Al Depletion Research Articles

Interfacial fracture toughness of APS bond coat/substrate under high temperature

Thermal barrier coating (TBC) is an essential requirement of a modern gas turbine engine. The TBC failure is the delamination and spallation. The failure mechanism is interfacial expansion mismatch and oxidation of bond coat (BC). The oxidation damage under high temperature results in the reduction of interfacial adhesion. The interfacial fracture toughness is an important property to analyze the TBC failure. Using the simple tensile test, pushout test method and three-point or four-point-bending test and so on, the interfacial fracture toughness of ceramic top coat/BC has been researched in the past. However, the fracture toughness of the BC/substrate due to the Al depletion was very few studied. In this study, a NiCrAlY bond coat by air plasma spray (APS) was deposited. The substrate is directionally solidified superalloy (DZ40M). The Young’s modulus of bond coat was obtained by the nanoindentation and average Young’s modulus of bond coat is 66.9 GPa. Isothermal oxidation was performed at 1,050◦C for 100 h. Using the HXZ-1000 micro-hardness equipment and fracture mechanics approach, the five different times was chosen to test the hardness and the crack length, and then the fracture toughness was obtained. While the oxidation exposure time increased at 1,050◦C, the hardness of the substrate close to the bond coat decreased with the increase of the bond coat in hardness. Meanwhile, the interfacial fracture toughness of the bond coat–substrate decreased because of the Al depletion.

A numerical model for the description of the lamellar and massive phase transformations in TiAl alloys

A phenomenological modelling approach has been developed to describe the massive transformation and the formation of lamellar microstructures during cooling in binary γ titanium aluminides. The modelling approach is based on a combination of nucleation and growth laws which take into account the specific mechanisms of each phase transformation. Nucleation of massive and lamellar γ is described with classical nucleation theory, accounting for the fact that nuclei are formed predominantly at α/α grain boundaries. Growth of the massive γ grains is based on theory for interface-controlled reactions. A modified Zener model is used to calculate the thickening rate of the γ lamellar precipitates. The model incorporates the effect of particle impingement and coverage of the nucleation sites by the growing phase. The driving pressures of the phase transformations are obtained from Thermo-Calc based on the actual temperature and matrix composition. CCT diagrams and lamellar spacings calculated with the model are in good agreement with experimental data obtained from dedicated heat treatment experiments and from the literature. The model permitted investigating the influence of cooling rate, alloy chemistry and average α grain size upon the amount of massive γ and the average thickness and spacing of the lamellae. In particular it indicates that the Al depletion of the α phase during lamellar precipitation seems to play an important role in the suppression of the massive transformation at moderate cooling rate and in the large lamellar spacings observed at low cooling rate.

Numerical Modelling of Diffusion Coupled with Cyclic Oxidation. Application to Alumina-Forming Coatings Used for Industrial Gas Turbine Blades

A model using a finite elements code is developed to simulate degradation due to combined cyclic oxidation and interdiffusion in materials used in high temperature components of gas turbines. Coating recession due to cyclic oxidation (oxide growth and spalling) is also modelled using a statistical approach. Interdiffusion between coating and superallloy is modelled to predict total Al depletion in the coating. The phase transformations in the alumina-forming coating and the effects of the precipitates at the coating / superalloy interface are investigated. The results of simulations are compared with experimental data. Effects of diffusion parameters and of cyclic oxidation kinetics are discussed.

Element-specific in situ corrosion behavior of Zr–Cu–Ni–Al–Nb bulk metallic glass in acidic media studied using a novel microcapillary flow injection inductively coupled plasma mass spectrometry technique

Element-specific time-resolved in situ corrosion susceptibility of the Zr 58.5Cu 15.6Ni 12.8Al 10.3Nb 2.8 bulk metallic glass was studied using a novel microcapillary flow injection inductively coupled plasma mass spectrometry (microcapillary FI-ICP-MS) technique. In comparison to the time consuming and sample demanding static immersion tests, the new set-up allows element-specific in situ investigations of corrosion dissolution rates with a high time resolution. Moreover, online monitoring of corrosion processes can be performed at a single sample spot. Time-resolved profiles of element release from Zr 58.5Cu 15.6Ni 12.8Al 10.3Nb 2.8 were investigated in 1 M HCl and 1 M HNO 3 under the open circuit potential conditions. Preferential dissolution of Cu and Al from the Zr-bulk metallic glass was determined. Moreover, X-ray photoelectron spectroscopy (XPS) was employed to characterize the composition of the oxide film before and after the immersion in acidic corrosive medium. Based on microcapillary FI-ICP-MS and XPS results the mechanisms for simultaneous Al and Cu depletion and Zr content enrichment in the oxide film were proposed.

The role that bond coat depletion of aluminum has on the lifetime of APS‐TBC under oxidizing conditions

AbstractBond coat oxidation as well as bond coat depletion of Al are still believed to be a major degradation mechanism with respect to the lifetime of thermal barrier coating (TBC) systems. In this study the top coat lifetime is described as being limited by both bond coat depletion of Al and mechanical failure of the top coat. The empirical results are introduced by considering three spallation cases, namely, Al depletion failure, thermal fatigue failure, and thermal aging failure. Al depletion failure occurs when the Al content within the bond coat reaches a critical value. In this paper bond coat depletion of Al is modeled by considering the diffusion of Al into both the thermally grown oxide (TGO) and substrate. The diffusion model results are compared to Al concentration profiles measured with an electron beam microprobe. These measured results are from oxidized air plasma sprayed TBC systems (APS‐TBC) with vacuum plasma sprayed (VPS) bond coats for exposures up to 5000 h in the temperature range of 950–1100 °C. This paper focuses on the Al depletion failure and how it relates to top coat spallation.

Effect of bond coat surface roughness on oxidation behaviour of air plasma sprayed thermal barrier coatings

Two layered thermal barrier coatings (TBCs) with different bond coat surface roughnesses were produced by air plasma spraying. Isothermal oxidation test was carried out to evaluate the durability and oxidation behaviour of these systems. The results show that the roughness of bond coat significantly affects the lifetime of TBCs. With the increase in bond coat roughness, the lifetime decreases exponentially. The durability is enhanced by surface grinding treatments that decrease diffusion paths on the surface of the bond coat and diminish Al depletion rate. The shape and path of cracks at failure are also affected by the bond coat surface roughness.

Phase transformations induced by nitridation of quasicrystalline AlCuFeB powders

Nitridation is an important reaction which has never been investigated for Al-based quasicrystals. Here we show that AlCuFeB quasicrystalline particles undergo phase transformations upon annealing at high temperature in pure nitrogen. A thick nitride layer grows on the surface of the particles, and Al depletion in the bulk results in transformation of the quasicrystalline phase first into the cubic β-phase and then into the monoclinic λ-phase. The kinetics of this transformation is much faster than that of oxidation.

Thermal stability and oxidation behavior of Al-containing nanocrystalline powders produced by cryomilling

Al-containing nanostructured coatings provide excellent protection from high temperature corrosion. Aluminum oxide scales generally provide better oxidation resistance and yield lower oxidation rates than other oxide scale compositions. In this study, nanocrystalline 316L stainless steel containing 6 wt.% Al was synthesized using cryogenic milling (cryomilling). Complete alloying was obtained after 32 h of milling and the average grain size was found to be 7 nm. High temperature thermal stability and oxidation kinetics of the alloyed powders were examined. The powder demonstrated good grain growth stability at 500 °C, at which point, the powders had been heat treated for 120 h and the average grain size was found to be 11.4 nm. The oxidation kinetics of the powder were studied for 48 h at 500, 800, and 1,000 °C, respectively. For comparison, conventional 316LSS powder was also tested. Nanocrystalline 316LSS-6 wt.% Al showed lower weight gain than the conventional 316LSS powders. During the oxidation of nanocrystalline 316LSS-6 wt. % Al at 500 °C, protective aluminum oxide scale formed at the surface. At 800 °C and 1,000 °C, most of the nanocrystalline 316LSS-6 wt.% Al particles showed completed outer aluminum oxide scale. However, at 800 and 1,000 °C, some particles showed growth of chromium oxide scale underneath the aluminum oxide scale. In those samples, Al depletion was also observed due to a non-homogenous distribution of Al during cryomilling. The activation energy of the oxidation reaction was calculated and was found to be affected by the enhancement of the grain boundary diffusion in nanostructured particles.

Preparation and cyclic oxidation of gradient NiCrAlYRe coatings on Ni-based superalloys

Arc ion plating (AIP) and chemical vapor deposition (CVD) aluminizing were combined together to produce a gradient NiCrAlYRe coating. Elements in this coating were chemically graded with Al enrichment in the outer zone and Cr enrichment in the intermediate zone. Cyclic oxidation tests were performed at 1100 °C for up to 22 cycles. The results showed that the gradient NiCrAlYRe coatings performed better resistance to spallation and Al depletion than the conventional NiCrAlYRe coatings. This favorable oxidation behavior was attributed to the high Al distribution in the near surface region of the gradient coating.

Reactive Wetting of an Iron-Base Superalloy MSA2020 and 316L Stainless Steel by Molten Zinc-Aluminum Alloy

The reactive wetting behaviors of MSA2020, an Fe-based superalloy, and 316L stainless steel in contact with a molten Zn-Al alloy were investigated by the sessile drop method. This investigation led to the following findings. (1) 316L not only suffered considerable wetting, but also reacted with the molten Zn-Al alloy at a higher rate than MSA2020. (2) The contact angle of MSA2020 wet by the molten Zn-Al alloy dropped to an acute angle when the temperature was increased to 500 °C. (3) The surface reaction was found to initiate even though the liquid droplet and substrate were observed as nonwetting (contact angle larger than 90 deg). (4) The reaction mechanisms were identified in three stages. Initially, the Al diffused into the substrate to form an Fe-aluminide layer, which acted as the reaction front. Next, the reaction front penetrated the substrate through inward diffusion of Al. Finally, Zn-rich zones formed behind the reaction front as a result of Al depletion. (5) The alloying constituents (W, Mo, and Cr) in MSA2020 stably segregating on the surface reduced the wettability by molten Zn-Al by covering the reactive sites on the solid-liquid interface.

Cyclic and Isothermal Oxidation at 1,100 °C of a CVD Aluminised Directionally Solidified Ni Superalloy

CVD aluminide coatings deposited on a Directionally Solidified (DS) substrate were oxidized at 1,100 °C up to 240 h under isothermal and cyclic oxidation conditions to study the growth mechanisms of the oxide scales and the possible degradation of the coatings. The specimens were investigated using light and scanning electron (SEM) microscopy, energy-dispersive spectrometry (EDS) and X-Ray Diffraction (XRD). The results indicate that the coatings provide a much greater beneficial effect under isothermal conditions than upon cycling. The cycled specimens undergo oxide-scale spallation and increased roughening, which can derive from growth and thermal stresses as well as from the NiAl → Ni3Al phase transformation associated with Al depletion. Under isothermal conditions, typical oxide scales formed with the appearance of some rumples. However, the origin of rumpling is uncertain from these experimental results.

Al Diffusion in Polycrystalline Cu

ABSTRACTThe diffusion of aluminum (Al) from a source sandwiched between polycrystalline copper (Cu) thin films was investigated as a function of time and temperature through secondary ion mass spectroscopy (SIMS) and continuum simulations. Extracted diffusion coefficients for the bulk were in line with literature values. In order to simulate the experimentally derived diffusion profiles at temperatures where bulk diffusion is not the dominant diffusion mechanism (room temperature to 350 °C), it was necessary to explicitly include the re-distribution of Al as a result of Cu grain growth during anneal. Aluminum has the tendency to segregate to the Cu/liner and Cu/etch stop (ES) interface. The tendency of Al to segregate to the liner is ten times stronger for ruthenium (Ru) than for tantalum (Ta). In 100 nm wide dual damascene structures lined with Ru, this segregation behavior was responsible for the Al depletion in bulk Cu and for the Al depletion at the Cu/ES interface.

Spinel formation in thermal barrier systems with a Pt-enriched γ–Ni + γ′–Ni 3Al bond coat

The oxidation of electron beam physical vapour deposited thermal barrier coatings with a Pt-enriched γ–Ni + γ′–Ni 3Al bond coat was investigated. Due to the growth of the thermally grown oxide (TGO), γ–Ni formed underneath the TGO as a result of Al depletion. Phase characterisation by X-ray diffraction, as well as microstructural observations, indicated that a NiAl 2O 4 spinel phase formed at the TGO/bond coat interface after prolonged oxidation. It is proposed that the formation of spinel occurs when local cracks present at the interface and the underlying bond coat is Al-depleted. The cracks provide a direct path for oxygen and nickel oxide forms at the bond coat surface. With further oxidation, the spinel forms at the interface through solid state reaction between the TGO and nickel oxide.

Interfacial Fracture Toughness of APS Thermal Barrier Coating under High Temperature

Thermal barrier coating (TBC) is an essential requirement of a modern gas turbine engine. The TBC failure is the delamination and spallation. The oxidation damage under high temperature results in the reduction of interfacial adhesion. The interfacial fracture toughness is an important property to analyze the TBC failure. The interfacial fracture toughness of ceramic coating - bond coating has been researched in the past. However, the facture toughness of the bond coating - substrate due to the Al depletion was very few studied. In this study, a NiCrAlY bond coating by air plasma spray (APS) was deposited. The substrate was directionally solidified superalloy (DZ40M). Isothermal oxidation was performed at 10500 for 100h. Using the HXZ-1000 micro-hardness equipment, the five different times was chosen to test the hardness and the crack length, and then the fracture toughness was obtained. While the oxidation exposure time increased at 10500, the hardness of the substrate close to the bond coating decreased with the increase of the bond coating’ hardness. Meanwhile, the interfacial fracture toughness of the bond coating - substrate decreased because of the Al depletion.

Aluminium depletion in NiCrAlY bond coatings by hot corrosion as a function of projection system

Three different projection system are used to prepare NiCrAlY bond coats over metallic substrates: atmospheric plasma spray (APS), high velocity oxyfuel (HVOF) and high frequency pulse detonation (HFPD). These coatings were tested in hot corrosion experiments with sprayed Na 2SO 4 at 1000 °C for 20 and 100 h experiments in air. Coatings surface composition after thermal treatment was characterised by XRD and SEM. Cross section of coatings were analysed by SEM-EDX. A relationship between microstructural characteristics of initial coatings and final performance in hot corrosion was found in terms of porosity percentage: plasma sprayed coatings present higher percentage of porosity compared to HVOF and HFPD projection systems for the same composition and Al is heavily consumed in interparticle oxidation. This Al depletion in turn involves intrinsic chemical failure and surface layer is comprised by a porous spinel of mixed oxides. On the other hand, high energy projection systems produce dense coatings allowing the Al migration to external alumina layer, particularly in the case of HVOF coating.

Oxidation of arc-evaporated Al1−xCrxN coatings

The recently introduced Al1−xCrxN coatings are characterized by superior thermal stability and oxidation resistance. Although several studies on the microstructure and mechanical properties of these coatings have been published, the oxidation behavior and mechanisms were only investigated for Al1−xCrxN with Cr∕Al ratio bigger than 1. Within this work, the oxidation of arc-evaporated Al1−xCrxN coatings was investigated as a function of the Al content. Coatings were deposited onto cemented carbide substrates using an industrial-scale arc evaporation system (Balzers rapid coating system) and alloyed Al∕Cr targets yielding Al1−xCrxN coatings ranging from x=0.29 to x=0.79. Heat treatments in air were conducted in the temperature range between 900 and 1100°C. For comparison, samples were also annealed in Ar between 900 and 1200°C. Elemental distributions within the coating and the oxide layers were determined by secondary ion mass spectroscopy. After annealing in Ar, Cr was found to segregate to the coating surface following the decomposition of the face-centered cubic Al1−xCrxN phase into hcp AlN and, under release of N2, Cr2N and later on Cr. Simultaneously, Al depletion was observed within the Cr enriched near-surface zone. In contrast, the decomposition of the cubic Al1−xCrxN phase is severely hindred during annealing in air. Outward diffusion of Cr and Al, N depletion, and inward diffusion of O were observed, manifesting diffusion mechanisms depending on the initial coating composition. A protective mixed Cr2O3–Al2O3 scale, at the coating surface mainly dominated by Cr2O3, was formed for all coating compositions investigated. This protective oxide scale prevents the decomposition of the cubic Al1−xCrxN phase during high temperature exposure.The recently introduced Al1−xCrxN coatings are characterized by superior thermal stability and oxidation resistance. Although several studies on the microstructure and mechanical properties of these coatings have been published, the oxidation behavior and mechanisms were only investigated for Al1−xCrxN with Cr∕Al ratio bigger than 1. Within this work, the oxidation of arc-evaporated Al1−xCrxN coatings was investigated as a function of the Al content. Coatings were deposited onto cemented carbide substrates using an industrial-scale arc evaporation system (Balzers rapid coating system) and alloyed Al∕Cr targets yielding Al1−xCrxN coatings ranging from x=0.29 to x=0.79. Heat treatments in air were conducted in the temperature range between 900 and 1100°C. For comparison, samples were also annealed in Ar between 900 and 1200°C. Elemental distributions within the coating and the oxide layers were determined by secondary ion mass spectroscopy. After annealing in Ar, Cr was found to segregate to the coating sur...

Evaluation of the hot corrosion resistance of commercial β-NiAl and developmental γ′-Ni 3Al + γ-Ni-based coatings

This study evaluates the performance of commercially available β-NiAl and developmental Pt + Hf-modified γ′-Ni 3Al + γ-Ni coatings under Type II (705 °C) hot corrosion conditions. A further variable studied was the effect(s) of pre-oxidation treatment on subsequent hot corrosion resistance. The hot corrosion performances (with and without pre-oxidation treatment) were assessed by analyzing weight-change behavior and cross-sectional images of the corroded samples. It was found that the γ′ + γ coating exhibited superior Type II hot corrosion resistance with the pre-oxidation treatment; while, the Pt-modified β coating exhibited the best Type II hot corrosion resistance when no pre-oxidation treatment was given. Interestingly, pre-oxidation of the Pt-modified β coating was not beneficial to Type II hot corrosion resistance, due, it is believed, to Al depletion from the coating during the pre-oxidation treatment. It was also found that Pt addition is beneficial in improving the hot corrosion resistance of β coatings; whereas, the presence of Hf and a relatively high Al content in the γ′ + γ coatings is beneficial. Moreover, the results from this study clearly revealed the need to pre-oxidize the γ′ + γ coatings at elevated temperatures ( i.e., greater than ∼ 1050 °C) in order to achieve excellent hot corrosion resistance.

The Transient Stages of Inclusion Evolution During Al and/or Ti Additions to Molten Iron

This study investigates the evolution of inclusions as a result of Al and Ti additions to molten Fe at 1 873 K with the objective of elucidating the transient stages of inclusion formation during ladle processing of IF steel melts. The effects of order of addition, time after addition, Al/Ti ratio and oxygen content are evaluated through an experimental approach that involves de-oxidation and sampling inside a vacuum induction furnace under conditions where the total oxygen concentration of the melt and samples are maintained constant. Each sample is analyzed for chemistry and resulting inclusion characteristics (size, morphology and chemistry). All experiments were carried out under the thermodynamic condition that Al2O3 was the only stable inclusion. The following results were found. Firstly, the equilibration time of Al was found to be faster than that of Ti under the present experimental conditions and as a result Al2O3 forms initially when Al and Ti were simultaneously added. The addition of Ti results in the formation of oxide inclusions containing Ti up to 20 mol% as a result of local Al depletion and Ti-supersaturation. This was enhanced in terms of the fraction of Ti containing inclusions as the Ti content was increased. Ultimately, in all samples, the thermodynamically stable inclusion Al2O3 was predominant within 5 min, but the morphology of these final inclusions were of polygonal shape rather than the spherical inclusions that formed immediately after Al de-oxidation. This modification in shape could have consequences on clogging, during post ladle teeming and pouring, in continuous casting as the tendency for agglomeration would be expected to increase.

INFLUENCE OF ION IMPLANTATION ON OXIDATION BEHAVIOR OF TiAl

The influence of alloying elements on oxidation behavior of TiAl has been investigated using an ion-implantation technique and the mechanisms were discussed. The influence can be classified into several groups according to their effects. The implantation of β-forming elements, halogens, Cu and Zn results in a significant improvement of the oxidation behavior through formation of Al 2 O 3 layer in the initial stage of oxidation. The improvement by Zn is attributed to the formation of complex oxide of Zn and selective oxidation of Al beneath the layer. The implantation of Al , Si or P is also effective. On the other hand, implantation of Ag , Se and other several elements enhance the oxidation. The deterioration by Ag or Se is explained in terms of Al depletion in the implanted layer.

Influence of Ion Implantation of Several Elements on Oxidation Behavior of TiAl

TiAl-based alloys have attractive properties as light weight heat-resisting material. In the present work, the influence of Cu, Zn, Ag and Se on the oxidation behavior of TiAl was investigated by ion implantation at acceleration voltage of 50 kV and ion doses of 1019 to 2x1021 ions/m2. The oxidation behavior was assessed by a cyclic oxidation test at 1200 K in a flow of purified oxygen under atmospheric pressure. The oxidation products were analyzed by conventional methods including X-ray diffractometry, SEM and EPMA. The implantation of Zn and Cu improves the oxidation resistance significantly by forming virtually Al2O3 scales, while Ag and Se enhance the oxidation. The improvement by Zn is attributable to the formation of complex oxide of Zn in the initial stage of oxidation. The oxygen partial pressure under the layer seems to be very low, resulting in the formation of alumina scale due to a selective oxidation of Al. The influence of Cu is not certain. The influence of Ag and Se is explained in terms of Al depletion in the implanted layer.