Ti-rich Oxide Research Articles

Oxide Growth Behaviour of Fe-Ni-Cr Alloy at High Temperature Oxidation

The isothermal oxidation test has been investigated on two types of Fe-Ni-Cr alloy, namely Fe-33Ni-19Cr and Fe-40Ni-24Cr alloys. Both alloys were undergoing an isothermal oxidation test at temperature of 500? for 500 hours exposure time. The weight change per surface area of the oxidized samples has been recorded to calculate the oxidation kinetics of both alloys. The oxide growth behaviour of oxidized samples has been examined using scanning electron microscope (SEM) equipped with energy dispersive x-ray (EDX) spectrometer. The oxide phase formed on the sample surface has been analyzed using x-ray diffraction (XRD) technique. The results show that both alloys were followed a parabolic rate law, indicating a diffusion-controlled oxide growth mechanism. In addition, the oxidation kinetics indicating an increasing weight gain trend as the exposure time increase. Several oxide phases had formed on the oxidized surface of both alloys, consists of Cr-rich, Ti-rich, Fe-rich and spinel oxide structure. The surface morphology of both alloys demonstrated a continuous oxide scale formed on the alloy surface. Additionally, Fe-33Ni-19Cr alloy recorded a formation of Ti-rich oxide, whereas, Fe-40Ni-24Cr alloy displayed a formation of overgrown Nb-rich oxide particle which. Roles of the precipitates in oxidation mechanism give new insights into the alloy optimization.

Internal Oxidation Behavior of Fe-33Ni-19Cr Alloy

Fe-33Ni-19Cr alloy is the Ni-based alloy used at high temperature condition due to its excellent ability to form protective oxide layer at high temperature. Fe-33Ni-19Cr alloy was undergo a series of solution treatment process to vary the grain size of the alloy. The Fe-33Ni-19Cr alloy was solution-treated at 3 different temperature, namely 1000°C, 1100°C and 1200°C, for 3 hours soaking time, followed by water quench. The solution-treated alloys were then experienced an isothermal oxidation test at 900°C for 500 hours in laboratory air. The oxidized Fe-33Ni-19Cr alloy were characterized in terms of phase analysis and cross-sectional analysis using XRD and SEM-EDX to investigated the effect of different grain size alloy to the oxidation behavior. The solution treatment process was produced varies grain size of Fe-33Ni-19Cr alloy. The solution-treated Fe-33Ni-19Cr alloy at 1000°C exhibited the fine grain size, while solution-treated Fe-33Ni-19Cr alloy at 1200°C produced a coarse grain size. The oxidized Fe-33Ni-19Cr alloy recorded a formation of several oxide phases consists of Cr-rich oxide, Fe-rich oxide, Ti-rich oxide and spinel oxides structure. The cross-sectional analysis displays a several layer of oxide scales formed on the alloy surface with evidence of internal oxide penetration through the grain boundary area.

Characteristics of oxide pegs in Ti- and Y-doped CoNiCrAl alloys at 1150 °C

Oxide pegging is a widely accepted mechanistic model explaining the reactive element effect on the improved adherence of scale. However, previous models for the oxide peg formation process have not considered the effects of more than one active element added into the alloy during the peg formation. This study proposes a new model of oxide peg formation and growth for the doping of two reactive elements in an alloy (the precipitated Y and solid solute Ti). Different amounts of Ti and Y were added to a CoNiCrAl alloy, and the characteristics of the resulting oxide pegs, such as their linear density, size, and forming process, are obtained by examining alloy samples subjected to an isothermal oxidation operation at a temperature of 1150 °C. It is found that the amount of Y determines the density of the oxide pegs, and Ti does not form a Ti-rich oxide core if a Y-rich oxide exists in the sample. In samples with the same Y content, the oxide pegs primarily grow in length, and with increased Ti content, they grow along the β-phase boundary and into the alloy. Based on these results, a three-step model for oxide peg formation and growth is conceived.

Melt chemistry and redox conditions control titanium isotope fractionation during magmatic differentiation

Titanium offers a burgeoning isotope system that has shown significant promise as a tracer of magmatic processes. Recent studies have shown that Ti isotopes display significant mass-dependent variations linked to the crystallisation of Fe-Ti oxides during magma differentiation. We present a comprehensive set of Ti isotope data for a range of differentiation suites from alkaline (Ascension Island, Afar and Heard Island), calc-alkaline (Santorini) and tholeiitic (Monowai seamount and Alarcon Rise) magma series to further explore the mechanics of Ti isotope fractionation in magmas. Whilst all suites display an increase in δ49/47Ti (deviation in 49Ti/47Ti of a sample relative to the OL-Ti reference material) during magma differentiation relative to indices such as increasing SiO2 and decreasing Mg#, our data reveal that each of the three magma series have contrasting δ49/47Ti fractionation patterns over comparable ranges of SiO2 and Mg#. Alkaline differentiation suites from intraplate settings display the most substantial range of variation (δ49/47Ti = +0.01 to +2.32‰), followed by tholeiites (−0.01 to +1.06‰) and calc-alkaline magmas (+0.06 to +0.64‰). Alkaline magmas possess high initial melt TiO2 contents which enables early saturation of ilmenite + titanomagnetite and a substantial degree of oxide crystallisation, whereas tholeiitic and calc-alkaline suites crystallise fewer oxides and have titanomagnetite as the dominant oxide phase. Positive slopes of FeO*/TiO2 vs. SiO2 during magma differentiation are related to high degrees of crystallisation of Ti-rich oxides (i.e. ilmenite). Bulk solid-melt Ti isotope fractionation factors co-vary with the magnitude of the slope of FeO*/TiO2 vs. SiO2 during magma differentiation.This indicates that the modal abundance and composition of the Fe-Ti oxide phase assemblage, itself is controlled by melt composition, governs Ti isotope fractionation during magma differentiation. In addition to this overall control, hydrous, oxidised calc-alkaline suites display a resolvable increase in δ49/47Ti at higher Mg# relative to drier and more reduced tholeiitic arc suites. These subparallel Ti isotope fractionation patterns are best explained by the earlier onset of oxide segregation in arc magmas with a higher oxidation state and H2O content. This indicates the potential of Ti isotopes to be utilised as proxies for geodynamic settings of magma generation.

On the Microstructure and Properties of Nb-12Ti-18Si-6Ta-2.5W-1Hf (at.%) Silicide-Based Alloys with Ge and Sn Additions.

In this paper two Nb-silicide-based alloys with nominal compositions (at.%) Nb-12Ti-18Si-6Ta-2.5W-1Hf-2Sn-2Ge (JZ1) and Nb-12Ti-18Si-6Ta-2.5W-1Hf-5Sn-5Ge (JZ2) were studied. The alloys were designed using the alloy design methodology NICE to meet specific research objectives. The cast microstructures of both alloys were sensitive to solidification conditions. There was macro-segregation of Si in JZ1 and JZ2. In both alloys the βNb5Si3 was the primary phase and the Nbss was stable. The A15-Nb3X (X = Ge,Si,Sn) was stable only in JZ2. The Nbss+βNb5Si3 eutectic in both alloys was not stable as was the Nb3Si silicide that formed only in JZ1. At 800 °C both alloys followed linear oxidation kinetics and were vulnerable to pesting. At 1200 °C both alloys exhibited parabolic oxidation kinetics in the early stages and linear kinetics at longer times. The adhesion of the scale that formed on JZ2 at 1200 °C and consisted of Nb and Ti-rich oxides, silica and HfO2 was better than that of JZ1. The microstructure of JZ2 was contaminated by oxygen to a depth of about 200 μm. There was no Ge or Sn present in the scale. The substrate below the scale was richer in Ge and Sn where the NbGe2, Nb5(Si1-xGex)3, W-rich Nb5(Si1-xGex)3, and A15-Nb3X compounds (X = Ge,Si,Sn) were formed in JZ2. The better oxidation behavior of JZ2 compared with JZ1 correlated well with the decrease in VEC and increase in δ parameter values, in agreement with NICE. For both alloys the experimental data for Si macrosegregation, vol.% Nbss, chemical composition of Nbss and Nb5Si3, and weight gains at 800 and 1200 °C was compared with the calculations (predictions) of NICE. The agreement was very good. The calculated creep rates of both alloys at 1200 °C and 170 MPa were lower than that of the Ni-based superalloy CMSX-4 for the same conditions but higher than 10−7 s−1.

Warkite, Ca2Sc6Al6O20, a new mineral in carbonaceous chondrites and a key-stone phase in ultrarefractory inclusions from the solar nebula

Warkite (IMA 2013-129) is a new Sc-rich ultrarefractory mineral in the rhönite group of the sapphirine supergroup. It has a P1¯ aenigmatite-type structure with a = 10.367 Å, b = 10.756 Å, c = 8.895 Å, α = 106°, β = 96°, γ = 125°, and Z = 2, and general formula of Ca2(Sc,Ti,Al,Mg,Zr)6Al6O20. Warkite occurs as micrometer-sized crystals in eleven ultrarefractory Ca,Al-rich inclusions (UR CAIs) from the CM, CV, CO, and CH chondrites. In the CM, CO, and CV CAIs, warkite in the cores coexists with a Ti-rich oxide, either perovskite or kangite; the cores are generally mantled and rimmed by davisite and/or Sc-diopside. In the CH CAIs and one CO CAI, warkite in the cores coexists with perovskite and grossite; the cores are mantled by grossite ± gehlenite, and rimmed by low-Sc, Al-diopside. Therefore, there are two basic families of warkite-bearing inclusions, those containing Sc-rich clinopyroxene but no grossite and those containing grossite but no Sc-rich clinopyroxene. Scandian clinopyroxene in warkite-bearing CAIs generally formed by the reaction of warkite, which supplied most or all of the Sc, Ti, and Al, and a nebular gas that supplied much of the Ca and O and virtually all of the Mg and Si. The presence of Sc-rich clinopyroxenes may reflect exposure of some warkite-bearing CAIs to a dust-rich environment, which would enhance partial pressures of Si-, Mg-, and Ca-bearing species in the vapor and make it oxidizing relative to a dust-poor gas. Warkite in grossite-bearing inclusions is generally Ti3+-enriched relative to those in davisite-bearing inclusions, consistent with their formation in relatively dust-poor, more-reducing environments. Warkite compositions are sensitive to the presence or absence of spinel, melilite, Sc-rich clinopyroxenes, and grossite. Compositional variations of perovskite and warkite grains indicate a connection but, except for late-stage Fe exchange, they did not equilibrate with each other. The presence of at least two trends in Y-Sc among perovskites without corresponding trends in warkite suggests that at least some perovskite formed separately.Warkite-bearing CAIs from CM2s and CO3.0s are uniformly 16O-rich (Δ17O ∼ −23‰), whereas those from metamorphosed COs and CVs are isotopically heterogeneous: warkite, kangite, perovskite, melilite, and davisite are 16O-depleted to various degrees (Δ17O range from −22 to −2‰) relative to hibonite, spinel, and forsterite, all having 16O-rich compositions (Δ17O ∼ − 25 to −20‰). We infer that warkite-bearing CAIs originated in an 16O-rich nebular gas. Subsequently, CAIs from metamorphosed CVs and COs experienced O-isotope exchange with an 16O-depleted external reservoir, most likely aqueous fluids on the CV and CO chondrite parent asteroids; however, O-isotope exchange in the solar nebula cannot be excluded.

Spalling resistance of thermally grown oxide based on NiCoCrAlY(Ti) with different oxide peg sizes

NiCoCrAlY is usually used as the material to bond-coat in thermal barrier coating systems under high temperatures. Using NiCoCrAlY and adding different amounts of the active element Ti (0, 0.1, 0.3, and 0.5; wt%), the relationship between oxide peg size and thermally grown oxide spalling resistance is ascertained by examining specimens subjected to a cyclic oxidation experiment. Oxide pegs, consisting of a Y- and Ti-rich oxide core and an outer Al2O3 sheath, are formed by Ti and Al successively reacting with oxygen after the rapid generation of Y-rich oxide. The size of the oxide pegs with higher Ti concentrations is on average bigger in the alloy. Severe thermally grown oxide spalling occurred in the 0.5 wt% Ti specimen, but in the 0.3 wt% Ti specimen, it displayed the best adherence with the alloy due to the presence of fine pegs. Cavities existed at the interface between the inner oxide core and outer Al2O3 sheath of larger pegs serve as microcrack sources and further induce severe scale failure combined with the initial interface crack between the scale and alloy.

Precipitation Behavior of Laves Phase in the Vicinity of Oxide Film of Ferritic Stainless Steel: Selective Oxidation-Induced Precipitation

Anti-corrosion behavior of Fe–Cr alloys at elevated temperature is greatly influenced by the protective nature of oxide scale formed on them. However, precipitates formed underneath the scale can affect the diffusion of reactive elements, growth mechanism and failure mechanism of oxide film. Here, we elucidate the precipitation mechanism of Laves phase in the vicinity of oxide film during oxidation at 950–1050 °C in air. Two types of automotive exhaust manifold-used ferritic stainless steels were designed by replacing Mo with different contents of W and adding small content of Ce. By systematic investigations of the precipitates in the vicinity of steel/oxide interface, we conclude that Laves phase precipitated near the oxide film is related to the selective oxidation of chromium, which is dramatically different from conventional precipitation mechanism. Nb was localized at the interface due to rejection from the growing Cr2O3 scale, which promoted the formation of Laves phase. The experimental steels were characterized by excellent oxidation resistance and Ti–rich oxide formed in the internal oxidation zone that hindered the further diffusion of oxygen.

Organic Matter Preservation and Incipient Mineralization of Microtubules in 120 Ma Basaltic Glass

Hollow tubular structures in subaqueously-emplaced basaltic glass may represent trace fossils caused by microbially-mediated glass dissolution. Mineralized structures of similar morphology and spatial distribution in ancient, metamorphosed basaltic rocks have widely been interpreted as ichnofossils, possibly dating to similar to 3.5 Ga or greater. Doubts have been raised, however, regarding the biogenicity of the original hollow tubules and granules in basaltic glass. In particular, although elevated levels of biologically-important elements such as C, S, N, and P as well as organic compounds have been detected in association with these structures, a direct detection of unambiguously biogenic organic molecules has not been accomplished. In this study, we describe the direct detection of proteins associated with tubular textures in basaltic glass using synchrotron X-ray spectromicroscopy. Protein-rich organic matter is shown to be associated with the margins of hollow and partly-mineralized tubules. Furthermore, a variety of tubule-infilling secondary minerals, including Ti-rich oxide phases, were observed filling and preserving the microtextures, demonstrating a mechanism whereby cellular materials may be preserved through geologic time.

The Effect of Ti on the Early Stages of Oxidation of an Alumina-Forming NiCrAl Alloy

Model γ-NiCrAl alloys with and without titanium were oxidized for short times up to 32 h at 1000 °C and subsequently characterized by scanning electron microscopy, transmission electron microscopy, and atom probe tomography in order to clarify the role of Ti on the oxidation behavior of alumina-forming nickel alloys. At the longer oxidation times, Ti was found to have no significant effect on the major protective oxide phases formed and overall scale and individual oxide layer thicknesses. Ti was observed to segregate to interfaces within the scale and form a Ti-rich oxide at the scale surface over time. At the shortest oxidation times before a continuous Al2O3 layer formed, the Ti-containing alloys exhibited thicker Cr2O3 layers, suggesting that Ti accelerates Cr2O3 growth kinetics. Additionally, oxide nodules resulting from fast oxidation of TiN particles were observed on the Ti-containing alloys. These may be linked to increased spalling propensity of Ti-containing alloys.

Addition of Ta and Y in a hard Ti-Al-N PVD coating: Individual and conjugated effect on the oxidation and wear properties

A comparative study of as-deposited structural properties, oxidation and wear properties of industrially arc-deposited Ti-Al-N, Ti-Al-Y-N, Ti-Al-Ta-N and Ti-Al-Ta-Y-N is presented. In the as-deposited coatings, Ta leads to more compressive stress while Y reduces the compressive stress but increases the nanohardness. The nanohardness of Ti-Al-Ta-Y-N is shown to be dominated by Y while its oxidation rate is dominated by Ta. Interestingly, Ta specifically favours the growth of oxide at the interface between Al-rich and Ti-rich oxide layers.In 850°C ball-on-disk wear tests, there is a pronounced effect of the running-in stage, which in turn depends strongly on the oxidation properties of the coating.The transferability of these findings to industrial machining at higher temperatures are discussed.

Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb1.7Si2.4Ti2.4Al₃Hf0.5 and Nb1.3Si2.4Ti2.4Al3.5Hf0.4 Alloys.

Nb–silicide based alloy will require some kind of coating system. Alumina forming alloys that are chemically compatible with the Nb–silicide based alloy substrate could be components of such systems. The intermetallic alloys Nb1.7Si2.4Ti2.4Al3Hf0.5 and Nb1.3Si2.4Ti2.4Al3.5Hf0.4 were studied in the cast, heat treated and isothermally oxidised conditions at 800 and 1200 °C to find out if they are alumina scale formers. The alloys were designed using the alloy design methodology NICE and were required (i) not to have stable solid solution phase in their microstructures; (ii) not to pest and (iii) to form alumina scale. Their microstructures consisted of silicides and aluminides. Both alloys satisfied (i) and (ii) and formed thin scales at 800 °C. At 1200 °C the former alloy suffered from internal oxidation and formed alumina intermixed with Ti rich oxide beneath a thick “layered” scale of mixed oxides that contained Ti and/or Al and/or Si. There was no internal oxidation in the latter alloy that formed a thin continuous well adhering α-Al2O3 scale that was able to repair itself during oxidation at 1200 °C. In both alloys there was severe macrosegregation of Si, which in Nb1.3Si2.4Ti2.4Al3.5Hf0.4 was almost double that in Nb1.7Si2.4Ti2.4Al3Hf0.5. The severe macrosegregation of Si contributed to the formation of a “layered” structure in the former alloy that was retained at 800 and 1200 °C. Both alloys met the “standard definition” of High Entropy Alloys (HEAs). Compared with the range of values of the parameters valence band (VEC), δ and Δχ of bcc solid solution plus intermetallic(s) HEAs, only the Δχ of the alloy Nb1.7Si2.4Ti2.4Al3Hf0.5 was within the range and the parameters VEC and δ of both alloys respectively were outside and within the corresponding ranges. The alloy Nb1.3Si2.4Ti2.4Al3.5Hf0.4 exhibited strong correlations between the parameters Δχ, δ and VEC, and the range of values of each parameter was wider compared with the alloy Nb1.7Si2.4Ti2.4Al3Hf0.5. There was a strong correlation only between the parameters Δχ and δ of the latter alloy that was similar to that of the former alloy.

High-temperature sliding wear behavior of an intermetallic γ–based TiAl alloy

The high-temperature sliding wear behavior of the γ–based TiAl alloy TNM-B1 was investigated using a pin-on-disk apparatus up to 800 °C in air under variation of the counterpart material. Sliding TNM-B1 against a Ni-based pin led to approximately twice as high wear losses compared to like-on-like wear tests up to 400 °C. These severe wear losses were verifiably attributed to a strong material transfer from the TiAl surface to the pin already existing at low temperatures. Moreover, it was proven that there is a distinctive temperature dependent transition from severe to mild wear for both sliding couples due to the formation of protective oxide scales above 600 °C. In this regard, Al- and Ti-rich oxide scales formed on the worn surfaces minimizing direct metal-to-metal contact.

A Study of the Effect of 2 at.% Sn on the Microstructure and Isothermal Oxidation at 800 and 1200 °C of Nb-24Ti-18Si-Based Alloys with Al and/or Cr Additions.

Alloying with Al, Cr, Sn, and Ti significantly improves the oxidation of Nb silicide-based alloys at intermediate and high temperatures. There is no agreement about what the concentration of Sn in the alloys should be. It has been suggested that with Sn ≤ 3 at.% the oxidation is improved and formation of the brittle A15-Nb3Sn compound is suppressed. Definite improvements in oxidation behaviour have been observed with 5 at.% Sn or even higher concentrations, up to 8 at.% Sn. The research reported in this paper is about three model alloys with low Sn concentration and nominal compositions Nb-24Ti-18Si-5Cr-2Sn (ZX3), Nb-24Ti-18Si-5Al-2Sn (ZX5), and Nb-24Ti-18Si-5Al-5Cr-2Sn (ZX7) that were studied to understand the effect of the 2 at.% Sn addition on as-cast and heat-treated microstructures and isothermal oxidation in air at 800 and 1200 °C for 100 h. There was macrosegregation of Si and Ti in the alloys ZX3 and ZX5 and only of Si in the alloy ZX7. The Nbss was stable in all alloys. Tin and Ti exhibited opposite partitioning behaviour in the Nbss. The βNb5Si3 was the primary phase in all three cast alloys and had partially transformed to αNb5Si3 in the alloy ZX3. Aluminium in synergy with Sn increased the sluggishness of the βNb5Si3 to αNb5Si3 transformation during solidification. After the heat treatment the transformation of βNb5Si3 to αNb5Si3 had been completed in all three alloys. Fine precipitates were observed inside some αNb5Si3 grains in the alloys ZX5 and ZX7. In the latter alloys the A15-Nb3X (X = Al, Si, and Sn) formed after the heat treatment, i.e., the synergy of Al and Sn promoted the stability of A15-Nb3X intermetallic in these Nb-silicide-based alloys even at this low Sn concentration. A Nbss + Nb5Si3 eutectic formed in all three alloys and there was evidence of anomalous eutectic in the parts of the alloys ZX3 and ZX7 that had solidified under high cooling rate and/or high melt undercooling. A very fine ternary Nbss + Nb5Si3 + NbCr2 eutectic was also observed in parts of the alloy ZX3 that had solidified under high cooling rate. At 800 °C none of the alloys suffered from catastrophic pest oxidation; ZX7 had a smaller oxidation rate constant. A thin Sn-rich layer formed continuously between the scale and Nbss in the alloys ZX3 and ZX5. At 1200 °C the scales formed on all three alloys spalled off, the alloys exhibited parabolic oxidation in the early stages followed by linear oxidation; the alloy ZX5 gave the smallest rate constant values. A thicker continuous Sn-rich zone formed between the scale and substrate in all three alloys. This Sn-rich zone was noticeably thicker near the corners of the specimen of the alloy ZX7 and continuous around the whole specimen. The Nb3Sn, Nb5Sn2Si, and NbSn2 compounds were observed in the Sn-rich zone. At both temperatures the scales formed on all three alloys consisted of Nb-rich and Nb and Si-rich oxides, and Ti-rich oxide also was formed in the scales of the alloys ZX3 and ZX7 at 1200 °C. The formation of a Sn-rich layer/zone did not prevent the contamination of the bulk of the specimens by oxygen, as both Nbss and Nb5Si3 were contaminated by oxygen, the former more severely than the latter.

Spallation resistance of oxide scales on Alloy 617 enhanced by boron addition

We report on the effects of B additions to Alloy 617 on its high-temperature oxidation behavior and on the spallation of oxide scales. Alloy 617 and a B-added derivative (Alloy 617B) were oxidized at 1050 °C in air and comparatively studied. Cr2O3 scales formed on both alloys with a spinel phase and a Ti-rich oxide on top. While both alloys showed similar scale growth kinetics, Alloy 617B showed substantially enhanced resistance to oxide scale spallation. B segregation at Cr2O3 grain boundaries, detected by atom probe tomography, was found to be the reason for the enhanced spallation resistance of Alloy 617B.

Investigation on the formation mechanism of Ti-bearing non-metallic inclusions in Fe-Al-Ti-O-N alloy by inductive separation method

Formation mechanism of Ti-bearing non-metallic inclusions was studied by inductive separation method, and characterization method was selected according to the type of each inclusion. The Al+Ti oxide was heterogeneous consisting of Al2O3 with Ti rich oxide layer. Compositional and size distribution was characterized by automated SEM observation and EDS analysis. It is formed as the reduction of Al2O3 by Ti in molten alloy at 1873K. The Al-Ti oxide was homogenous. It was liquid at 1873K characterized by EBSD. The Al/Ti molar ratio of oxide was consistent with that in each alloy as analyzed by manual SEM/EDS analysis. Its formation mechanism is proposed to be the reaction among dissolved Ti, Al, and O in molten alloy at 1873K. TiN precipitated during solidification. The fraction of TiN smaller than 1μm was larger in alloy2 than that in alloy1 as analyzed by automated SEM/EDS analysis. It is resulted from larger supersaturation degree in alloy2 during solidification.

Development of a Novel Melt Spinning-Based Processing Route for Oxide Dispersion-Strengthened Steels

Melt spinning of an Fe-5Y and Fe-1Y-1Ti (wt pct) alloy produced a relatively uniform spatial distribution of Y and Ti in solid solution and ribbons with consistent yield (> 60 pct by weight), fast processing time (< 10 seconds), good scalability (up to > 100 g feedstock material), and repeatability. Heat treatment in the presence of Fe2O3 as an oxygen source (Rhines pack method) at 973 K validated the potential of forming < 20 nm Y-rich oxides in the Fe-5Y ribbons. Pulverized Fe-1Y-1Y ribbons were consolidated to bulk using the field-assisted sintering technique (FAST) incorporating nano-sized Fe3O4 powder as the oxygen source. After FAST at 1273 K, 50 MPa, and 30 minutes, a comparatively high number density of sub-micron Y and/or Ti-rich oxides were developed. Further formation of fine-scale oxides took place during post-FAST annealing, resulting in an approximate 20 pct increase in hardness at temperatures below 573 K, but with a reduced hardening effect above 673 K due to a small fraction of persistent porosity and mechanically weak prior ribbon boundaries that were decorated with Ti-rich oxides.

Thermal stability and oxidation resistance of V-alloyed TiAlN coatings

V-containing nitride coatings recently attract a wide range of research interests owing to their excellent tribological properties. To evaluate their comprehensive properties, a comparative study on the intrinsic thermal stability and oxidation resistance of TiAlN and TiAlVN coatings are conducted here. Ti0.56Al0.44N, Ti0.50Al0.44V0.06N, and Ti0.40Al0.50V0.10N coatings, deposited by cathodic arc evaporation, exhibit a single-phase face-centered cubic structure with a hardness of 28.9–29.8GPa. The V-containing coatings show a pronounced age-hardening upon annealing, which contributes to a hardness increase of 3.7 and 4.8GPa at 800°C for Ti0.50Al0.44V0.06N and Ti0.40Al0.50V0.10N, respectively, corresponding to 2.9GPa for Ti0.56Al0.44N. Also, alloying with V retards the formation of wurtzite AlN upon annealing, especially in Ti0.50Al0.44V0.06N, and thus contributes to a higher hardness above 30GPa even annealing at 1100°C, while the hardness of Ti0.56Al0.44N significantly reduces to 27.8 ± 0.6GPa. However, alloying with V into TiAlN leads to an earlier formation of rutile TiO2 and also Ti-rich oxide top-layer on the outside surface instead of dense Al2O3, and thus degrades the oxidation resistance. When exposed to air at 700°C for 10h, the Ti0.50Al0.44V0.06N and Ti0.40Al0.50V0.10N coatings suffer from a severe oxidation, whereas only a compact oxide scale with a thickness of ~ 80nm for Ti0.56Al0.44N is formed.

Preparation of (FeV80)48Ti26+xCr26(x=0–4) alloys by the hydride sintering method and their hydrogen storage performance

Abstract The V-Ti-based hydrogen storage alloy (FeV80) 48 Ti 26 Cr 26 was synthesized by the hydride powder sintering method. X-ray diffraction (XRD) analysis results show that the alloy consists of a main BCC phase, a La 2 O 3 phase and a Ti-rich oxide phase. The existence of the Ti-rich oxide phase leads to a loss of Ti element content in the main BCC phase and further results in a lattice contraction. Thus, the hydrogen absorption capacity of the alloy synthesized by the hydride powder sintering method is only 2.8 wt%, with a hydrogen desorption capacity of 1.5 wt%. To address this problem, the effects of Ti element compensation on the microstructure and the hydrogen storage capacities of the alloy were investigated. It was found that the lattice constant of the BCC phase increases and the amount of Ti-rich oxide phase is almost unchanged with increased Ti element compensation, leading to an improved hydrogen storage capacity. When Ti element compensation reaches 4 at%, the hydrogen absorption capacity of the alloy reaches 3.3 wt% with a desorption capacity of 2.0 wt% at room temperature and at a pressure of 10 −4 MPa. The hydrogen desorption plateau slope factor of the (FeV80) 48 Ti 26+x Cr 26 (x = 0–4) alloy prepared by the hydride powder sintering method is as small as that of the alloy prepared by the arc-melting method. Thus the sintering method is a suitable option for the manufacture of V-Ti-based hydrogen storage alloys.

Oxide evolution on Alloy X-750 in simulated BWR environment

In order to simulate the environment experienced by spacer grids in a boiling water reactor (BWR), specimens of the Ni-based Alloy X-750 were exposed to a water jet in an autoclave at a temperature of 286 °C and a pressure of 80 bar. The oxide microstructure of specimens exposed for 2 h, 24 h, 168 h and 840 h has been investigated mainly using electron microscopy. The specimens suffer mass loss due to dissolution during exposure. At the same time a complex layered oxide develops. After the longest exposure the oxide consists of two outer spinel layers consisting of blocky crystals, one intermediate layer of nickel oxide interspersed with Ti-rich oxide needles, and an inner layer of oxidized base metal. The evolution of the oxide leading up to this structure is discussed and a model is presented.