Reactive Element Effect Research Articles

Effect of reactive elements in MCrAlX bond coat for durability improvement of thermal barrier coatings

The effects of reactive elements (RE) in MCrAlX alloy (M: Metals, X: RE) as the bond coat for thermal barrier coatings durability were investigated by both experimental evaluation and density functional theory (DFT) calculations. Cyclic oxidation tests showed that the lifetime of the sample with Y, Hf, and Si as RE displayed approximately twice of the sample with Y only. Furthermore, the interface energies between oxide (α-Al2O3) and bond coat with various RE (Si, Sc, Ti, Y, Zr, Hf, La, and Ce) were calculated by DFT. DFT results indicate that Hf and Si doping effectively reduce the interface energy.

Contribution of Y2O3/Hf co-doping to alumina scale growth on oxide dispersion strengthened Co-based superalloy

Effect of Y2O3/Hf co-doping on the growth of the alumina scale formed on an oxide dispersion strengthened (ODS) Co-10Ni-20Cr-15Al (at%) superalloy at 900 °C in air was investigated. On both superalloys with and without Y2O3/Hf co-doping, a distinctive layered alumina scale consisting of an outer θ-alumina and an inner α-alumina was formed. Y2O3/Hf co-doping could reduce the oxidation growth rate of the θ- and α-alumina layers and improve the spallation resistance of the scale by suppressing the formation of interfacial pores. The reactive element effect in the θ-alumina has been initially elucidated in detail with the aid of 3D-AP characterization.

Effects of Lu on the α-Al2O3/β-NiAl interface adhesion from first-principles calculations

The effects of the reactive element lutetium (Lu) addition on the α-Al2O3(0001)/β-NiAl(110) interface adhesion is investigated from first-principles calculations. Our results show that Lu can improve the α-Al2O3/NiAl interface adhesion by three mechanisms: (i) trapping impurity sulfur (S) in the NiAl bulk by forming Lu-S bonds thereby reducing detrimental S segregation to the Al2O3/NiAl interface, (ii) promoting Lu segregation to the α-Al2O3/NiAl interface, thus blocking the segregation of S toward the oxide/alloy interface, (iii) enhancing the α-Al2O3/NiAl interfacial bonding strength by forming Lu-O and Lu-Ni bonds across the interface. These results rationalize the reactive element effect on the high-temperature oxidation resistance of alumina-forming alloys.

Antioxidant therapy against TGF-β/SMAD pathway involved in organ fibrosis.

Fibrosis refers to excessive build-up of scar tissue and extracellular matrix components in different organs. In recent years, it has been revealed that different cytokines and chemokines, especially Transforming growth factor beta (TGF-β) is involved in the pathogenesis of fibrosis. It has been shown that TGF-β is upregulated in fibrotic tissues, and contributes to fibrosis by mediating pathways that are related to matrix preservation and fibroblasts differentiation. There is no doubt that antioxidants protect against different inflammatory conditions by reversing the effects of nitrogen, oxygen and sulfur-based reactive elements. Oxidative stress has a direct impact on chronic inflammation, and as results, prolonged inflammation ultimately results in fibrosis. Different types of antioxidants, in the forms of vitamins, natural compounds or synthetic ones, have been proven to be beneficial in the protection against fibrotic conditions both invitro and invivo. In this study, we reviewed the role of different compounds with antioxidant activity in induction or inhibition of TGF-β/SMAD signalling pathway, with regard to different fibrotic conditions such as gastro-intestinal fibrosis, cardiac fibrosis, pulmonary fibrosis, skin fibrosis, renal fibrosis and also some rare cases of fibrosis, both in animal models and cell lines.

Effects of reactive elements and Pt doping on the adhesive strength and tension property of α-Al2O3/γ'-Ni3Al-S interface

The enhancement of cyclic oxidation performance in ultra-high temperature protective coatings for turbine blades in new generation aero-engines can be achieved through the reactive elements or Pt doped β-NiAl single phase. However, it is important to note that the content of Al in NiAl gradually decreases during high temperature service, leading to an inevitable phase transition from β-NiAl to γ’-Ni3Al. We only studied the reactive element effects on the adhension and stability of the α-Al2O3/β-NiAl interface previously, but the influence mechanism for α-Al2O3/γ'-Ni3Al interface is not clear. Therefore, it is imperative to investigate the adhension and stability of the γ’-Ni3Al/oxide interface. This paper investigates the effect of Pt or reactive elements (Hf, Zr, Dy, Y, La) doping on the α-Al2O3/γ'-Ni3Al interfacial adhesion and tensile properties with or without impurity element S, using the first-principles calculation method. The results indicate that the influence of Pt or REs doping on interface adhension is contingent upon the existence of S, and the effect on static interface bonding or dynamic tensile properties varies as well. It will improve the comprehensive design system of Ni-Al coatings, and provide theoretical guidance for the development of new metallic protective coating materials.

Study of acoustic and aerodynamic performance of reactive silencer with different configurations: Theoretical, modeling and experimental

Periodic sound reduction at some frequencies and high pressure loss could be considered as the disadvantages of simple expansion silencers. Generally, reactive elements are applied in silencers to optimize their acoustic and aerodynamic performance. In this paper, we were going to investigate the combined and independent effects of reactive elements, baffle and extended tubes at the inlet and outlet of simple expansion silencer, on the acoustic performance and pressure loss of simple expansion silencer. Firstly, simple expansion silencer dimensions were determined based on theoretical calculations to attenuate the noise by 10 dB. Secondly, the simulation was performed using COMSOL acoustical module software based on finite element method to predict silencer sound transmission loss and pressure loss, respectively, before and after adding reactive elements. Then, according to ISO7235, ISO3744 and ISO3746 standards noise insertion loss of various silencers under study was measured in a free field and in the following that silencers pressure loss evaluated. Finally, the predicted results with the software were compared with the experimental ones. The addition of the extended tubes increased the transmission loss and insertion loss at low and medium frequencies and mitigated the pressure loss of the silencer compared to the simple expansion silencer. The baffle increased the transmission loss curve in the medium frequencies compared to the simple expansion silencer. The combination of baffle and extended tube elements caused an extraordinary increase in insertion loss and transmission loss at the medium frequencies and a decrease in pressure loss. It could be concluded from the present study that using extended tube might be probably the best choice to reduce pressure loss and increase the acoustic performance of simple expansion silencer at low and medium frequencies and what's more is that the best acoustic performance in medium frequencies can be achieved by using a combination of baffle and extended tube.

A comparative study on the oxidation behavior of Y-, Hf- and YHf-doped NiCoCrAl alloys: Effect of reactive elements

This study investigates the effect of Y, Hf and Y/Hf addition on oxidation behavior of NiCoCrAl alloys at 1100 °C. It is demonstrated that Hf is most effective in reducing the oxide growth rate, while co-doping Y and Hf doesn’t result in a synergistic effect in slowing down oxidation rate. Formation of interfacial voids is inhibited in NiCoCrAlY and NiCoCrAlYHf, suggesting that Y is more effective in improving the resistance to oxide spallation. A new model based on the ionic radii of reactive elements (REs) and bonding strength of RE-O is proposed to guide alloy design for improved oxidation resistance.

The Reactive Element Effect

Rationalizing the reactive element effect operative in alumina- and chromia-forming alloys upon oxidation under oxidizing hot gas atmospheres, referring to investigations on zirconium oxide-coated test samples for gas turbine alloys. This retrospective study uses the results of cyclic furnace lifetime tests conducted at 1100 °C on ZrO2-coated Ni-base alloys with Y− or Y+ Hf-doped bond coats and correlates them with the parabolic oxidation rate constant at 1100 °C of binary NiAl alloys doped with Y, Zr, or Hf. Parallel results at higher temperatures allow the respective oxidation processes during the cyclic lifetimes to be assigned to cation-dominated or anion-dominated transport processes. The correlations document the close interrelationship betweenthe refractory element content (Mo, Re, Ta, W) in the substrate alloythe total content of the two reactive elements Y and Zr in the mixed zone of the scale, representing a Me3+ iso-valence valueindividual relative lifetime parameters in pct for EBPVD thermal barrier coating systems associated with cation-dominated transport processes.

Preparation and oxidation behavior of novel CeO2/Y2O3-modified aluminide coatings on Ni using sol–gel derived/pack cementation process

CeO2/Y2O3-modified aluminide coatings were prepared on Ni substrate using a sol–gel derived/pack cementation process. For comparison, simple aluminide coatings were also produced on Ni substrate using the same pack cementation process. The microstructure and high temperature oxidation behavior of three aluminide coatings at 900 °C for 50 h in air were comparably analyzed using SEM/EDS and XRD. The results indicated that both CeO2/Y2O3-modified aluminide coatings exhibited better oxidation resistance than the CeO2/Y2O3-free one, because of various effects of CeO2/Y2O3 on the oxidation including the grain refinement effect of the aluminide, the “reactive element effect”(REE) and the epitaxial template effect of Y2O3 to promote the growth of α-Al2O3.

Negative effect of Hf and Zr co-doping on alumina film growth of nickel-aluminium single-crystals during thermal cyclic oxidation

ABSTRACT The microstructure and cyclic oxidation behaviour of Hf and Zr co-doped B2 NiAl single-crystal alloys at 1473 K were investigated. Hf and Zr co-precipitated into one phase during alloy manufacturing. The RE-free single-crystal alloy obtained a double-layered alumina film comprised of outer equiaxed grains and inner columnar grains and exhibited lower oxidation rate than the corresponding poly-crystal alloy, while the co-doped single-crystal alloy displayed higher oxidation rate than the RE-free single-crystal alloy and no synergistic effect was discovered. The effect of Hf and Zr co-doping on the growth behaviour of the alumina film was discussed. It is proposed that the existence of grain boundaries is an important precondition for activating the reactive element effect. The absence of grain boundaries significantly weakened the positive effect of Hf and Zr co-doping and distinctly magnified its negative effect on the cyclic oxidation resistance of the alloy in the meantime. Highlights Cyclic oxidation behaviour of Hf and Zr co-doped NiAl single-crystals is compared. The undoped alloy obtains a double-layered alumina film during oxidation. The co-doped alloy shows higher alumina film growth rate than the undoped alloy. The absence of alloy grain boundaries suppresses the reactive element effect.

The effect of Mo and Ge reactive elements on high-temperature oxidation of higher manganese silicide

Higher manganese silicide (HMS) alloys (Mnx-αMoαSiy-βGeβ (x = 0.99–1.011, α = 0.005–0.02, y = 1.75, β = 0.005–0.01)) were studied to elucidate the effect of Mo and Ge pertaining to oxidation. Oxidation experiments were conducted using thermogravimetry and characterized using x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Isoconversion experiments below 450 °C, shows that doping (up to 2 at%) raises the oxidation potential of HMS. Isothermally, the oxidation rate reduces buy one order of magnitude by doping on Mn and/or Si sites from 0.5 to 2 at%, revealing that the dopants-based oxides do not lessen the robustness of SiO2 oxide.

Microstructure formation and high temperature oxidation behavior of Ti-Al-Cr-Y-Si coatings on TiAl

The paper presents the application of Closed Hollow Cathode Physical Vapor Deposition (CHC-PVD) method for the depostion of Ti-Al-Cr-Y-Si coatings on γ-TiAl 48-2-2 alloy for high temperature oxidation protection. The study concerned the analysis of the coating’s growth mechanism, initial microstructure as well as phase transformations investigations using high resolution Scanning Transmission Electron Microscopy (STEM) and high temperature X-ray diffraction (HT-XRD). The coated alloy was subjected to high temperature oxidation test at 850 °C where a fivefold lower mass gain compared to bare 48-2-2 alloy was observed. Detailed microstructural investigations allowed to characterize the thermally grown oxide scale, which was found to be composed of nanometric layers of titania, equiaxed (Al,Cr)2O3 and columnar alumina. These investigations provided microstructural evidence for the Cr effect on the formation of Al2O3, which was postulated previously. Yttrium was found to segregate to the grain boundaries of alumina oxide scale during high temperature oxidation, indicating the occurence of the reactive element effect.

On aliovalent cations control of α-alumina growth on doped and undoped NiAl

Alumina forming Ni-base superalloys are essential due to their oxidation resistance at elevated temperatures. A two-step procedure allows to assess the outward growth of the oxide scale from the resulting oxide ridges that form at 1100°C and cap the alumina grain boundaries. Employing undoped 50Ni50Al (at%) as reference, the impact of reactive elements on the diffusion processes, here Zr and Hf, is quantified using atom probe tomography. Unexpectedly, we find that up to one monolayer of Ni may co-decorate the alumina grain boundaries. Additionally, a decrease in Al-, and Ni-diffusivity of two orders of magnitude is observed owing to the reactive element effect. We employ density functional theory calculations to better understand the role of aliovalent cations, here Ni(II), Zr(IV), and Hf(IV) in the α−alumina scale. The calculations show that Ni may not only decorate the alumina grain boundaries but also facilitates transport of electrons as well as oxygen vacancies. Thereby oxide scale growth becomes enhanced. In turn, the dual impact of reactive elements, i.e. to annihilate oxygen vacancies and to remove impurity states in the band gap, explains the reduced scale growth rate.

Origins of a Low-Sulfur Superalloy Al2O3 Scale Adhesion Map

Low-sulfur single-crystal Ni-base superalloys have demonstrated excellent cyclic oxidation resistance due to improved Al2O3 scale adhesion. This derives from preventing deleterious interfacial sulfur segregation that occurs at common ppm levels of S impurity. Multiple hydrogen-annealing desulfurization treatments were employed to produce a continuum of levels demonstrating this oxidative transition, using 1 h cyclic oxidation at 1100 °C for 500 h to 1000 h. The sulfur content was determined by glow discharge mass spectrometry. The complete gravimetric database of 25 samples is revealed and correlated with sulfur content. Maximum adhesion (i.e., no weight loss) was achieved at ≤ 0.3 ppmw S, significant spallation (20–30 mg/cm2) above 2 ppmw, with transitional behavior between 0.3 and 2 ppmw S. A map suggested that adhesion was enabled when the total sulfur reservoir was less than one S atom per Ni interface atom. Equilibrium models further suggest that segregation may be minimized (~1% at 0.2 ppmw bulk), regardless of section thickness. 1st order adhesion effects have thus been demonstrated for PWA 1480 having no Y, Zr, or Hf reactive element dopants and no possibility of confounding reactive element effects. The results are compared with 2nd generation PWA 1484, Rene’N5, N6, and CMSX-4® SLS, all having Hf dopants.

Microstructural characteristics and oxidation behavior of the modified MCrAlX coatings: A critical review

Currently, the service temperature of the MCrAlX coating (M = Ni, Co, or NiCo; X = minor elements such as Y, Ce, Si, Ta) either as an overlay or as a bondcoat in some critical engineering parts for gas turbine and aero-engine applications is more than 1000 °C. Therefore, the improvement of thermal stability and service lifetime of the MCrAlX coatings are the key factors that need to be emphasized. Among them, the high-temperature oxidation resistance is one of the key properties of MCrAlX coatings. The key modification processes can be employed to improve the structural properties and oxidation behavior of MCrAlX coatings. These methods mostly include techniques such as vacuum heat treatment, nano-crystallization, pre-oxidation, alloying with the reactive element or rare earth elements, dispersion of oxide particles, laser treatment, spark plasma sintering, hot isostatic pressing, and using multilayered/graded coating systems. In recent decades, investigations on the increasing of the oxidation resistance of MCrAlY coatings were mostly focused on the modification of these coatings using reactive elements, laser treatment, and development of advanced multilayered or gradient coatings. Recent investigation exemplified that reactive elements (or their oxides( as well as nano-crystallization can increase high-temperature corrosion and oxidation resistance of MCrAlX coatings. This enhancement may be associated with the structure refinement, grain boundary purification, and adhesion improvement of the Al 2 O 3 oxide scale on the coating surface. However, there are conflicting opinions regarding the effect of reactive elements on the oxidation behavior of the MCrAlY coatings. The controversial findings in the literature indicated that the dominant mechanism of the enhancement of oxidation resistance for the reactive element or rare earth element modified MCrAlY coatings is still unclear and complicated. Therefore, a basic conception of the mechanism of the oxide scale growth in MCrAlY coating under short- and long-term oxidation is still far from obvious. The present work is a review with critical viewpoints in the case of the improvement of MCrAlY coatings and investigation of their structural characteristics and oxidation behavior, focused on different types of modification techniques. • Different modification techniques can be used to improve the structural characteristics of the MCrAlX coatings. • Reactive element modified MCrAlX coatings have higher oxidation resistance than conventional coatings. • The enhancement of oxidation properties may be attributed to the higher oxide scale adhesion in modified MCrAlX coating.

Review: Mechanism of Reactive Element Effect—Oxide Pegging

Review: Mechanism of Reactive Element Effect—Oxide Pegging

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

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

Pinning effect of reactive elements on structural stability and adhesive strength of environmental sulfur segregation on Al2O3/NiAl interface

It has been proved that the reactive elements improve the adhesion strength of the α-Al2O3/β-NiAl interface significantly, and Hf may be the best choice among the common doping elements, including Hf, Zr, Dy, Y, La for the interface without impurity. However, when impurity sulfur has to be considered, it is found that La and Y enhance interface adhesion and mechanical stability strength of α-Al2O3/β-NiAl–S interface most obviously, rather than Hf. The fundamental mechanism of reactive elements tying up S in the interface and the selection of reactive elements have been studied via a combinational analysis of bond topology and electronic structure.

Do Mass Transport Kinetics Always Control Al2O3 Scale Growth? An Alternative Explanation for the Efficacy of the Reactive-Element Effect

Do Mass Transport Kinetics Always Control Al2O3 Scale Growth? An Alternative Explanation for the Efficacy of the Reactive-Element Effect

Reactive elements dependence of elastic properties and stacking fault energies of γ-Ni, γ′-Ni3Al and β-NiAl

Abstract The reactive element effects (REEs) have been proven to improve the thermal oxidation resistance of β-NiAl. However, their effects on mechanical properties, which are significant implications for the Ni-based superalloys substrate and β-NiAl bond coat of thermal barrier coatings (TBCs), have not been well understood so far. In this work, the basic elastic properties, ideal strengths, and stacking fault energies of reactive elements (REs: Hf, Zr, Dy, Y, La) doping γ-Ni, γ′-Ni3Al, β-NiAl systems have been calculated using the first-principles method. The elastic properties results suggest that all five REs can enhance the ductilities of γ-Ni, β-NiAl, and γ′-Ni3Al, of which La performs best. More interesting, the complicated stacking fault (CSF) and anti-phase boundary (APB) energies of γ-Ni and β-NiAl decrease with the addition of REs in order of La > Y(Dy)>Zr > Hf, showing a totally opposite trend to that of commonly accepted the REEs on oxidation. The significant charge density redistributions could explain the physical origin of the improvement of plastic deformation ability for β-NiAl. This work gives insight into the mechanism of RE effects on the ductility and strength of γ-Ni, γ′-Ni3Al, β-NiAl materials, providing theoretical guidance for Ni-based superalloys design via a combinational analysis of bond topology and electronic structure.