Outer TiO2 Layer Research Articles

High Temperature Oxidation of Ti45.4Al4.8Nb and Ti46Al2Mo2Nb Alloys

The Ti46Al2Nb2Mo and Ti45.4Al4.8Nb alloys were oxidized isothermally and cyclically in air between 800 and 1000oC, and their oxidation characteristics were investigated. Nb and Mo were beneficial to oxidation resistance. The initially formed thin TiO2-rich scale changed to an outer, superficial TiO2 layer, a thick Al2O3-rich middle layer, and an inner (TiO2-rich, Al2O3-deficient) layer, as the extent of oxidation progressed. The dissolved ions of Mo and Nb had a tendency to be expelled from the outer TiO2 layer, which was formed by the outward diffusion of Ti ions, to the inner (TiO2-rich, Al2O3-deficient) layer, which was formed by the inward transport of oxygen, owing to the nobility of Mo and Nb when compared to Ti and Al.

Oxidation behavior of a single phase γ-TiAl alloy in low-pressure oxygen and hydrogen

The microchemistry and morphology of the oxide layer produced on a single phase γ-TiAl alloy (Ti–52.1Al–2Ta at.%) following anneals at 823 and 1173 K in low-pressure oxygen and hydrogen environments were studied. The oxide structure on the electropolished surface consisted of two layers, an outer one of Al2O3 and an inner one of TiO2. Annealing in low partial pressures of oxygen retained the same oxide structure but increased the total thickness. After the low-pressure oxygen anneals at 823 K, the oxide surface was smooth, whereas after anneals at 1073 K and above the surface consisted of small, round particles, whose size and density increased with increasing annealing temperature and oxygen partial pressure. In contrast, the oxide structure produced by annealing in a hydrogen environment after the low-pressure oxygen treatment consisted of an outer layer of TiO2 and a sub-oxide of Al2O3. The morphology of this oxide consisted of elongated, rod-like particles, whose size increased with annealing temperature. Absorption of H from gaseous hydrogen atmospheres is discussed in relation to the surface and sub-surface oxide structures.

ＴｉＡｌの酸化特性に及ぼす窒素イオン注入の影響

TiAl was implanted with nitrogen and its effect on cyclic oxidation behavior of TiAl has been investigated in purified oxygen at 1200 K and in static air at 1100 K. The nitrogen implantation was carried out at room temperature, 973 K or 1173 K. The acceleration voltage was 50 kV and the ion dose was 1020 m-2 or 1021 m-2. For the specimen implanted with nitrogen at room temperature with a dose of 1021 m-2, the nitrogen distributed into the depth of about 200 nm with a maximum concentration at about 120 nm, and the concentration changed gradually in the penetration depth. The implantation of nitrogen resulted in the formation of Ti2AlN. The amount of the nitride was larger for the implantation carried out at room temperature than that at 973 K or 1173 K because of the nitrogen diffusion into the matrix during high temperature implantation. The nitrogen implantation deteriorates the oxidation behavior of TiAl in oxygen due to enhanced scale spallation, while the cyclic oxidation behavior of the nitrogen-implanted specimen in air was comparable to that of TiAl without implantation. From the specimen oxidized in oxygen for a very short period, small amount of Ti2N was detected along with TiO2 by X-ray diffraction, indicating that the nitride formed by implantation is not stable during oxidation. The scale formed on the specimen oxidized in oxygen had thick outer layer of TiO2 and stratified inner layer with micro pores. This scale structure seems to be responsible for the lower scale strength. The present results suggest that nitride layer formed by implantation is not continuous thus ineffective for improvement of the oxidation resistance of TiAl.

High temperature oxidation of Ti−48%Al−2%W intermetallics

Powder metallurgically produced Ti-48% Al-2%W alloys were oxidized between 800 and 1050°C in air. The W-addition was quite effective in providing isothermal and cyclic oxidation resistance. The alloys oxidized parabolically up to 1050°C during isothermal oxidation, with small weight gains. The scales were adherent up to 900°C during cyclic oxidation. Oxide scales consisted primarily of an outer TiO2 layer, an intermediate Al2O3 layer, and an inner (TiO2+Al2O3) mixed layer. Tungsten was present below the intermediate Al2O3 layer. and also at the scale-matrix interface as W-enriched compounds. Below the oxide scale, a Ti3Al zone containing some W and O existed.

TEM study of mantle in TiNi wire

The near-surface area of the commercial TiNi wire has been thinned by ion milling and investigated by transmission electron microscopy (TEM). The local chemical composition has been characterized by the combination of Energy-Dispersive-X-ray-Spectroscopy (EDX) and Electron-Energy-Loss-Spectroscopy (EELS) enabling us to measure simultaneously the content of heavy Ti, Ni, Cu and light O, C elements. It has been found that the wire mantle consists of the following consequent layers: 1. Austenitic TiNi with a small amount of fine martensitic plates. Lamellar shaped voids are observed near the edge of the TiNi phase. 2. Few distinct titanium oxide layers with the total thickness of about 5 μm and the thickness of the individual layers of 0.2-0.5 μm. The stoichiometry of titanium oxides is close to 50:50 but varies noticeably from layer to layer. EELS elemental mapping allows to visualize clearly the Ti and O distribution along the layers. 3. Outer TiO2 layer of I μm in thickness. The developed technique of ion milling and the combination of TEM,EDX and EELS has been shown to be suitable for characterization of commercial TiNi products and predicting the in-vivo release of Ni-ions from their surface.

Microstructure and oxidation behavior of Ti–Si–B alloys

Recently, the existence of a new intermetallic ternary phase, with a composition close to Ti6Si2B, was observed in the Ti–Si–B ternary system. The aim of this work is to identify the phase transformations during the oxidation experiments in air of Ti–Si and Ti–Si–B alloys containing Tiss+Ti5Si3 (eutectic), Tiss+Ti6Si2B+Ti5Si3 (eutectic) and Tiss+Ti6Si2B (after heat treatment at 1250°C for 16h). The alloy ingots were produced in an arc-melting furnace using water-cooled copper hearth and a non-consumable tungsten electrode gettered by titanium. In this sequence, oxidation experiments in air were carried out at 900 and 1100°C for 12, 24 and 48h. Samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and microanalysis via wave-length dispersive spectrometry (WDS). After oxidation experiments in air at 900°C during 48h, the following phases of the Ti–Si, Ti–B, Ti–Si–B systems were observed in the microstructures of the studied alloys: Tiss+Ti3Si in the binary eutectic (Tiss+Ti5Si3), Tiss+Ti3Si+Ti6Si2B in the ternary eutectic (Tiss+Ti6Si2B+Ti5Si3), and Tiss+Ti6Si2B in the two-phase heat-treated alloy (Tiss+Ti6Si2B). In all the alloys, the scale was composed of an outer layer of coarse grain TiO2 (rutile), an intermediate layer of a mixture of fine-grained TiO2 and SiO2 (tridymite) and other nitrite phases, and an inner layer formed by the TiSi and TiSi2 phases. Phases of Ti–Si, Ti–B and Ti–Si–B systems were not observed after oxidation experiments in air at 1100°C for 48h, due to the effects of atmospheric contamination.

Cracking behavior of oxide scale formed on Ti3SiC2-based ceramic

The cyclic oxidation and acoustic emission (AE) tests were carried out for studying cracking behavior of oxide scales formed on Ti3SiC2-based ceramic at 1100°C. A duplex oxide scale with an outer layer of pure TiO2 and an inner layer of a mixture of TiO2 and SiO2 was formed. The oxide scale did not spall from substrate during the cyclic oxidation at 1100°C for 360 times. However, a great number of micro-cracks penetrating whole inner oxide layer were detected. AE test showed that the oxide scale did not crack during the isothermal oxidation at 1100°C for 1 h, however, the scale cracked during the cooling stage. Comparing the growth rate and thickness between the oxide layers formed during the isothermal oxidation and cyclic oxidation, respectively, indicated that cracks in the inner oxide layer served as paths mainly for outward diffusion of titanium and for inward diffusion of oxygen, resulting in increased growth rate of the outer oxide layer. Because of entire and compact TiO2 consisted of outer oxide layer, and low thermal stress resulting from small mismatch of thermal expansion coefficients between the oxides and the substrate, Ti3SiC2 exhibited excellent cyclic oxidation resistance at 1100°C for 360 cycles.

High-Temperature Oxidation Behavior of Ti2AlC in Air

The isothermal oxidation behavior of bulk Ti2AlC in air has been investigated in temperature range 1000–1300°C for exposure time up to 20 hr by TGA, XRD, and SEM/EDS. The results demonstrated that Ti2AlC had excellent oxidation resistance. The oxidation of Ti2AlC obeyed a cubic law with cubic rate constants, kc, increasing from 2.38×10-12 to 2.13×10-10 kg3/m6/sec as the temperature increased from 1000 to 1300°C. As revealed by X-ray diffraction (XRD) and SEM/EDS results, scales consisting of a continuous inner α-Al2O3 layer and a discontinuous outer TiO2 (rutile) layer formed on the Ti2AlC substrate. A possible mechanism for the selective oxidation of Al to form protective alumina is proposed in comparison with the oxidation of Ti–Al alloys. In addition, the scales had good adhesion to the Ti2AlC substrate during thermal cycling.

Effect of the surface oxide layer on transformation behavior and shape memory characteristics of Ti-Ni and Ti-Ni-Mo alloys

Oxidation behaviors of Ti-49Ni and Ti-48.3Ni-0.7Mo (at.%) alloys in dry air from 723 K to 1273 K have been investigated by means of scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction, and then effects of oxidation on transformation behavior and shape memory characteristics of the alloys were studied by means of differential scanning calorimetry and thermal cycling tests under constant load. Three-layered surface scale was formed in both alloys oxidized at temperatures higher than 1023 K, consisting of an outer TiO2 layer, an intermediate layer of mixture of TiO2 and Ni and an inner TiNi3 layer. Thickness of the surface oxide layer increased from 1 μm to 50 μm with raising oxidation temperature from 923 K to 1273 K. The surface oxide layer raised transformation temperatures associated with the B2-B19′ and the R-B19′ transformation, while it did not almost change transformation temperatures associated with the B2-R transformation. Recoverable elongation was not changed in the alloys oxidized at temperatures below 823 K with raising oxidation temperature, whereas it decreased in the alloys oxidized at temperatures above 923 K. Transformation hysteresis was not almost changed by oxidation in a Ti-49Ni alloy, but it decreased largely in a Ti-48.3Ni-0.7Mo alloy.

High temperature oxidation of thermomechanically treated Ti-47 Al−2Cr−2Nb

Thermomechanically treated Ti−47Al−2Cr−2Nb alloys were oxidized at 800°C, 900°C and 1000°C in air. Various heat treatments done to obtain duplex microstructure and stabilize microstructures did not affect the overall oxidation kinetics and the oxide morphology of the alloys. Alloys oxidized at similar rates formed oxide scales consisting primarily of an outer TiO2 layer, an intermediate Al2O3 layer, and an inner (TiO2+Al2O3) mixed layer. A small amount of dissolved Cr ion was preferentially segregated in the inner oxide layer. Niobium, however, was segregated in not only the inner, but also the outer oxide layer.

Cyclic-Oxidation Behavior of Ti3SiC2-Base Material at 1100°C

The cyclic-oxidation behavior of Ti3SiC2-base material was studied at 1100°C in air. Scale spallation and weight loss were not observed in the present tests and the weight gain would just continue if the experiments were not interrupted. The present results demonstrated that the scale growth on Ti3SiC2-base material obeyed a parabolic rate law up to 20 cycles. It then changed to a linear rate with further increasing cycles. The scales formed on the Ti3SiC2-base material were composed of an inward-growing, fine-grain mixture of TiO2+SiO2 and an outward-growing, coarse-grain TiO2. Theoretical calculations show that the mismatch in thermal expansion coefficients between the inner scale and Ti3SiC2-base matrix is small. The outer TiO2 layer was under very low compressive stress, while the inner TiO2+SiO2 layer was under tensile stress during cooling. Scale spallation is, therefore, not expected and the scale formed on Ti3SiC2-base material is adherent and resistant to cyclic oxidation.

Cyclic oxidation behavior of TiAl composites incorporated with TiB2 dispersoids

TiAl alloys incorporated in (0,3,5,10) wt.% TiB2 dispersoids were manufactured via mechanical alloyingspark plasma sintering (MA-SPS), and their cyclic oxidation characteristics were studied at 800, 900 and 1000°C in air. The cyclic oxidation resistance of the prepared TiAl-TiB2 composites effectively increased with increases in TiB2 content. The oxide scale formed consisted of an outer TiO2 layer, an intermediate Al2O3 layer, and an inner (Al2O3+TiO2) mixed layer. The scale adherence was relatively good, and much thinner oxide scales, when compared to TiB2-free TiAl alloys, were formed on the prepared composites. The incorporated TiB2 dispersoids oxidized to TiO2 and B2O3 which evaporated during oxidation.

The Oxidation of TiB2 Particle-Reinforced TiAl Intermetallic Composites

The oxidation kinetics of TiAl alloys with and without (3, 5, 10 wt.%) TiB2 dispersoids were studied between 1073 and 1273 K in atmospheric air. The inert TiB2 dispersoids effectively increased the oxidation resistance of TiAl alloys. The higher the TiB2 dispersoids content, the more pronounced the effect. The oxide scale formed on TiAl–TiB2 composites was triple-layered, consisting mainly of an outer TiO2 layer, an intermediate Al2O3 layer, and an inner (TiO2+Al2O3) mixed layer. No B2O3 was observed within the oxide scale because of its high vapor pressure. A thin Ti3Al sublayer and discrete TiN particles were found at the oxide–substrate interface. During the oxidation of TiAl alloys with and without TiB2 dispersoids, titanium ions diffused outwardly to form the outer TiO2 layer, while oxygen ions transported inwardly to form the inner (TiO2+Al2O3) mixed layer. The increased oxidation resistance by the addition of TiB2 was attributed to the enhanced alumina-forming tendency and thin and dense scale formation.

The Effect of Si3N4 Additions on the Oxidation Resistance of TiAl Alloys

The oxidation kinetics of TiAl alloys with and without 3 and 5 wt.%additions of Si3N4 particles were studied at 1173 and1273 K in 1 atm of air. The Si3N4 dispersions wereunstable in the matrix phase, so that some of them reacted with titaniumduring sintering to form Ti5Si3 and dissolvednitrogen. The oxide scale formed on TiAl–Si3N4alloys consisted of an outer TiO2, an intermediate(Al2O3+TiO2), and an inner(TiO2+Al2O3) mixed layers. The enhancedalumina-forming tendency, the presence of discrete SiO2 particlesbelow the outer TiO2 layer, and the improved scale adhesion bySi3N4 dispersions were attributable mainly to theincreased oxidation resistance compared to the Si3N4-freeTiAl alloys. Marker experiments showed that, for TiAl–Si3N4 alloys, the primary mode of scale growth was the outward diffusion oftitanium ions for the outer scale and the inward transport of oxygen ionsfor the inner scale.

Influence of tungsten species on the structure of anodic titania

A sputter-deposited Ti-20 at.% W alloy has been anodized at high efficiency in order to examine the influence of alloy species on the structure of anodic films on titanium. Interestingly, amorphous oxide films develop on the alloy to voltages of about 200 V, in contrast with crystalline film formation on relatively pure titanium at voltages less than 20 V. The amorphous films consist of an outer TiO2 layer and an inner layer containing units of TiO2 and WO3. The layered films form as a consequence of the migration of Ti4+ ions about 2.4 times faster than that of W6+ ions, probably owing to the relative strengths of the W6+−O and Ti4+−O bonds. Film growth proceeds by migration of both the O2− and/or OH− ions and the metal cations, typical of amorphous anodic films, with the transport number of cations, determined using a silicon species marker, of 0.39. During anodizing above about 200 V, local regions of thicker film material develop, possibly owing to nucleation and growth of crystalline material. The vo...

Oxidation of TiAl based intermetallics

The high temperature oxidation behaviour of the binary and ternary alloys of the Ti–48Al system was studied at different temperatures. The primary objectives of this work were the establishment of the activation energies, the migration tendencies of the alloy species, mechanism of oxidation and chemistry of the oxide scales. The ternary additions were Cr (1.5 at 19%), V (2.2 at%), W (0.2 at%) and Mn (1.4 at%). The addition of ternary additions did not play a significant role in the oxidation behaviour at 704°C. At 815°C the alloys with Cr and V exhibited linear oxidation behaviour with large weight gains while the base Ti–48Al alloys exhibited the best behaviour. At 982°C the Mn-containing alloy was the worst, exhibiting a linear oxidation behaviour while the alloy with V and W and the base alloy with 400 p.p.m. oxygen exhibited the best oxidation behaviour. At 982°C the outermost oxide layer in contact with air is always near stoichiometric TiO2. In all the alloys a layer of porosity is created just below the outer TiO2 layer by the Kirkendall mechanism due to the rapid outward diffusion of Ti atoms. The addition of trivalent atoms like Cr in small amounts appear to be detrimental to the oxidation process as they can generate additional oxygen vacancies while the addition of atoms with valence of 5, such as V, and 6, such as W, appear to have beneficial effect on the oxidation behaviour at 982°C by tying up oxygen vacancies.

Influence of Moisture on the oxidation of γ‐TiAl

AbstractThe influence of water vapour on the oxidation of Ti‐50 at.% Al was studied at 900°C. Thick, well‐adherent oxide scales were formed consisting of an outer TiO2 layer and an inner heterogeneous mixture of TiO2 and Al2O3. The interface between these layers is marked by large pores and Al2O3 particles embedded in TiO2. No compact Al2O3 barrier layer was observed. The oxidation leads to formation of a two phase, oxygen enriched subsurface zone, which is Al‐depleted by inner oxidation of Al to Al2O3.The oxidation kinetics were followed by continuous thermogravimetry. Exposure in moist oxygen leads to an accelerated attack compared with oxidation in dry oxygen. In moist oxygen the rate is linear after a short transition period. The oxidation rate is influenced by water partial pressure and oxygen partial pressure.

High-temperature corrosion of Ti and Ti-6Al-4V alloy

Pure titanium and Ti-6Al-4V were exposed at 750°C in an H2/H2O/H2S PO2≈10−18 Pa and PS2≈10−1 Pa), H2/H2O (PO2≈10−18 Pa) and air environments for up to 240 hr. The corrosion kinetics, obtained by the discontinuous gravimetric method, showed that the sulfidation/oxidation kinetics were linear for Ti and linear-parabolic for Ti-6Al-4V in the H2/H2O/H2S environment. Both materials obeyed parabolic rate laws in the H2/H2O atmosphere after a transient period, and linear-parabolic rate laws in air. After exposure to the H2/H2O/H2S atmosphere, the titanium specimen displayed a double scale of TiO2 with an intervening TiS2 film between the double-layered scale of TiO2 and the substrate. Ti-6Al-4V also contained a double layer of TiO2 together with a stratum consisting of Al2S3, TiS2 and vanadium sulfide at the junction of the inner TiO2 layer and substrate. Some Al2O3 precipitated in the external portion of the outer TiO2 layer. Following oxidation in the low-PO2 atmosphere a double-layered oxide of TiO2 scale formed on both Ti and Ti-6Al-4V. The scale on Ti-6Al-4V also contained an α-Al2O3 film situated between the outer and inner (TiO2) layers. For both materials, multilayered-scale formation characterized air oxidation. In detail a multilayered oxide scale of TiO2 formed on the air-oxidized Ti, while a multilayered oxide scale with alternating layers of Al2O3/TiO2 developed on Ti-6Al-4V oxidized in air.

Monocyte activation on titanium-sputtered polystyrene surfaces in vitro: the effect of culture conditions on interleukin-1 release

The release of interleukin-1α (IL-1α) by human peripheral blood monocytes cultured for 24 and 48 h on polystyrene (PS) and titanium-sputtered polystyrene (Ti) was evaluated. Magnetron sputtering of the PS surfaces resulted in a formation of a 50-nm-thick coat, consisting of an outer layer of TiO2. Monocytes released IL-1α without the addition of exogenous stimuli. A doubling of the culture time from 24 to 48 h did not have a major effect on the amount of IL-1α released. The IL-1α levels were increased by addition of lipopolysaccharide (LPS). High concentrations of PS particles (1 and 3 μm diameter) were equally effective stimuli for IL-1α release as LPS. Preadsorption of fibronectin to culture plates augmented LPS-stimulated IL-1α secretion, whereas preadsorbed fibrinogen had an inhibitory effect. Our observations indicate a direct activation of monocytes by PS and Ti, resulting in IL-1α secretion, which is modified by protein adsorption and exogenous stimuli.

Oxidation of ductile particle reinforced Ti-48Al composite

The oxidation behavior of a ductile TiNb particle reinforced Ti-48A1 (at. pct) system was studied at different temperatures. The objective of this work was to establish the role played by the TiNb reinforcement on the oxidation process. The activation energy for oxidation of Ti-48A1 was 324 kJ/mol and that of Ti-48Al-20 vol pct TiNb was 315 kJ/mol. The kinetics of oxidation of both the material systems are similar. At 982 °C, the outermost oxide layer in contact with air was always near stoichiometric TiO2. In all the alloys, a layer of porosity was created just below the outer TiO2 layer by the Kirkendall mechanism as a result of the rapid outward dif-fusion of Ti atoms. TiNb particles are more oxidation resistant than Ti-48A1. The boundary of the TiNb particles was oxidized to form TiO2. However, most of the TiNb particles were un-touched and existed as islands embedded within the Ti and Al oxides formed by oxidation of Ti-48A1. Oxidation in pure oxygen lowers the oxidation kinetics, as compared to oxidation in pure air for identical gas flow rates at 982 °C.