Reactive Element Effect Research Articles

Y/Hf-doped AlCoCrFeNi high-entropy alloy with ultra oxidation and spallation resistance

In this study, we report a type of ultra oxidation and spallation resistant Y/Hf-doped AlCoCrFeNi high-entropy alloy (HEA). The alloy exhibits a significantly lower oxidation rate at 1100 °C with a kp value of about 1.9 × 10−13 g2 cm-4s-1, which is one magnitude lower than those of the conventional NiCoCrAlY alloys. Additionally, the alloy shows a strong resistance to the oxide scale spallation. The superior oxidation performance for the Y/Hf-doped HEA can be attributed to the fast establishment of exclusive α-Al2O3 scale, the beneficial reactive element effect and the large columnar grain size (width) in the oxide scale.

Low temperature formation of aluminide coatings on the electrodeposited nanocrystalline Ni and its oxidation resistance with La2O3/CeO2 nanoparticle dispersion

A systematic study was made to determine the effects of aluminizing temperature and time on the growth of the Ni2Al3 coating on the electrodeposited nanocrystalline (NC) Ni with and without La2O3/CeO2 nanoparticles by the pack aluminizing process at 550 °C/600 °C. The results show that the growth of Ni2Al3 coating obeyed the parabolic law after a delay time (t0), which decreased with increment of aluminizing temperature. The addition of La2O3/CeO2 nanoparticles significantly refined the grain of aluminide. The La2O3/CeO2-dispersed Ni2Al3 coatings exhibit a lower oxidation rate than the La2O3/CeO2-free one in air at 900 °C, because of various effects of La2O3/CeO2 on the oxidation including the grain refinement effect of the aluminide, the “reactive element effect” (REE) and the epitaxial template effect of La2O3 to promote the growth of α-Al2O3. Furthermore, the accumulation of La2O3 particles at the degradation front slightly inhibited the interdiffusion between the aluminide and Ni; while the formation of a CeO2-rich layer between aluminide and Ni–CeO2 film significantly mitigated the degradation of aluminide coatings.

Reactive Element Effect Applied by Alloying and SiHfBCN Coating on the Oxidation of Pure Chromium

To reduce scale spallation, scale volatilization, and nitrogen embrittlement in Cr-based systems, reactive elements such as Y, Zr, Hf, La, and Ce were introduced as oxide dispersions into pure chromium. In addition, Hf coating systems were investigated. One was a Hf sputter layer with varying thickness, and the other one a Hf-containing precursor ceramic, i.e., SiHfBCN, which may be considered as suitable material for environmental barrier coating applications. Oxidation tests at $$1050\,^{\circ }\hbox {C}$$ in synthetic air for 50 h were carried out using thermogravimetric analysis. The samples were analyzed via X-ray diffraction, optical microscope, electron microprobe analysis, and scanning electron microscope. All reactive elements led to a decrease in total mass gain after oxidation compared to pure Cr, with Y and Zr showing the strongest effect. Improvements in oxide attachment, oxide growth rate, volatilization rate as well as nitridation resistance were observed. Concerning these experiments, Y showed the most promising results. Concerning Hf, coating systems, especially SiHfBCN, showed a higher effect on improving the oxidation resistance. The reason for this outcome might be that not only Hf is active in the precursor ceramic layer. A more complex oxide layer has formed, which consisted of not only Cr2O3 but also of Hf and Si oxides. This layer prevents the material from any nitridation under the selected oxidation conditions.

Characteristics and high temperature oxidation behavior of Ni-Cr-Y2O3 nanocomposite coating prepared by cathode plasma electrolytic deposition

Ni-Cr-Y2O3 nanocomposite coating, with Y2O3 nano-particles finely dispersed, was prepared by cathode plasma electrolytic deposition from environmental-friendly nickel-trivalent chromium sulfate electrolyte directly. The coating is dense and uniform with typically molten morphology, possessing a nanocrystalline structure with an average grain size of about 30–50 nm. Owing to the synergy effect of the fine-grained structure of the coating and the reactive-element effect of the addition of Y2O3 nano-particles, a thin, compact, uniform and continuous Cr2O3 oxide scale can be formed during 200 h oxidation at 750 °C. Such Cr2O3 oxide scale provides effective protection to the inner alloy and substrate.

Characterization of a Cr2O3 scale formed on chromium with implanted yttrium ions by advanced electron microscopy

Although the advantageous effect of reactive elements on the oxidation resistance of alloys that form protective Cr2O3 scales was discovered 80 years ago, the underlying mechanism has still not been fully explained. One of its aspects, which was investigated in depth in the presented paper, is the structure of grain boundaries in the oxidation product. Atomic-resolution scanning transmission electron microscopy ((S)TEM) was used to examine the micro- and nanostructures of Cr2O3 formed on chromium implanted with yttrium ions. Oxidation was performed in air at either 1073 K or 1273 K. A FEI (S)TEM Titan Cubed G2 60–300 equipped with a ChemiSTEM was used to examine scale's nanostructure at a resolution down to 70 picometres. Lamellae were cut using a ZEISS NEON 40 EsBCrossBeam, a scanning electron microscope with focused ion beam (FIB). Phase composition was identified via selected area diffraction (SAED), and the chemical composition was determined via energy-dispersive X-ray spectroscopy (STEM-EDS). For both oxidation temperatures, chromium was found in the interior of the Cr2O3 scale in the form of the YCrO3 perovskite. The STEM-HAADF atomic-resolution microscopic image was found to be fully consistent with a computer-simulated crystal structure of the perovskite structure obtained by means of the JEMS software. If the layer containing yttrium is treated as a marker, then it may be assumed that the thinner outer layer grew due to the outward diffusion of metal via point defects in the cation sublattice, while the thicker inner layer formed as a result of the inward diffusion of oxygen via point defects in the anion sublattice. For the higher oxidation temperature, yttrium enrichment was also observed in the grain boundary area in chromia. However, STEM observations did not reveal the presence of an intermediate layer with a perovskite structure between Cr2O3 grains. Sulfur segregation at the chromia/metal interface was observed.

Effect of reactive alloy elements on friction stir welded butt joints of metallurgically immiscible magnesium alloys and steel

Magnesium alloys (AZ31 and WE43) and steel (316 stainless steel, SS316) were butt welded through friction stir welding. Even though Mg and Fe are metallurgically immiscible, intermetallic compounds (IMCs) formed at the welded interface due to the existence of alloying elements in both magnesium alloys and steel. The formation of IMCs among alloying elements and the matrix were verified by thermodynamic and kinetic calculations and microstructural characterization. The elastic modulus mismatch and resultant stress concentration during tensile testing lead to interfacial fracture and the very existence of IMCs impaired dissimilar joint strength due to the brittleness of the IMCs.

Influence of composition and heating schedules on compatibility of FeCrAl alloys with high-temperature steam

FeCrAl alloys are proposed and being intensively investigated as alternative accident tolerant fuel (ATF) cladding for nuclear fission application. Herein, the influence of major alloy elements (Cr and Al), reactive element effect and heating schedules on the oxidation behavior of FeCrAl alloys in steam up to 1500 °C was examined. In case of transient ramp tests, catastrophic oxidation, i.e. rapid and complete consumption of the alloy, occurred during temperature ramp up to above 1200 °C for specific alloys. The maximum compatible temperature of FeCrAl alloys in steam increases with raising Cr and Al content, decreasing heating rates during ramp period and doping of yttrium. Isothermal oxidation resulted in catastrophic oxidation at 1400 °C for all examined alloys. However, formation of a protective alumina scale at 1500 °C was ascertained despite partial melting. The occurrence of catastrophic oxidation seems to be controlled by dynamic competitive mechanisms between mass transfer of Al from the substrate and transport of oxidizing gas through the scale both toward the metal/oxide scale interface.

High-temperature oxidation of CrAlYN coatings: Implications of the presence of Y and type of steel

Nanolayered CrAlN and CrAlYN/CrAlN (average contents of Al ≈ 25 at.% and Y ≈ 1.6 at. %) coatings are deposited on M2 and 316 steel substrates and heated to 1000 °C in air for 2 h to study their oxidation mechanism, the thermal stability and the reactive element (RE) effect of yttrium. CrAlN on M2 develops a Cr2O3/Al2O3 passivation layer that preserves in high degree the fcc-CrAlN structure however iron ions leave the substrate and travel to the surface along the column boundaries. The CrAlYN/CrAlN coatings deposited on steels are not stable at 1000 °C, and the initial fcc-CrAlN phase is partially transformed to hcp-Al(O)N and Cr-Fe phases (M2) and Cr2N and Al2O3 (316). The addition of Y changes the predominant scale growth direction. Inward oxygen diffusion becomes dominant but a reduction of the oxide scale thickness as compared to CrAlN is not observed. The advanced microstructural analysis made by transmission electron microscopy combined with electron energy-loss spectroscopy determined that yttrium migrates mainly to the oxide scale (forming mixed oxides with substrate elements - V and Mo, either as dispersed particles or segregated at the grain boundaries) in M2, and to the oxide interface and column boundaries (forming Al-Y oxides and YN, respectively) in 316 steel. The benefits of addition of Y in improving the oxidation resistance are discussed comparatively with literature data. The RE effect of yttrium is thus observed to be dependent on the substrate, film architecture and composition.

High-temperature oxidation behaviour of minor Hf-doped β-NiAl single crystals in dry and humid atmospheres

Oxidation behaviour of undoped β-NiAl and β-NiAl-0.05Hf single crystals were investigated at 1100 °C in 20 vol% O2 + Ar and 15 vol% H2O + Ar, respectively. On contrary to the well-known beneficial effect of Hf additions on the oxidation behaviour of polycrystalline β-NiAl, severe-scale spallation is observed for β-NiAl-0.05Hf single-crystal alloy, especially in dry atmosphere. It is proposed that the presence of the grain boundaries in the polycrystalline substrate enables proper content of Hf segregating at the grain boundaries of the oxide scale and at the scale–substrate interface, which promotes the conditions required for reactive element effect. Once the reactive element effect of Hf does not work, its strong affinity with oxygen will accelerate the process of high-temperature oxidation. However, the absence of the grain boundaries as fast diffusion paths partially compromises the adverse influence of water vapour on cyclic oxidation resistance of β-NiAl single crystal, resulting in considerable suppression of the oxidation rate and less loss of protection compared with that in dry atmosphere. In addition, interfacial cavities are observed in dry and humid atmospheres, which may eventually result in the spallation of the oxide scale.

Interfacial phenomena and evolution of modified aluminide bondcoatings in Thermal Barrier Coatings

The paper presents results of microstructural investigations of TBCs on Pt- and PdPt-aluminide bondcoatings with focus on the interfacial phenomena that take place during the pre-oxidation treatment as well as further thermal cycling at 1100 °C. The Pt and Pd coatings were produced using a Physical Vapor Deposition (PVD) method while aluminizing was performed using a high activity Vapor Phase Aluminizing (VPA). The ceramic yttria stabilized zirconia (YSZ) top coating was deposited using Electron Beam Physical Vapor Deposition (EB-PVD) method. Prior to the EB-PVD process the bondcoatings were pre-oxidized at 1140 °C for 2 h in air atmosphere in order to form a stable and adherent α-Al2O3 TGO with minimum transient oxidation. Special effort has been done in order to investigate the microstructure of the Thermally Grown Oxide (TGO) formed during pre-oxidation treatment prior to YSZ deposition and the phenomena occurring at the interface between the YSZ, TGO, and the bondcoatings. Microstructure evolution of the TGOs is described and related to the conditions of the pre-oxidation treatment and the chemistry of the bondcoatings. During thermal cycling at 1100 °C the TGO grows by inward oxygen diffusion forming distinctive columnar grains which grow deeper at triple grain junctions with the bondcoating. Therefore the TGO/bondcoating interface evolves from flat to a convoluted during high temperature oxidation. Several segregation phenomena can be found in the TBCs that include i.a. the Reactive Element Effect involving the presence of Hf and Y on α-alumina grain boundary disconnections in the TGO. It was found that compared to bare René N5 superalloy the Hf segregation is hindered in the presence of modified aluminide bondcoatings.

The reactive element effect of yttrium and yttrium silicon on high temperature oxidation of NiCrAl coating

The microstructure formed on the bond coat affects the oxidation resistance, particularly the formation of a protective oxide layer. The adhesion of bond coat and TGO increased significantly by addition of reactive element. In the present work, the effect of yttrium and yttrium silicon as reactive element (RE) on NiCrAl coating was investigated. The NiCrAl (without RE) and NiCrAlX (X:Y or YSi) bond coating were deposited on Hastelloy C-276 substrate by High Velocity Oxygen Fuel (HVOF) method. Isothermal oxidation was carried out at 1000 °C for 100 hours. The results showed that the addition of RE could prevent the breakaway oxidation. Therefore, the coating with reactive element were more protective against high temperature oxidation. Furthermore, the oxidation rate of NiCrAlY coating was lower than NiCrAlYSi coating with the total mass change was ±2.394 mg/cm2 after 100 hours of oxidation. The thickness of oxide scale was approximately 1.18 μm consisting of duplex oxide scale of spinel NiCr2O4 in outer scale and protective α-Al2O3 in inner scale.

High temperature isothermal oxidation behaviour of an oxide dispersion strengthened derivative of IN625

Gas atomised IN625 powder was mechanically alloyed with <1·0 Wt.% nano-yttria and consolidated by spark plasma sintering (SPS) to produce an oxide dispersion strengthened (ODS) alloy. The isothermal oxidation rate constant of the ODS alloy, and wrought IN625, was determined by thermogravimetric analysis. This was performed at 900 °C in static laboratory air for exposure times of up to 1000 h. It was found that the ODS alloy oxidised ~40x slower than wrought IN625, which is attributed to the reactive element effect. It is further proposed that the improvement in oxidation resistance of the ODS alloy, and the superior morphology of the oxide scale formed on the ODS alloy, may be related to the presence of Nb carbide, rather than δ-phase, in the ODS alloy.

Ceria coating for controlling the isothermal oxidation behaviour of Ni-based alloy 625

A ceria coating is electro-deposited on a chromium oxide forming Ni-based alloy to investigate the mechanisms of the reactive element effects. It is demonstrated that the ceria coating not only improves the oxidation resistance but also enhances the adherence between the oxide layer and the substrate alloy. The formation of iron oxide results in porous and cracked oxide layers on the bare sample. Oxide spallation starts to occur on the bare sample after exposure at 900 °C for 500 h. In contrast, a mixed iron-free oxide layer forms on the coated sample which is believed to enhance the oxidation resistance of the substrate. The change in oxidation mechanism of the coated sample from outward diffusion to inward diffusion is attributed to grain-boundary segregation of ceria particles. Furthermore, the formation of fine oxide grains on the coated sample improves the adhesion between the oxide layer and the matrix alloy. Schematic models of the oxidation mechanism are developed to interpret the oxidation behaviour of the bare and coated samples.

Effect of CeO2 Coating on the Isothermal Oxidation Behaviour of Ni-Based Alloy 230

Ceria coating was deposited on alloy 230 using an electro-deposition process in a cerium nitrate electrolyte. Reactive element effects were investigated by comparing the oxidation behaviour of the samples with and without the ceria coating. The prepared ceria coating reduced the oxidation rate and improved the adherence of the oxide layer. A thicker oxide layer with large spallation areas formed on the uncoated sample, while a thin and protective oxide layer was found on the coated sample after oxidation for 1000 h at 900 °C. The oxidation mechanisms of alloy 230 with and without ceria coating were discussed. Furthermore, as the ceria coating changed the grain shapes of chromium oxides from columnar to an equiaxed structure, discussion also advanced proposals regarding the mechanisms of formation of these different oxides. The equiaxed grains enhanced the adhesion of the oxide to the alloy surface.

Effects of Y2O3/Y on Si-B co-deposition coating prepared through HAPC method on pure molybdenum

In order to clarify the effects of reactive element Y on the properties of Si-B co-deposition coating on Mo substrate, the Si-B-Y2O3 and Si-B-Y co-deposition coatings were prepared at 1300 °C for 5 h by using the pack mixtures 16Si-4B-xY2O3/Y-4NaF-(76-x)Al2O3 (wt.%, x=0, 0.5, 1, 2, 4, 8). X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and wavelength dispersive spectroscopy (WDS) techniques were used to analyze the structure and oxidation behavior of these coatings. The results revealed that the Si-B-Y2O3 and Si-B-Y co-deposition coatings had the same structure with that of the Si-B co-deposition coating. However, Y was incorporated into these coatings and the thicknesses of these coatings were thicker than that of the Si-B co-deposition coating. In addition, the Si-B-Y co-deposition coating demonstrated better cyclic oxidation resistance than the Si-B co-deposition coating at 1100 °C. The modifying mechanism of Y on the Si-B co-deposition coating was discussed.

Challenges in Developing Oxidation-Resistant Chromium-Based Alloys for Applications Above 900°C

Chromium-based alloys are potential candidates for high-temperature structural applications. This article reviews the challenges of chromium and Cr-alloys used at temperatures higher than 900°C with the focus on their oxidation behavior. First, latest findings on the key environmental factors affecting the oxidation resistance such as volatilization and the impact of nitrogen in air are summarized. Oxidation resistance is addressed with regards to the effects of major alloying elements and reactive elements as well as its correlation with microstructure in multi-phase alloys. Secondly, the existing challenges to develop chromium alloys with enhanced high-temperature oxidation resistance are discussed. It is shown that volatilization and nitridation, the two major obstacles for the use of chromium alloys in air, can be significantly improved by alloy design.

The reactive element effect of ceria particle dispersion on alumina growth: A model based on microstructural observations.

The oxidation kinetics of alumina-forming metals can be affected by adding a small amount of a reactive (normally rare earth) element oxide (RExOy) and the segregation of the reactive element (RE) ions to the growing alumina grain boundaries (GBs) has been considered as a responsible reason. However, this interpretation remains a controversial issue as to how RE ions are produced by RExOy which is thermodynamically and chemically stable in metals. The question is answered by a model that is based on transmission electron microscopy (TEM) investigation of a CeO2-dispersed nickel aluminide oxidized in air at 1100 °C. The CeO2 dispersion is incorporated into the alumina scale by the inward growth of inner α-Al2O3, where it partially dissolves producing tetravalent Ce cations which then transform to trivalent cations by trapping electrons. The trivalent cations segregate to the α-Al2O3 GBs and diffuse outward along first the GBs and later the twin boundaries (TBs) in the outer γ-Al2O3 layer, being precipitated as Ce2O3 particles near surface.

Current Thoughts on Reactive Element Effects in Alumina-Forming Systems: In Memory of John Stringer

In memory of John Stringer (1934–2014), one of the leaders in studying the reactive element (RE) effects, this paper reviews the current status of understanding of the effect of RE dopants on high-temperature oxidation behavior, with an emphasis on recent research related to deploying alumina-forming alloys and coatings with optimal performance in commercial systems. In addition to the well-known interaction between indigenous sulfur and RE additions, effects have been observed with C, N, and O found in commercial alloys and coatings. While there are many similarities between alumina-forming alloys and coatings, the latter bring additional complicating factors such as the effects of O incorporation during thermal spraying MCrAlY coatings, coating roughness, and heat treatments that must be considered in optimizing the beneficial dopant addition. Analogies can be seen between RE effects in alloys and in the substrates beneath diffusion M–Al coatings. Recently, there has been more interest in the influence of mixed oxidant environments, since these may modify the manifestation of the RE effect. Finally, some thoughts are provided on optimizing the RE benefit and modeling oxidation of RE-doped alloys.

The Band Structure of Polycrystalline Al2O3 and Its Influence on Transport Phenomena

The electronic (band) structure of polycrystalline Al2O3, in particular the density of near‐band edge grain‐boundary localized states, plays a significant role in a host of high‐temperature phenomena, including sintering, high‐temperature creep, oxygen permeability in dense “dry” Al2O3 ceramics, and Al2O3 scale formation on Al2O3 scale‐forming alloys. All these phenomena involve creation or annihilation of charged point defects (vacancies and/or interstitials) at grain boundaries and interfaces, and must of necessity involve electrons and holes. Thus, the density of states associated with grain boundaries in Al2O3 assume great importance, and has been calculated using DFT for both nominally undoped and Y‐doped Σ7 bi‐crystal boundaries. These quantum mechanical calculations must be taken into account when considering why Y2O3 segregation to Al2O3 grain boundaries is so effective in enhancing high‐temperature creep resistance of polycrystalline Al2O3, and in understanding the reactive element effect in Al2O3 scale‐forming alloys. Finally, a case will be made that grain‐boundary diffusion is mediated by the migration of a class of grain‐boundary ledge defects called disconnections, which are characterized by a step height h and a Burgers vector b.

Pinning effect of reactive elements on adhesion energy and adhesive strength of incoherent Al2O3/NiAl interface.

The profound effects of reactive elements (REs) on the adhesion energy and adhesive strength of the α-Al2O3/β-NiAl interface in thermal barrier coating (TBC) systems have attracted increasing attention because RE-doping has played a significant role in improving the thermal cycling lifetime of TBCs. However, the fundamental mechanism is, so far, not well understood due to the experimental difficulty and theoretical complexity in interface modelling. For this purpose, in the present study we have performed comprehensive density functional theory calculations and information targeted experiments to underline the origin of the surprising enhancement of interface adhesion, stability and mechanical strength of the α-Al2O3/β-NiAl interface by different RE doping levels. Our results suggest that the interface failure firstly appears within the NiAl layer adjacent to the Al-terminated oxide under mechanical loading, while the formation of O-RE-Ni bond pairs at the interface can effectively hinder the interface de-cohesion, providing a higher mechanical strength. By comparing several typical REs, it is observed that Hf can emerge not only with the highest interface adhesion energy, but also the highest mechanical strength; in agreement with our experimental results. By continuously increasing the dopant concentration, the strengthening effect may increase correspondingly, but is limited by the solute solubility. These results shed light into the effect of REs on the stability and strength of the α-Al2O3/β-NiAl interface, providing theoretical guidance for interface design via a combinational analysis of bond topology and electronic structure.