zr ODS steels, the proportions of Y–Zr complex oxides are ∼87.4% and ∼54.4% while the number fractions of Y–Al complex oxides are ∼3.5% and ∼5.2%, respectively, indicating that 0.5–0.6 wt.% Zr inhibits the formation of Y–Al complex oxides remarkably while prompts the significant occurrence of Y–Zr complex oxides. For 3.7Al–0.1Ti–0.6Zr and 3.8Al–0.12Ti–0.5Zr ODS steels, the proportions of Y–Ti complex oxides are ∼5.7% and ∼36.6%, respectively, indicating it is effective for increasing the proportion of Y–Ti complex oxides by increasing the content of Ti from 0.09 wt.% to 0.12 wt.% with the content of Zr decreased from 0.63 wt.% to 0.49 wt.%. The crystallographic orientation correlations of the oxides and matrix were determined. The formation and refinement mechanisms of oxides and, moreover, the reasons of the unusual irradiation tolerance and thermal stability of the ODS steel were discussed based on the results.

Crystal and metal/oxide interface structures of nanoparticles in Fe–16Cr–0.1Ti–0.35Y2O3 ODS steel

Oxide dispersion strengthened (ODS) steel is one of the most promising candidate structural materials for the cladding of generation IV nuclear fission reactors and the blanket of future fusion reactors because it has excellent resistance to creep, irradiation, corrosion and, moreover, strong capability for managing helium due to the ultrahigh density nanometer-scale oxides. Crystal and metal/oxide interface structures of the nanoparticles in Fe–16Cr–0.1Ti–0.35Y2O3 ODS steel have been studied by high resolution transmission electron microscopy (HRTEM). The phases of 79 nanoparticles, which have diameter of 4–9 nm and peak number fraction and, therefore, represent the oxides contributing most significantly to the macroscopic properties of the ODS steel, were identified and, moreover, the proportions of various types of oxides were determined. About 59.5% of the nanoparticles are composed of Y–Ti complex oxides. Small amount of Ti addition, i.e., 0.1 wt.%, into Fe–Cr ODS steel with 0.35 wt.% Y2O3 and, moreover, the much lower content of excess oxygen prompts the significant formation of orthorhombic and hexagonal Y2TiO5 (45.6%), instead of cubic Y2Ti2O7 (2.5%). Orthorhombic YTi2O6 (2.5%) and tetragonal YCrO4 (7.6%) oxides were firstly found in Fe–Cr–Ti–Y2O3 ODS steel. Almost all the nanoparticles are either coherent or semi-coherent with the bcc steel matrix. The crystallographic orientation correlations of the oxides and matrix were determined. The mechanisms of the formation and polymorphic transition of various kinds of oxides and, moreover, the reasons of the unusual strengthening, thermal stability and radiation tolerance of the ODS steel were discussed based on the comprehensive database developed here.

Formation and evolution of macro inclusions in IF steels during continuous casting

The evolution of macro inclusions during continuous casting was investigated by large-area inclusion characterization using ASPEX and analysis of nozzle blockage deposits. Six kinds of inclusions over 5 μm were observed in samples taken from the tundish and the slabs: single alumina particles, alumina dendrites, refractory-related alumina, alumina associated with bubbles, alumina clusters and Al–Ti complex oxides. By examining the morphology of the nozzle blockage deposit, it was concluded that the refractory-related alumina in the slabs came from the decarburization layer washed away by the steel stream. Some of the alumina clusters that came from the nozzle blockage deposit dislodged by the steel flow, were formed by the agglomeration and sintering of 5–20 μm inclusions that were carried over from the tundish. Two kinds of Al–Ti oxides were found in the tundish, and their evolution mechanisms during the casting process were proposed.

Toughness improvement by Zr addition in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

ABSTRACTThe effect of Zr addition on the microstructure and impact toughness in the coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels subjected to 100 kJ cm-1 heat input was investigated. The second- phase particles were mainly Al–Ti complex oxides and (Ti,Nb)N precipitates in Zr-free steel, whereas lots of finer Zr–Al–Ti complex oxides and (Al,Ti,Nb)N precipitates were formed in Zr-bearing steel because of Zr addition. These finer oxides and precipitates effectively restricted the austenite grain growth by pinning effect during welding thermal cycle, and smaller and more uniform prior austenite grains were obtained in CGHAZ of Zr-bearing steel. Furthermore, more acicular ferrite grains nucleated on Zr–Al–Ti complex oxides, inducing formation of fine-grained microstructure in CGHAZ of Zr-bearing steel. The toughness improvement in CGHAZ of Zr-bearing steel with dimple fracture surface was attributed to the grain refinement by pinning effect and acicular ferrite formation.

In-situ microscopy study of grain refinement in the simulated heat-affected zone of high-strength low-alloy steel by TiN particle

ABSTRACTHigh-strength low-alloy steels subjected to high heat input welding are susceptible to failure due to low toughness caused by grain coarsening. The effect of TiN on grain refinement in the simulated heat-affected zone (HAZ) was investigated. Because of small amount of Ti addition, abundant dispersed nanoscale TiN precipitates were formed. The TiN precipitates tended to be stable at high temperature and effectively retarded the austenite grain growth by refining the grain size during thermal cycle. Furthermore, the TiN also covered on the surface of Al–Ti complex oxide with MnS and caused low interface energy with ferrite. The acicular ferrite grains nucleated on complex inclusion in austenite grains at intermediate temperature and induced the austenite grain transform to the fine-grained mixed microstructure of acicular ferrite and bainite. The crystallographic grain size became small in the simulated HAZ due to the effective pinning effect and acicular ferrite formation.

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

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

A Study on the Transient Inclusion Evolution during Reoxidation of a Fe–Al–Ti–O Melt

The effect of a simulated reoxidizing environment on the chemical and morphological evolution of non-metallic oxide inclusions was studied. Additions of 545 ppm and 274 ppm of soluble oxygen were introduced to an Al killed melt containing approximately 600 ppm of Ti and 600 ppm of Al. It was found that inclusion chemistry evolved from Al2O3, Al2TiO5 and eventually to Ti3O5 for the higher oxygen addition case and to Al–Ti complex oxides for the lower oxygen addition one. Morphologically, it was observed that irregular inclusions gradually were replaced by spherical ones during the reoxidation process. These changes are discussed through the coupling of thermodynamic prediction and experimental conditions, and considerations on the local variations of O and metallic element activities.

A Ce–Cu–Ti oxide catalyst for the selective catalytic reduction of NO with NH3

Abstract A Ce–Cu–Ti complex oxide catalyst for the selective catalytic reduction of NO with NH 3 was prepared by coprecipitation method. XRD and H 2 –TPR reveal that the strong interaction between Ce and Cu results in the production of a new active oxygen species with high reducibility at low temperatures. Compared with Ce–Ti oxide catalyst, the Ce–Cu–Ti oxide catalyst produces better performance at the temperatures lower than 350 °C and higher SO 2 -resistent ability. H 2 O will reduce the SCR activity of Ce–Cu–Ti catalyst at low temperatures while promoting the catalyst performance at temperatures more than 350 °C.

Formation of residual ferrite in 9Cr-ODS ferritic steels

It is recognized that the high-temperature strength of 9Cr-ODS ferritic steels is maintained by the presence of a ferrite phase. In order to clarify the formation process of the ferrite phase, 9Cr-ODS ferritic steels containing various contents of Y 2O 3, i.e., 0 mass%, 0.1 mass%, 0.35 mass%, and 0.7 mass%, are prepared by means of mechanical alloying and hot-pressing. The ferrite phase is formed by the addition of 0.35 mass% and 0.7 mass% Y 2O 3; however, it is not formed for steels without and with 0.1 mass% Y 2O 3 normalized for 1 h at 1050 °C. It is considered from the thermodynamic analyses that the pinning of the α–γ interface motion by the dispersed Y–Ti complex oxide particles can be attributed to the retention of the ferrite phase normalized at 1050 °C, and this ferrite phase cannot be δ-ferrite that is in equilibrium with the γ-phase.

Thermodynamic Analysis of Mn-depleted Zone Near Ti Oxide Inclusions for Intragranular Nucleation of Ferrite in Steel

A series of thermodynamic analyses have been performed to elucidate possible mechanism of Mn-depleted zone (MDZ) development near Ti oxide inclusions as nucleation sites of Intragranular Acicular Ferrite (IGF) transformation in steels, using a computational thermodynamic approach (CALPHAD). It is demonstrated that thermodynamic calculations are able to reproduce experimentally known inclusions evolution in the steels. The MDZ development near the Ti2O3 inclusions is shown to be a result of Mn absorption into the Ti2O3. Moreover, from thermodynamic analysis of phase equilibria in the Mn–Ti complex oxide system, it is proposed that a phase change of inclusion from (Ti3O5) s.s. (pseudobrookite structure solid solution dissolving Mn) to (Ti2O3) s.s. (ilmenite structure solid solution dissolving Mn) accelerate the Mn absorption into the Ti2O3 inclusion in steel. From a series of thermodynamic calculations, optimum thermal heating condition to enhance Mn absorption into Ti2O3 inclusions as well as IGF formation is discussed.

Stability of Y–Ti complex oxides in Fe–16Cr–0.1Ti ODS ferritic steel before and after heavy-ion irradiation

The mechanical strength of oxide dispersion strengthened (ODS) ferritic steels depends on the dispersion and microstructural stability of the oxide particles. Yttria is normally selected as dispersion particles to strengthen ODS ferritic steels, and other additional elements such as titanium and aluminum are added to obtain finer dispersion particles for the improvement of the mechanical properties of the ODS steels. Kyoto University is involved with the development of new ODS ferritic steels. 16Cr–0.1Ti–0.35Y 2O 3 ODS steel is a fusion structural material. In this research, the irradiation resistance of 16Cr–0.1Ti–0.35Y 2O 3 (16Cr0.1Ti) ODS steel was investigated by high-resolution TEM and by performing an ion irradiation experiment. The ODS steel was consolidated by hot extrusion at 1150 °C and annealed at 1050 °C for 1 h for recrystallization. Oxides in 16Cr0.1Ti ODS steel were characterized by HRTEM and STEM-EDX and specified as (Y, Ti) complex oxides with an atomic ratio of Y:Ti = 2:1. A 1.7 MeV tandem accelerator for heavy-ion irradiation at the dual-beam material irradiation facility for energy technology (DuET), Kyoto University, was used to introduce irradiation damages in materials. The comparison of the dispersion, shape, and chemical composition of the oxides between the pre- and post-irradiated materials revealed that (Y, Ti) complex oxides were stable under the ion irradiation up to 60 dpa at 650 °C.

Microstructural development of a heavily neutron-irradiated ODS ferritic steel (MA957) at elevated temperature

Microstructural observation was performed on a neutron-irradiated oxide dispersion strengthened (ODS) ferritic steel with emphasis on oxide behavior, including phase stability under irradiation at elevated temperature (∼973 K). Transmission electron microscopy observation of the Y–Ti complex oxide particles showed they were fine (∼40 nm) whereas the Ti-oxide particles were relatively coarse (∼300 nm). Dispersion parameters of oxide particles, such as mean size and number density, changed due to irradiation. This implies recoil resolution of the oxide particles. When irradiated at 973 K, some Y–Ti complex oxides survived and interacted with the dislocation structures, which delayed the dislocation recovery and stabilized the elongated grain structure. It is considered that oxide particles could be effective pinning points of dislocations in motion under irradiation to a dose of ∼100 dpa.

Photocatalytic Reaction on Layered Cs–Nb–Ti Complex Oxide

Abstract Ion-exchangeable layered titano-niobates, CsNbTi2O7 and CsNbTiO5, were prepared and the photocatalytic properties of those oxides as well as of the H+-exchanged forms were investigated. H2 or O2 evolution reaction from an aqueous methanol solution or from an aqueous silver nitrate solution was carried out under a high pressure mercury lamp irradiation. For H2 evolution, Pt (0.1 wt%)-loaded H+-exchanged forms showed remarkable activity both for CsNbTi2O7 and CsNbTiO5, while higher rates of O2 evolution were obtained for H+-exchanged forms than those for the original forms.