Outward Diffusion Of Ti Research Articles

Pt Effect on the Bond-Coating/Superalloy Interdiffusion: Case of a Pt-Modified NiCoCrAlYTa Coating

Heat treatments and oxidation tests were performed on systems composed of a single-crystal AM3 superalloy with a NiCoCrAlYTa+Pt coating to investigate the effect of Pt on Ta and Ti diffusion at high temperature. Experimental results were compared to an AM3 superalloy directly coated with Pt. For all the studied systems, the effect of Pt on elemental activities was evaluated through thermodynamic calculations. Pt diffusion was found to be faster in the NiCoCrAlYTa coating than in AM3. High Ta contents were measured in the Pt-rich γ′ phase below the surface and significant Al and Cr transport toward the surface was observed, in agreement with thermodynamic calculations which predicted an important decrease in their activities in the presence of Pt. An outward diffusion of Ti was also noticed, whereas calculations did not show a decrease in Ti activity due to Pt. Other discrepancies between experiments and thermodynamic calculations were noted and are discussed in this work.

Enhancing oxidation resistance via grain boundary engineering in L12-strengthened medium entropy alloys

The concept of grain boundary engineering (GBE) has been successfully applied to L12-strengthened (CoCrNi)94Al3Ti3 medium entropy alloy, with the aim of improving the oxidation resistance by increasing the ratio of special boundaries and suppressing discontinuous precipitation. Surprisingly, our results reveal that GBE treatment not only slows down the oxidation kinetics and but also alters the oxide scale from TiO2 and multi-defect Cr2O3 to continuous and protective Cr2O3 and Al2O3, thereby contributing to an enhanced oxidation and anti-spalling resistance. The GBE treatment reduces the oxidation weight gain of the current alloy from 1.950 mg cm–2 to 1.211 mg cm–2 after 100 h of cyclic oxidation at 800 °C. The findings show that the extensive outward diffusion of Ti accelerates ion transport and promotes microporosity, thus leading to more defects being formed in the oxide film. The GBE treatment suppresses the discontinuous precipitation of the Ti-bearing L12 phase and breaks the random large angular grain boundaries network, inhibiting the diffusion of Ti and ultimately enhancing the oxidation properties of the alloy. The current work provides an idea of oxidation resistance enhancement for Ti-bearing L12-strengthened alloys without changing the alloy composition.

Multiple ceramic particles help to improve oxidation resistance of TiAl alloy

A novel microstructural regulation strategy was utilized to improve the oxidation resistance of TiAl alloy by the controllable multiple ceramic particles. TiAl composites were prepared using spark plasma sintering (SPS) by introducing SiC and graphene oxide (GO). TiAl composites showed nearly full lamellar microstructure, and the Ti5Si3 particles in-situ precipitated at the lamellar colony boundaries and Ti2AlC in-situ precipitated at the α2/γ interfaces. Grain refinement of TiAl composite was obvious, displaying a decrease of lamellar colony size from 259 to 81 μm after addition 0.3 wt% SiC and 0.5 wt% GO. The isothermal oxidation results showed that the mass gain of TiAl composite was decreased by 56% compared to TiAl alloy after oxidation at 950 ℃ for 100 h. This was mainly attributed to the in-situ precipitated Ti2AlC and Ti5Si3 ceramic particles inhabiting the interior diffusion of O atoms and the outward diffusion of Ti and Al atoms, then the activity of O and Ti was decreased. Furthermore, the Ti2AlC and Ti5Si3 ceramic particles contributed to release the internal stress between the oxide layer and matrix, strengthening the bondability between the oxide layer and matrix.

Atomic Scale Insight into the Oxidation Mechanism of Nb/Ta‐Doped Ti3SiC2/Al2O3 Ceramics

The low oxidation resistance of Ti3SiC2/Al2O3 composites limits their applications as high‐temperature structural materials. To improve the oxidation resistance, Nb and Ta are introduced into the substrate herein. The Nb/Ta‐doped Ti3SiC2/Al2O3 composite shows superior oxidation resistance than clean Ti3SiC2/Al2O3 following the oxidation process at 800–1000 °C for 100 h. The oxidation mechanism is determined via first‐principles calculations. The oxidation process of the composite is controlled by the inward diffusion of O and outward diffusion of Ti. When Nb/Ta elements are doped, their larger atomic radii and stronger binding produce a higher diffusion barrier for O and Ti atoms, which prevents element migration and reduces the thickness of the oxidation layers.

Influence of minor tensile stress on the oxidation behavior of powder metallurgy superalloy

Oxidation behavior of a novel powder metallurgy superalloy was investigated at 900 °C under minor tension of 0, 30, 50, and 70 MPa, respectively. Minor tensile stress was proved helpful in enhancing the oxidation resistance of superalloy. With increasing stress, oxide scale became thinner, more compact, more continuous and flatter. This was also accompanied by a reduced internal oxidation in alloy matrix. At the same time, more Ti was accumulated into the interior of Cr2O3 layer than in its surface, due to the inhibited outward diffusion of Ti by the more compact oxide scale at higher tensile stress.

Oxidation mechanism of in-situ TiC/Ni composites at 1073 K

The oxidation behavior at 1073 K of in-situ TiC/Ni composites prepared by reactive sintering obey parabolic kinetics. The oxidation activation energy is 210 ± 30 kJ•mol−1, closed to that of O diffusion through rutile. Oxide scale formed on composites includes two layers: continuous outer layer consisted by upper TiO2 +NiTiO3 +NiO and nether TiO2, and inner layer composed of TiO2 islands dispersed in Ni matrix. The oxidation process is determined by outward diffusion of Ti and Ni and inward diffusion of O, and the later one plays a predominant role, leading to the faster growth rate of inner layer than outer layer.

Oxidation performance and interfacial reaction behavior of glass-ceramic coating on TiAl alloy with electrodeposited SiO2 interlayer

A duplex composite coating consisting of electrodeposited SiO 2 interlayer and glass-ceramic coating was fabricated on TiAl alloy. The oxidation behavior of the composite coating was studied in air at 1000 °C. The composition, morphology, and microstructure of the oxide scale were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS). Results showed that the composite coating exhibited outstanding resistance against high temperature oxidation. After 100 h oxidation, no scale spallation was observed and the weight gain was only 1.16 mg cm −2 . The inward diffusion of oxygen and outward diffusion of Ti were efficiently prevented. A continuous and adherent Al 2 O 3 layer was formed at the coating/substrate interface. The electrodeposited SiO 2 interlayer could improve the adhesion between the substrate and glass-ceramic coating. • Glass-ceramic coating and SiO 2 interlayer were fabricated on TiAl alloy. • SiO 2 interlayer can dissolve into the glass-ceramic coating and reduce its mobility. • The adhesion property of the glass-ceramic coating was enhanced. • The formation of nitride at the coating/substrate interface was suppressed.

Microstructure and Oxidation Behavior of NiCoCrAlY Coating With Different Sm2O3 Concentration on TiAl Alloy

In order to improve the oxidation resistance of TiAl alloys, NiCoCrAlY coatings with different amounts of Sm2O3 were prepared by laser cladding on TiAl alloys. The microstructure and oxidation behavior of the coatings were investigated by isothermal oxidation tests at 900°C. The results indicated that the grains of the coatings were refined by Sm2O3. The fine grain reduced the crack sensitivity of the doped coatings and promoted the transformation of θ-Al2O3 to α-Al2O3. Therefore, the internal oxidation of the coatings and the growth rate of the oxide films were reduced. The segregation of Sm at grain boundary inhibited the outward diffusion of Ti, thus reducing the excessive oxidation of Ti. In addition, the oxidation mechanism of the coating was changed from simultaneous diffusion of Al and O to predominant inward diffusion of O. The oxidation resistance of the doped coatings was significantly improved. However, excessive Sm2O3 is detrimental to the improvement of the oxidation resistance. The oxidation resistance of 3 wt% Sm2O3 is the best.

Effect of titanium addition on the oxidation resistance of Fe–13Cr-5Al-0.3Ti alloy in air between 700 °C–1100 °C

Cyclic oxidation tests were carried out on a Fe–13Cr–5Al–0.3Ti alloy over a temperature range of 700 °C–1100 °C in air. The composition and structure of the oxide layer were characterized by SEM and XRD, and the influence of temperature and Ti on the oxidation mechanism was analyzed. The results demonstrated that a dense α-Al2O3 oxide layer was formed on the surface of the substrate at 900 °C, which inhibited the outward diffusion of Ti and made the alloy more resistant to oxidation. However, when the temperature was increased to 1000 °C, the oxide layer could not effectively inhibit the outward diffusion of the Ti. In particular, the oxide formed on alloys treated 1100 °C was divided into two layers, where the inner layer was a dense α-Al2O3 oxide layer and the outer layer was TiO2 oxide layer.

The Effects of Grain Size and Twins Density on High Temperature Oxidation Behavior of Nickel-Based Superalloy GH738.

In the present study, surface treatment techniques such as room temperature machining (RTM) and low temperature burnishing (LTB) processing have been used to improve the microstructure of GH738 superalloy. Nano-grains and nano-twins are obtained on the top surface of RTM and LTB specimens. It is found that although the grain size of RTM and LTB specimens is almost the same, different types of nano-twins have been produced. Moreover, the effect of RTM and LTB processing on high temperature oxidation behavior of nickel-based superalloy GH738 at 700 °C is investigated. The result shows that LTB specimen has the best high temperature oxidation resistance owing to the formation of nano-grains and higher twins density, which induce to form a continuous protective Al2O3 layer at the interface between outer oxide layer and matrix. It is observed that this layer inhibits the inward diffusion of O and outward diffusion of Ti and significantly improves oxidation resistance of LTB specimen. Furthermore, the effects of nano-grains and crystal defects on the diffusion mechanism of elements are clarified during the high temperature oxidation test.

High-temperature oxidation behaviour of TiAl alloys with Co addition

The challenge of enhancing the high-temperature oxidation resistance of TiAl alloys is hereby addressed by Co addition. Isothermal oxidation tests were conducted on the newly designed TiAl-Co alloys in laboratory air at 900 °C up to 100 h. Sintered microstructure, oxidation kinetics, scale structure, spallation resistance and oxidation mechanisms were systematically investigated. Results show that the original sintered microstructure of TiAl alloys mainly consists of matrix phases α2-Ti3Al/γ-TiAl lamellae, while the Co addition leads to the formation of the two additional Co-rich phases of CoAl2Ti and Ti (Al, Co, Cr and Nb) at grain boundaries. The Co-doped TiAl alloys exhibit an improved high-temperature oxidation resistance compared with the Co-free alloy. The presence of the Co-rich phases network along the grain boundaries and Co-rich layer at the scale/substrate interface can hinder the inward diffusion of oxygen and the outward diffusion of Ti and Al, thereby suppressing the growth of oxide scale and improving the spallation resistance of TiAl alloys. As a result, the TiAl-3Co alloy possesses excellent oxidation resistance, with the minimum mass gain of 4.08 mg/cm2, thinnest scale thickness of 17.8 μm and without surface spallation or crack formation after isothermal oxidation for 100 h. This result would pave the way for designing high-temperature oxidation-resistant TiAl-based alloys.

Constructing self-supplying Al2O3-Y2O3 coating for the γ-TiAl alloy with enhanced oxidation protective ability

The failure of the ceramic coating usually occurs at high temperatures due to the appearance of the cracks, which are caused by the accumulation of thermal stress. Here a self-supplying Al2O3-Y2O3/Al-Y coating was prepared by magnetron sputtering on the γ-TiAl alloy. After oxidation, the coating cross-section showed that three layers were formed, and the main phases consisted of Al2O3, Y2O3, and Y3Al5O12 (YAG). The isothermal oxidation test revealed that the coating efficiently prevented γ-TiAl alloy from degeneration for 100 h with a mass gain of 0.34% compared to the uncoated sample with a mass loss of 1.51% for 60 h at 950 °C. The study focused on the microstructure evolution of the coating during the oxidation process. We found that the improvement of oxidation resistance primarily could be attributed to three reasons: 1) the Y2O3 phase was more stable than Al2O3 and TiO2 at elevated temperature. 2) Al-Y layer, as the intermediate layer, can compensate for the element consumption of the ceramic layer caused by oxidation. 3) the Al3Ti layer efficiently suppressed the outward diffusion of Ti.

Corrosion of Multilayered TiAlSiN Films at 800–1000 °C in N2/0.1%H2S Gas

A multilayered TiAlSiN thin film consisting of alternating nanocrystalline Ti(Si)N and Al(Si)N nanolayers was deposited on steel by arc ion plating. The film composition was 26Ti–16.3Al–1.2Si–56.50N in at%. The film was corroded at 800–1000 °C for 4–100 h in N2/0.1%H2S gas to study its corrosion behavior in hostile (H, S)-containing environments. The corrosion was primarily governed by oxidation, because oxides of Ti and Al were much more stable than the corresponding sulfides. The oxygen source for oxidation was impurity oxygen in N2/0.1%H2S gas. Initially, a superficial Al2O3 scale formed. Soon, the scale developed into the outer TiO2-rich layer and the inner Al2O3-rich layer, beneath which formed an oxygen affected zone. As corrosion progressed, Si tended to accumulate in the lower part of the inner Al2O3-rich layer owing to its thermodynamic nobility. Preferential oxidation of Al to Al2O3, formation of fine, dense Al2O3 and TiO2 grains in the oxide scale, and strong Ti–Si, Al–N and Ti–N bonds in the TiAlSiN film caused the scale to grow quite slowly and suppressed fast inward diffusion of sulfur and hydrogen as well as fast outward diffusion of Ti, Al, and Si. Therefore, the film displayed good corrosion resistance at 800–900 °C for up to 100 h. However, it corroded completely, with partial scale spallation and whisker growth at 1000 °C for 50 h.

Aluminization of a Si–Y co-deposition coating to protect a Ti2AlNb based alloy from high temperature oxidation

Coatings were prepared by aluminization of a Si–Y co-deposition coating on a Ti2AlNb based alloy using halide activated pack cementation technique. The microstructure evolution and the high temperature oxidation behavior of the coatings were investigated. During the aluminization, the (Ti,Nb)5Si4 inner layer of the Si–Y co-deposition coating was consumed faster than the (Nb,Ti)Si2 outer layer. A (Ti,Nb)Al3 superficial layer and a (Nb,Ti)3Si5Al2 + (Ti,Nb)Al3 middle layer successively formed. Besides, (Nb,Ti)3Si5Al2 pieces appeared inside the outer layer when solid solution Al in (Nb,Ti)Si2 reached saturation. Both inward diffusion of active [Al] atoms and outward diffusion of Ti and Nb atoms controlled the coating growth. The outward diffusion of Ti and Nb atoms caused Al beneath the inner layer was enriched continuously. The Si–Y co-deposition + aluminization coating could provide better protection for the Ti2AlNb based alloy from oxidation at 1123 K than the Si–Y co-deposition coating, which was attributed to preferential formation of Al2O3 on the surface to prohibit formation of loose TiO2. After the oxidation for 100 h, a continuous, compact and adhesive oxide scale consisting of an Al2O3 main part and an Al2O3 + TiO2 + SiO2 lower part formed on the coating.

Lifetime study of sputtered PtAl coating on γ-TiAl with and without TBC topcoat at high temperatures

In this study, a PtAl based coating was tested under high temperature conditions at 900 °C and 1000 °C. The composition of Pt-53Al was chosen because of its excellent results in previous studies at another temperature. Due to the formation of a thin continuous alumina layer on top and the phase stability of the oxidation resistant coating, a lifetime of 1500 1 h-cycles was achieved.Depending on the testing temperature and time, the coating underwent an evolution of different Ti-Pt-Al phases due to the outward diffusion of Ti and Nb from the substrate and inward diffusion of Pt. The oxidation resistance decreases with a continued interdiffusion and a decreasing Pt content in the Ti-Pt-Al phases.Additionally, the Pt-53Al coating was successfully deposited with a Thermal Barrier Coating top coat by EB-PVD. The oxidation test at 1000 °C also revealed a lifetime up to 1500 cycles without failure, particularly based on the good adhesion between the TGO and TBC.Mass gain measurements were done at each temperature and for all coating systems. Further microstructural examination methods to study the coating evolution, such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD) for phase analysis, were used.

Cathodic arc deposition of NiCrAlY coating: oxidation behaviour and thermodynamic

ABSTRACTNiCrAlY coating was deposited on the Ni-base superalloy in the cathodic arc deposition process. The isothermal oxidation test was carried out at 1000°C in air. The cross-section of the coatings was studied by the field emission scanning microscope electron and energy dispersive spectroscopy. The formation and growth of the internal oxide were addressed during the high temperature oxidation. The outward diffusion of Ti occurred from the substrate to the coating, leading to the formation of TiO2 beneath the thermally-grown oxide scale. Thermodynamic measurements of TiO2 formation were performed and the role of the internal oxide in the formation of TiO2 was determined.

High-Temperature Corrosion of Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C Alloy in N2/0.1%H2S Gas

Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C alloy was corroded at 800–1100 °C for 200 h in N2/0.1%H2S gas to characterize its corrosion behavior in an aggressive H2S-containing environment. The alloy displayed superior corrosion resistance because Ti and Al preferentially reacted with impurity oxygen in the gas to form TiO2 and Al2O3. It corroded primarily by outward diffusion of Ti, Al, W, and Cr in addition to inward transport of sulfur, nitrogen, and oxygen. Scales were adherent and consisted of an outer TiO2 layer, an intermediate Al2O3 layer, and an inner (TiO2, Al2O3)-mixed layer. TiN and Ti2AlN formed at the scale/matrix interface where sulfur, Nb, W, and Cr segregated.

Al-Si and Al-Si-Y coatings deposited by HS-PVD for the oxidation protection of γ-TiAl

Many attempts have been made to extend the service temperatures of lightweight materials based on γ-TiAl alloys due to their insufficient oxidation resistance at T>850°C for turbine applications. Among them, deposition of high-temperature oxidation protective coatings has been proved to be a promising approach. The key purpose hereby is the formation of protective oxide scales such as Al2O3, Cr2O3 and SiO2, which offer a barrier against diffusion of oxygen into the bulk material. Besides, a dense microstructure for a suppressed interdiffusion and an adequate coating-substrate adhesion in terms of the chemical compatibility and the similarity of the coefficients of thermal expansion are indispensable.In the present work, a total of five coating systems containing Al-Si and Al-Si-Y with different chemical compositions were deposited on γ-TiAl substrate by high speed physical vapor deposition (HS-PVD), which offers high deposition rates owing to hollow cathode discharge (HCD) and gas flow sputtering (GFS). The as-deposited coatings were characterized with regards to their coating thickness, morphology and phase composition. Subsequently, thermal cyclic oxidation tests were carried out at T=950°C in air. The results confirmed the high oxidation resistance of the Al-Si and Al-Si-Y coatings compared to the uncoated γ-TiAl substrates. Both Si and Y were found beneficial for the enhancement of oxidation resistance of the aluminide coatings by promoting the formation of α-Al2O3, YAlO3 and a continuous intermetallic scale of TiSi. Significantly suppressed inward oxidation and outward diffusion of Ti could be obtained by the sample coated with Al-Si-Y during cyclic oxidation test. The results of the conducted research reveal a high potential of the HS-PVD Al-Si-Y coating system for the oxidation protection of γ-TiAl at T>850°C in turbine applications.

Investigation on the effect of Nb doping on the oxidation mechanism of Ti3SiC2

Nb doping substantially changes the oxidation mechanism and significantly enhances its oxidation resistance of Ti3SiC2 at 800 °C. After Nb doping, the oxidation of Ti3SiC2 is only controlled by the inward diffusion of O, while the outward diffusion of Ti is restrained totally. The oxide layer structure changes from a duplex-layer of TiO2 outer layer and TiO2+SiO2 mixture inner layer to a single TiO2+SiO2 mixture layer. It is proposed that Nb doping decreases the concentrations of oxygen vacancies and Ti interstitials in the formed TiO2, leading to the completely restrained outward diffusion of Ti and the decreased oxidation rate.

Exploring a novel ceramic (Ti,W)3SiC2 for interconnect of intermediate temperature solid oxide fuel cell

A solid solution (Ti,W)3SiC2 possessing good oxidation resistance and low area-specific resistance (ASR) after oxidation has been synthesized by an in-situ hot pressing process. The oxidation rate constant at 800 °C in air is 6.29 × 10−14 g2 cm−4 s−1 for (Ti,W)3SiC2. The formed single-layer oxide is composed of W doped rutile TiO2 and amorphous SiO2. SiO2 is evenly inlaid in the communicative body frame of TiO2. W doped in TiO2 mainly exists as W6+. W doping not only hinders the outward diffusion of Ti by decreasing the concentration of native Ti interstitials in TiO2, but also restrains the inward diffusion of oxygen by decreasing the concentration of O vacancies. Furthermore, W dopant in TiO2 enhances the electrical conductivity of TiO2 by increasing the concentration of semi-free electron. Therefore, the low ASR of (Ti,W)3SiC2 after oxidation owes to high electrical conductivity of TiO2 as well as the reduced thickness of oxide scale. All the results render (Ti1-xWx)3SiC2 promising as interconnects for the intermediate temperature solid oxide fuel cell.