Al-depleted Zone Research Articles

Numerical analysis of Al2O3 and TiO2 growth and oxygen dissolution in a metal substrate during the isothermal oxidation of an α-Ti alloy at 973 K

Oxide growth of Al2O3 and TiO2 as well as O dissolution during the oxidation of an α-Ti alloy at 973 K were simulated using the finite volume method coupled with the calculation of phase diagrams (CALPHAD) method. The results indicated that the addition of 11.02 at.% Al to a Ti–O system resulted in the formation of an 18-nm-thick Al2O3 layer, which decelerated TiO2 growth and O dissolution in the substrate. An increase in the O concentration at the oxide–metal interface was also inhibited by Al2O3 formation. A thin Al-depletion zone was formed at the oxide–metal interface. Furthermore, Al2O3 formation was restricted by the low concentration and diffusivity of Al in the substrate.

High-temperature oxidation behavior of laser additively manufactured AlCrCoNiSi-based high-entropy alloys at 1100 ℃

High-temperature oxidation behavior of two laser additively manufactured AlCrCoNiSi-based high-entropy alloys (Al10 and Al15) are systematically studied in this work. These two alloys exhibit similar oxidation behaviors involving initial linear kinetics oxidation and subsequent parabolic kinetics oxidation at 1100 ℃. However, Al10 alloy has a superior oxidation resistance than Al15 alloy according to the oxidation rate constant. α-Al2O3 is the predominant phase of the oxide scales grown on both alloys. Cross-sectional views of the scales show that the scales consist of surface equiaxed grains and underlying columnar grains. For all oxidized alloys, a newly-formed fcc layer forms in the subsurface Al-depletion zone due to the preferential oxidation of aluminum. Al10 alloy exhibits a much thicker newly-formed fcc layer than Al15 alloy after the same oxidation time. This work ascribes the superior oxidation resistance of Al10 alloy to its thick newly-formed fcc layer, which suppresses the outward Al diffusion from the substrate to scale-metal interface by increasing the diffusion distance.

Evaluation of high temperature oxidation behavior of LZ, CSZ, and LZ-CSZ composite thermal barrier coatings

La2Zr2O7 (LZ) compound is a promising candidate for the new generation of thermal barrier coatings (TBCs). Despite its unique properties, this material suffers from a quite low coefficient of thermal expansion (CTE) and also brittleness which restricts its potential application. In this study, LZ was modified with ceria stabilized zirconia (CSZ), and LZ-CSZ composite powders with different compositions were prepared and applied with the air plasma spraying (APS) method. The oxidation behavior of samples was studied at 1100 °C, and the tests were continued until coatings spallation. Microstructural studies showed that the thermally grown oxide (TGO) layer in all samples consists of alumina and mixed spinel oxide layers. Due to lower oxygen diffusivity, the samples with a higher fraction of LZ contain a narrower TGO layer with a small spinel phase. The inward oxygen penetration in coatings with higher CSZ percentages was considerable, that led to the evolution of Al-depletion zone in the bond coat area, and increased the portion of spinel oxides in the TGO. The 50CSZ-50LZ coating, due to its better combination of CTE compatibility, thermal stability, and fracture toughness, showed better oxidation resistance and collapsed after 264 h with a TGO thickness of 5.4 μm.

Hot salt corrosion behavior of the Ti-44Al-4Nb-1.5Mo-(B,Y) alloy in the temperature range of 750–950 °C

Hot salt corrosion behavior of TNM alloy (Ti-44Al-4Nb-1.5Mo-(B, Y) (at.%)) was investigated in cyclic conditions at 750 °C, 850 °C and 950 °C for 100 h in the 75 wt% Na2SO4 + 25 wt% NaCl salt mixture. The hot salt corrosion resistance of γ、α2 and β phase in the alloy was studied by short-term quasi-in-situ hot corrosion experiments. Results show that the hot salt corrosion resistance of the three phases decreases in the following order: γ、α2、β. The weight changes of TNM alloy after hot salt corrosion at 750 °C, 850 °C and 950 °C for 100 h are 13.8 mg/cm2, -19.6 mg/cm2 and -26.7 mg/cm2. Under the condition of hot salt corrosion at 750 °C, the corrosion scales are mainly composed of a mixed corrosion scale of TiO2 and Al2O3, Al2O3 scale, and Al depletion zone, similar to the morphology of a high-temperature oxidation scale. The corrosion scales are seriously peeled off and destructive transverse cracks are generated inside the corrosion scales under the conditions of hot salt corrosion at 850 °C and 950 °C. The acid-base corrosion dissolution of oxides in the corrosive medium and the formation of volatile chlorides are the main causes of cracks in the TiO2 and Al2O3 scales. In addition, the Nb element in the TNM alloy is mainly enriched beneath the corrosion scales to form a NaNbO3 layer, which can inhibit the decomposition and penetration of the corrosive medium. Mo element reacts with the S element to form MoS2, which anchors the S element to a certain extent.

Crystallographic and TEM Features of a TBC/Ti2AlC MAX Phase Interface after 1300 °C Burner Rig Oxidation

A FIB/STEM interfacial study was performed on a TBC/Ti2AlC MAX phase system, oxidized in an aggressive burner rig test (Mach 0.3 at 1300 °C for 500 h). The 7YSZ TBC, α-Al2O3 TGO, and MAXthal 211TM Ti2AlC base were variously characterized by TEM/STEM, EDS, SADP, and HRTEM. The YSZ was a mix of “clean” featureless and “faulted” high contrast grains. The latter exhibited ferro-elastic domains of high Y content tetragonal t″ variants. No martensite was observed. The TGO was essentially a duplex α-Al2O3 structure of inner columnar plus outer equiaxed grains. It maintained a perfectly intact, clean interface with the Ti2AlC substrate. The Ti2AlC substrate exhibited no interfacial Al-depletion zone but, rather, numerous faults along the basal plane of the hexagonal structure. These are believed to offer a means of depleting Al by forming crystallographic, low-Al planar defects, proposed as Ti2.5AlC1.5. These characterizations support and augment prior optical, SEM, and XRD findings that demonstrated remarkable durability for the YSZ/Ti2AlC MAX phase system in aggressive burner tests.

The Formation Mechanism of a Multilayer-Structure Oxide Film during the Oxidation of FeCrAl in Air at 700 °C

A protective oxide film is the key to the corrosion resistance of the FeCrAl alloy. The mechanism of the formation of the multilayer oxide film of the FeCrAl alloy in 700 °C air was explored by studying the structure evolution of the oxide film and the oxidation kinetics of FeCrAl. The results show that a multilayer oxide layer is formed on the surface of the FeCrAl alloy after 1344 h, with a (Fe,Cr)2O3 layer, an Al-rich oxide layer, an Al-depleted zone, and a new Al-rich oxide layer sequentially arranged from the surface to the matrix. This indicates that the Al element plays an important role in the formation of the oxide film. The Al in the matrix is depleted to form the Al-rich oxide layer, resulting in the Al-depleted zone. The new Al-rich oxide layer is formed under the Al-depleted zone by internal oxidation. It should be noted that the precipitation of the AlN phase in the matrix is observed, which might be a probable factor for the Al-depleted zone in the matrix.

Columnar and nanocrystalline combined microstructure of the nitrided layer by active screen plasma nitriding on surface-nanocrystalline titanium alloy

The microstructure and formation mechanism of the nitrided layer by active screen plasma nitriding on surface-nanocrystalline TA17 titanium alloy were studied by TEM. The nitrided layer of both the original and shot-peened TA17 samples was composed of two sublayers: the outer TiN layer formed by deposition of titanium nitride particles and the inner Ti2N layer formed by nitrogen diffusion into the titanium substrate. The Ti2N layer was found to be an Al-depleted zone, which was a proof for its formation mode of nitrogen diffusion. Compared with the TiN layer of the original sample filled with columnar grains, the TiN layer of the shot-peened sample was composed of mainly equiaxed nanograins and a small amount of columnar grains. The large number of high-energy grain boundaries on the shot-peened surface provided numerous nucleation sites, resulting in the formation of equiaxed nanocrystalline TiN. During long term nitriding, most of the nano-scale TiN grains were maintained due to their high thermal stability, while the nano grains of the shot-peened substrate surface grew into microscale due to their low thermal stability. With the thickening of the nitrided layer, some equiaxed TiN nanograins grew into columns perpendicular to the substrate surface due to competitive growth.

Enhanced joint integrity of diffusion-welded Alloy 617 by controlling the micro-chemistry near the surface

The diffusion welding process is critical for manufacturing efficient gas-to-gas heat exchangers. Due to issues affecting structural integrity that were extensively reported for Alloy 617 diffusion weldments, the effect of pre-treatment on micro-chemistry near the surface was investigated to enhance the interface properties. During the pre-oxidation step (930 °C/200 h/static air), an Al-depleted zone was created through the formation of internal oxides. A fresh joining surface with a minimal Al content was, thereafter, exposed by conventional mechanical polishing. After diffusion welding (1150 °C/2 h/14.7 MPa) and post-weld heat treatment (1120 °C/20 h), an interface, in which Al-rich oxides were almost absent, and extensive grain boundary migration across the interface were observed. Tensile specimens were fractured based on a ductile behavior, with more than 60% strain at the rupture point, considering a test temperature of 900 °C. In the stress–rupture test at 800 °C, the times to rupture of the diffusion weldment were comparable to those of the as-received alloy, and the strain to rupture reached over 17%.

High temperature oxidation behavior of Al0.6CrFeCoNi and Al0.6CrFeCoNiSi0.3 high entropy alloys

Amongst the structural materials for high temperature applications that are currently in the focus of research, high entropy alloys (HEAs) exhibit very promising features, such as high strength, good fracture toughness, high temperature phase stabilities and excellent resistance against softening at elevated temperatures. Key influencing factors for high temperature applications are the mechanical properties at elevated temperatures as well as the oxidation behavior of a given material. AlxCrFeCoNi alloys have been mostly investigated among different HEAs. However, up to now there are limited reports dealing with the oxidation behavior of AlxCrFeCoNi–based HEAs, especially with respect to the effect of Si-alloying on the oxidation behavior at elevated temperatures. In this study, the oxidation behavior of two HEAs, Al0.6CrFeCoNi and Al0.6CrFeCoNiSi0.3, were comparatively investigated at T = 800 °C, 900 °C and 1000 °C in air to determine the effect of Si-alloying. The results show that oxidized layers typically consist of Cr2O3/spinel and Al2O3 with an underlying Al-depleted zone. AlN was found inside the Al-depleted zone with the exception of Al0.6CrFeCoNiSi0.3 oxidation at T = 800 °C. However, Si alloying only positively influenced the oxidation behavior at T = 800 °C. Here, it promoted the formation of a continuous Al2O3 layer. The Si-alloying did not enhance the oxidation resistance at T = 900 °C and 1000 °C.

Influences of Cr and Co on the Growth of Thermally Grown Oxide in Thermal Barrier Coating during High-Temperature Exposure

Thermal barrier coating (TBC) is a critical material in the aerospace domain to increase the lifetime of gas turbine components subjected to thermal load. The properties of TBC are strongly related to the growth of thermally grown oxide (TGO) whose main constituent is Al2O3. However, the oxidation of Cr and Co can affect the growth of TGO, which is not studied sufficiently. In this paper, high-temperature exposure at 1000 °C was performed to investigate the effect of Cr and Co oxides on TGO growth. The morphology and composition analysis of the interface between the ceramic top coat and the bond coat (TC/BC) were investigated by using scanning electron microscopy (SEM) and the energy dispersion spectrum (EDS). The thermodynamics and kinetics of oxidation were analyzed. The results indicated that the oxidation kinetics basically followed the sub-parabolic law with exposure time. Additionally, the major factor affecting the formation of oxides was the diffusion rate at the initial stage of exposure, then oxides depended on thermodynamics, and the oxidation was influenced by both of them in the last stage. The major elements to be oxidized were different at different stages. Moreover, the replacement reaction of Cr2O3 and the phase conversion of Al2O3 resulted in thickness variations of the TGO and Al-depleted zone during high-temperature exposure.

Lifetime of 7YSZ thermal barrier coatings deposited on fluorine-treated γ-TiAl-based TNM-B1 alloy

A thermal barrier coating (TBC) system was manufactured on a Mo-containing γ-TiAl-based alloy utilizing the halogen effect to ensure the formation of thin protective oxide scales. Samples of the TNM-B1 alloy were treated with fluorine using hydrofluoric acid, an F-polymer spray, or plasma immersion ion implantation. They were subsequently exposed to air at 900 °C for 24 h to form a thermally grown oxide (TGO) layer consisting mainly of alumina. Using electron-beam physical vapor deposition, ceramic topcoats of 7 wt% yttria partially stabilized zirconia were deposited on the pre-oxidized fluorinated samples. The lifetime of this TBC system was determined by performing thermal cycling tests at 900 and 1000 °C in air. At 900 °C, lifetimes exceeding 1000 cycles of 1 h dwell time at high temperature were measured. The F-polymer treatment proved to be the most effective fluorination method. For TNM-B1 samples fluorinated by the latter treatment, lifetimes up to 1000 cycles were also determined at 1000 °C. The TBC exhibited good adherence to the TGO consisting mainly of alumina. An Al-depleted zone developed with small precipitates of the AlF3 phase below the oxide scale. Transmission electron microscopy examinations of the Al-depleted zone revealed the formation of the cubic Z-phase (Ti5Al3O2) which transformed into the α2-Ti3Al phase with prolonged high temperature exposure.

A magnetron sputtered microcrystalline β-NiAl coating for SC superalloys. Part I. Characterization and comparison of isothermal oxidation behavior at 1100 °C with a NiCrAlY coating

A microcrystalline β-NiAl coating was prepared on a single-crystal (SC) superalloy substrate via magnetron sputtering and subsequent vacuum annealing. The grain sizes of the coating ranged from about 300nm to 1μm. A reference NiCrAlY coating, which was mainly comprised of γ′-Ni3Al and α-Cr, was prepared by means of vacuum arc evaporation (VAE). Isothermal oxidation tests were carried out at 1100°C in air for 50h. Both coatings formed thin and adherent α-Al2O3 scales during tests, while the oxide scales on the bare superalloy primarily consisted of spinel (Ni,Co)Al2O4 with underlying α-Al2O3 scale. The parabolic rate constant of the NiAl-coated specimens was about one order of magnitude lower than that of the NiCrAlY coated specimens. After oxidation tests, only a small amount of γ′ phase was detected at some columnar boundaries of the β-NiAl coating, and about 2/3 parts of the NiCrAlY coating transformed into γ phase which resolved the α-Cr precipitations, while an Al-depleted zone in thickness of about 10μm formed beneath the TGO of the bare superalloy. Inter-diffusion zones and secondary reaction zones were observed on the specimens coated by either β-NiAl or NiCrAlY. The oxidation mechanism and microstructure evolvement of the specimens during high temperature exposures were discussed.

Observation and modeling of α-NiPtAl and Kirkendall void formations during interdiffusion of a Pt coating with a γ-(Ni-13Al) alloy at high temperature

During the last 15years, Pt-rich γ–γ′ bond-coatings have been studied extensively for their corrosion and oxidation resistance, and as a lower cost alternative to β-(Ni,Pt)Al bond-coatings in thermal barrier coating systems. To optimize their fabrication and durability, it is essential to investigate their interdiffusion with Ni-based superalloys. This study reports on experimental results and modeling of the interdiffusion of the model Pt/γ-(Ni-13Al) alloy system. Pt coatings were deposited either by electroplating or by spark plasma sintering using a Pt foil. Heat treatments at 1100°C for 15min to 10h were performed either in a high-temperature X-ray diffraction device under primary vacuum or in a furnace under argon secondary vacuum. The α-NiPtAl phase with L10 crystal structure formed very rapidly, implying fast uphill Al diffusion toward the surface. For Pt electroplating, α-phase transformed to γ′-(Ni,Pt)3Al after only 45min–1h at 1100°C. The resulting two-phased γ–γ′ microstructure remained up to 10h. When using a Pt foil coating, the continuous layer of α-NiPtAl phase disappeared after 10h and the γ′-(Ni,Pt)3Al or γ-(Ni,Pt,Al) phase appeared, resulting in two different diffusion paths in the Ni–Pt–Al phase diagram. Voids also formed at the interdiffusion zone/substrate interface for both systems after 1h or more. Composition analyses confirmed that voids were located at the Pt diffusion front corresponding to the Al-depleted zone. Experiments performed with the samples coated with a Pt foil confirmed that voids are due to a Kirkendall effect and not to the Pt deposition process. Numerical simulations including the cross-term diffusion coefficients in the diffusion flux equations reproduced the experimental concentration profiles for the γ-phased systems.

Stepwise Depletion of Coating Elements as a Result of Hot Corrosion of NiCrAlY Coatings

Present investigation deals with the hot corrosion behaviour of the NiCrAlY coatings deposited by HVOF technique on Superni76 under cyclic conditions at 900 °C in the presence of Na2SO4 + 60% V2O5 salt. The weight change behaviour of the coatings was followed with time up to 200 cycles and Kp value was calculated for the hot corrosion process. Surface and cross-section of the corroded samples were examined by FESEM/EDS and XRD to follow the progress of corrosion up to 200 cycles. In earlier cycles, the corrosive species oxidised top surface of the coatings. With increasing number of cycles, oxidation of the coatings occurred up to 40-μm depth. A Cr-depleted band was seen below the oxide scale. Further increase in number of cycles led to migration and oxidation of Al to form Al2O3 sublayer at coating/scale interface, thereby leading to formation of Al-depleted zone in the coating below the Al2O3 sublayer. The corrosion resistance of the NiCrAlY coatings is attributed to the formation of the continuous and dense Al2O3 sublayer at the coating/scale interface, which acts as barrier to the migration of Cr to the surface. The appearance of Al3Y after 100 and 200 cycles also contributes to the increased corrosion resistance of coatings after 100 and 200 cycles.

INTER-DIFFUSION BEHAVIOR OF RECOATED CoCrAlY COATING/DZ125 DIRECTIONALLY SOLIDIFIED SUPERALLOY

The thermal barrier coatings (TBCs) are widely used in gas turbines which could lower the surface operating temperature and improve the service life of the coated part component. When repair or refurbishment of these coated components, it is necessary to remove the coating and replace it with a new coatings. The CoCrAlY coatings were deposited by EB-PVD on the directionally solidified nickel-based superalloy DZ125. The coatings were removed via physical methods in varying thicknesses and recoated subsequently. The results showed that the depth of the removal had a significant impact on the inter-diffusion behavior as well as the oxidation resistance. The thickness of Al-depleted zone formed in the recoated coatings after oxidation are thinner than the first coated coatings. Moreover, the more the coating was removed, the thinner the Al-depleted zone was formed. The recoated coatings deposited on the substrate removed the entirely inter-diffusion zone (IDZ) showed the best oxidation resistance, and the IDZ formed after oxidation was similar to the first coated coatings.

Isothermal Oxidation Behaviour of a Two-Phase γ/γ′ Precipitation-Hardened Quaternary Pt-Based Alloys in Air at 1,350 °C

High temperature oxidation behaviour of water-quenched and air-cooled Pt84:Al11:Cr3:Ru2 (at.%) alloy specimens in air at 1,350 °C, for up to 100 h exposure, was studied, and found to have properties similar to a Pt–10Al–4Cr alloy, but with no zone of discontinuous oxides. The mass gain of the alloy specimens as a function of time was monitored during isothermal exposure. The oxidized specimens were examined using a focused ion beam workstation with scanning electron microscopy, scanning electron microscopy with energy dispersive spectroscopy, optical microscopy, X-ray diffraction and Raman spectroscopy. The SEM images of specimen cross sections were used to determine the thicknesses of the Al2O3 scales for the various oxidation times. Well-adhering, continuous and protective α-Al2O3 scales with no spallation formed during oxidation, irrespective of the exposure time. There was neither a zone of discontinuous oxides, nor any other internal oxidation observed. The depth of the Al depletion zone was not detected. The oxide grain sizes increased with increased exposure time, and the scale growth kinetics obeyed the parabolic rate law. After 100 h exposure, scale of about 11.79 ± 1.39 μm thick had grown on the water-quenched specimen, while the scale thickness of the air-cooled specimen was about 13.84 ± 2.15 μm. The results suggest that the investigated alloy has a promising potential for high-temperature applications.

Insights into high temperature oxidation of Al2O3-forming Ti3AlC2

Insights into high temperature oxidation of Al2O3-forming Ti3AlC2 are made to understand the sub-parabolic oxidation rate and the absence of Al-depletion zone beneath the protective Al2O3 scale. The scale results from selective oxidation of Al that migrated from Ti3AlC2 according to Ti3AlC2+3/2y O2→Ti3Al1−yC2+1/2y Al2O3, leaving Al vacancy in the substrate. Inward diffusion of oxygen via grain boundaries of the Al2O3 scale predominates for the scale growth. The Al2O3 grains grow with time, yielding sub-parabolic oxidation kinetics. The Al diffuses in a fast manner, accounting for the absence of Al-depletion zone.

Influence of Aluminum on the Formation Behavior of Zn-Al-Fe Intermetallic Particles in a Zinc Bath

The shape, size, and composition of dross particles as a function of aluminum content at a fixed temperature were investigated for aluminum added to the premelted Zn-Fe melt simulating the hot-dip galvanizing bath by a sampling methodology. In the early stage, less than 30 minutes after Al addition, local supersaturation and depletion of the aluminum concentration occurred simultaneously in the bath, resulting in the nucleation and growth of both Fe2Al5Zn x and FeZn13. However, the aluminum was homogenized continuously as the reaction proceeded, and fine and stable FeZn10Al x formed after 30 minutes. An Al-depleted zone (ADZ) mechanism was newly proposed for the “η→η+ζ→δ” phase transformations. The ζ phase bottom dross partly survived for a relatively long period, i.e., 2 hours in this work, whereas the η phase disappeared after 30 minutes. In the early stage of dross formation, both Al-free large particles as well as high-Al tiny particles were formed. The dross particle size decreased slightly with increased reaction time before reaching a plateau. The opposite tendency was observed when the Al content was 0.130 mass pct; with a relatively high Al content, the nucleation of tiny η phase dross was significantly enhanced because of the high degree of supersaturation. This unstable η phase dissolved continuously and underwent simple transformation to the stable δ phase. The relationship between nucleation potential and supersaturation ratio of species is discussed based on the thermodynamics of classical nucleation theory.

Infrared Brazing Fe3Al Using Ag-Based Filler Metals

The microstructural evolution and bonding shear strength of infrared brazed Fe3Al using Ag and BAg-8 (72Ag-28Cu in wt pct) braze alloys have been studied. The Ag-rich phase alloyed with Al dominates the entire Ag brazed joints, and the shear strength is independent of the brazing time. The BAg-8 brazed joint contains Ag-Cu eutectic for all brazing conditions, and its shear strength increases slightly with increasing brazing time. The highest shear strength of 181 MPa is acquired from the joint infrared brazed at 1073 K (800 °C) for 600 seconds. A thin layer of Fe3Al is identified at the interface between the brazed zone and the substrate for both braze alloys. An Al depletion zone in the Fe3Al substrate next to the interfacial Fe3Al is identified as the α-Fe phase. The dissolution of Al from the Fe3Al substrate into the molten braze causes the formation of α-Fe in the Fe3Al substrate.

Solid Reactions between Enamel and O-Phase Ti-Al-Nb Intermetallics at 800 °C

The specimens of O-phase Ti-22Al-25Nb (at%) intermetallics coated with silica-based enamel received mass gains of about 1 mg/cm2, after 300 h of oxidation or hot corrosion at 800 °C. These rates were much faster than the growth rates of silica films at the same temperature. To understand this phenomena, the specimens were analyzed using SEM, XRD, EPMA and TEM. An oxide layer with thickness of several μm was observed at the enamel/substrate interface of the coated specimens after either oxidation or hot corrosion. XRD and TEM analysis revealed the newly formed oxide layer was composed of α-Al2O3, Al2SiO5, Al2TiO5, rutile-TiO2 and NbO2. It was shown by EPMA profiling that an Al-depleted zone was located just beneath the oxides. It was proposed that the solid reactions between the enamel coating and the O-phase Ti-Al-Nb played important roles for the oxidation and hot corrosion behavior of the coated specimens.