Reactive Element Additions Research Articles

High-temperature oxidation of AlCrFeNi-(Mn or Co) high-entropy alloys: Effect of atmosphere and reactive element addition

The effects of atmosphere (oxygen and steam) and reactive element addition (Zr and Y) on high-temperature oxidation behavior of Al0.5CrFeNiMn and Al0.5CrFeNiCo high-entropy alloys at 1000 °C and 1200 °C were studied. The Al0.5CrFeNiMn alloy displayed different oxidation mechanisms in oxygen and steam owing to differing oxygen partial pressures and preferential oxidation of Mn. A protective alumina scale established on Al0.5CrFeNiCo alloy in both atmospheres and the beneficial effects of Y addition are seen by remarkably improved oxide scale adherence and suppression of convoluted oxide scale morphology. However, doping with 1 wt% Zr deteriorates the oxidation resistance with substantial Al+Zr internal oxidation.

A study on the oxidation and electrical behavior of crofer 22 APU solid oxide fuel cell interconnects with Ni-Co-CeO2 composite coating

A protective/conductive coating can be used to limit the formation of chromia in ferritic steel interconnects in solid oxide fuel cells (SOFCs). One of the most promising candidates for restricting chromia formation is the addition of reactive elements oxides to the coating. Here in, Ni-Co-CeO2 composite coating was applied to Crofer 22 APU alloy via the electroless method. The oxidation and electrical behavior of uncoated and coated samples were evaluated at 800 °C. The coating structure was examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Results showed Cr2O3-scale thicknesses, and a reduction in oxidation rates for Ni-Co-CeO2-coated samples compared to uncoated samples. Also, area specific resistance (ASR) results indicated that the formation of NiFe2O4, (Mn,Ni)2O4, Co3O4, and CoFe2O4 compositions and the presence of CeO2 reduced ASR. The ASR values of Ni-Co-CeO2 coated and uncoated Crofer 22 APU alloy after 300 h of isothermal oxidation at 800 °C was 11.8 mΩ cm2 and 35.7 mΩ cm2, respectively.

The Inhibiting Effect of Reactive Element Oxides on the Pack Cementation Aluminide Coating Formation

Minor additions of reactive elements (Y, Ce, La, Zr, Hf, etc.) significantly affect the high-temperature oxidation behavior of protective oxide scales. Designing oxidation-resistant aluminide coatings having optimum content of reactive elements is challenging due to the observed hindering effect of these elements on the coating formation. In the current investigation, this effect has been studied by examination of a high activity aluminide coating in powder mixtures of Al, NH4Cl, Y2O3, and Al2O3 with different Y2O3 contents. Oxides of Zr, Ce, and La were also examined for comparison. Cross-sectional microstructure of coatings and weight changes of the specimens were studied at different aluminizing times. The results showed that the inhibition effect applied for all reactive elements under investigation with different strengths and intensifies with more additions of Y2O3 up to a saturation limit. The observed inhibition effect was explained based on the equilibrium amounts of gaseous halides responsible for transfer of coating elements. It was demonstrated that the addition of Y2O3 can decrease the amount of aluminum carrier halide significantly. The mechanism of inhibition was attributed to the large agglomerates rich in aluminum and reactive elements detected in the pack after aluminizing treatment.

The effect of Zr on precipitation in oxide dispersion strengthened FeCrAl alloys

Oxide dispersion strengthened (ODS) FeCrAl alloys are promising candidate materials for advanced nuclear reactor applications requiring high-temperature strength, corrosion resistance, and irradiation tolerance. As these alloys have increased in compositional complexity through attempts to use highly reactive elements such as Zr to refine particle sizes and optimize nanoprecipitate dispersion characteristics, much debate has ensued as to the effects of these alloying element additions on alloy properties. In an attempt to reconcile differences in nanoprecipitate distributions reported in the literature over the past decade, a detailed investigation of a recently developed ODS FeCrAl alloy with nominal composition Fe–10Cr-6.1Al-0.3Zr+0.3Y2O3 is presented using a combination of atom probe tomography (APT), scanning/transmission electron microscopy (S/TEM), and computational thermodynamics modeling. It is illustrated that based on the amount of Zr available in the lattice, Zr competes with Al and Cr to form carbides and nitrides as opposed to oxygen-rich precipitates. This alloy system has a high number density (>1023 m−3) of ∼2–4 nm diameter (Y,Al,O)-rich nanoprecipitates, but it is shown that due to the compositional spread and unknown partitioning of Al between the matrix and precipitates, significant challenges still exist for quantifying the exact compositions of these precipitates using APT. However, the noted compositional spread is supported by identified complex oxides yttrium aluminum monoclinic (YAM) and yttrium aluminum garnet (YAG) using S/TEM. As a result of these findings, researchers developing ODS FeCrAl with reactive element additions must pay careful attention to C and N impurities when optimizing reactive element additions.

CHAPTER 4 High Temperature Oxidation of Stainless Steels

This chapter is dedicated to the description of high temperature oxidation of both chromia and alumina forming alloys. The defect structures of iron and chromium are firstly reviewed. The effects of elements on stainless steel oxidation behaviour are further addressed. For the chromia-forming stainless steel, the oxidation rate is reduced with the increased silicon content but not in a monotonic manner. Titanium and niobium can reduce breakaway oxidation of Fe–18Cr–10Ni austenitic stainless steel. Titanium can enhance the adhesion of scale to the Fe–18Cr by mechanical keying effect of TiO2 formed at the steel/scale interface. For the alumina-forming stainless steel, the formation of alumina and its transformation during oxidation are reviewed. Chromium can be added to reduce the critical aluminium content in the steels in order to form alumina at high temperatures. The addition of reactive elements with appropriate level can improve scale adhesion and reduce the steel oxidation rate. Refractory element like molybdenum can increase strength of material but also accelerate the oxidation rate of the steels containing reactive elements. The development of new alumina-forming austenitic alloy grades is finally described.

Microstructure Analysis of the Pack Cementation Aluminide Coatings Modified by CeO2 Addition

Minor additions of reactive elements (Y, Ce, Hf, etc.) can improve the oxidation behavior of high temperature alloys and coatings such as aluminide coatings. In the current investigation cerium-modified aluminide coatings were developed via additions of CeO2 to the powder mixture of high activity pack cementation aluminizing. Nickel-based substrates were aluminized at 750 °C and then diffusion annealed at 1080 °C. Microstructure of the coatings was studied using scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The results were compared with the characterization results of specimens achieved by aluminizing an electrodeposited Ni–CeO2 interlayer at 1030 °C. The coatings modified by CeO2 addition to the pack showed a dense Al-rich NiAl surface layer with cerium distributed in the coating, however, the coatings modified by Ni–CeO2 interlayer revealed a porous Ni-rich aluminide layer containing CeO2 particles. Increasing the CeO2 content of the pack decreased the thickness of the final coating, but the presence of CeO2 particles in the interlayer increased the thickness of the final aluminide coating. The observed microstructural features were explained using thermodynamic analysis of equilibrium concentration of the involved gaseous species.

Review on Nickel Aluminide based Bond Coat Properties and Oxidation Performance for Thermal Barrier Coating (TBC) Application

Thermal Barrier Coating (TBC) as protective coatings are applied to maintain efficiency and prevent structural failures mainly in gas turbine system. This paper reviews on recent bond coating from Nickel aluminide bond coat with addition of Reactive Elements (RE). This paper also reviews the major concern in TBC with presence of different Reactive Element (RE) added in term of RE composition, properties and oxidation test performance. Recent studies are more focusing on few REs including Ce, Hf, La, Y and Zr based on oxidation property test results. The comparisons clearly show that ceramics addition are superior for bond coat mechanical and thermal properties improvement while RE addition such as Ce and Zr present excellent oxidation performance at 900°C and above.

Direct Active Soldering of PEO Coated and Uncoated AA6061 Aluminium Alloy

Active soldering is an emerging technology of joining a range of materials, which utilizes Sn-Ag composition solder alloy other than conventional Sn-Pb solder, along with reactive element additions (like Ti, Ta, Hf and Zr). In the present study, active soldering is employed for bonding AA6061 aluminium alloy using S Bond 220-50 and S Bond 220M active solders. Soldering was achieved by direct active soldering of AA6061 plates each other using S Bond 220-50 and PEO coated AA6061 aluminium alloy using S Bond 220M, taking into consideration of the application of active solder alloy in joining ceramics. Mechanical properties were evaluated using hardness and lap shear test. Microstructure analysis is carried out by SEM microscopy. The soldered area is examined using EDS giving fair idea regarding the composition of elements present in the solder. It is observed that direct bonding of aluminium alloy using S Bond 220-50 showed better bond strength and hardness when compared with joining using S Bond 220M of the PEO coated aluminium plate.

Transient porous nickel interlayers for improved silver-based Solid Oxide Fuel Cell brazes

Silver-copper oxide reactive air brazes are the most widely used Solid Oxide Fuel Cell (SOFC) brazes. However, the conventional Ag-4 wt.% Cu composition has a high wetting angle of ∼40° in air on yttria-stabilized-zirconia (YSZ) that leads to manufacturing defects (denoted as Type I pores). Also, many elements that oxidize during brazing to promote braze wetting (such as Cu) are easily reduced by SOFC fuels. This results in Type II pores that decrease the braze interfacial strength and provide a quick path for hydrogen permeation into the braze (where the hydrogen reacts with diffused oxygen to form gaseous water pockets, denoted as Type III pores). The present work demonstrates that transient porous nickel interlayers, instead of reactive element additions, can be used to promote Ag wetting on YSZ and produce high-quality YSZ-stainless steel braze joints. Mechanical tests on these reactive-element-free, silver-based SOFC braze joints, both before and after 500 h of 750 °C oxidation in air, show that the braze and braze interface strengths are higher than the underlying YSZ|NiO-YSZ substrate. The elimination of Type I and Type II porosity enabled by this new technique should result in improved braze lifetimes for SOFC and other ceramic-to-metal sealing applications.

Cerium doping of Ti-Al-N coatings for excellent thermal stability and oxidation resistance

Ti–Al–N thin films are well established due to their outstanding thermo-mechanical properties. Nevertheless, this system is still a subject of many research activities to further enhance their oxidation resistance and thermal stability. The addition of reactive elements, such as Cerium, can significantly improve especially the oxidation resistance of various materials. Therefore, we study in detail the impact of Ce (2 at.% alloyed to powder metallurgically prepared Ti0.50Al0.50 targets) on growth processes, structure, mechanical properties, thermal stability, and oxidation resistance of magnetron sputtered Ti1−x−yAlxCeyN coatings prepared with DC bias potentials of Ubias=−25, −50, −75, and −100V. The deposition rate is significantly increased by a factor of ~1.75 (Ubias=−25V) to 1.45 (Ubias=−100V) when using Ti0.49Al0.49Ce0.02 instead of Ti0.50Al0.50 targets. Furthermore, also the hardness of the resulting single phase face centered cubic Ti0.43Al0.55Ce0.02N is with ~35GPa above that of Ti0.42Al0.58N with ~34GPa, for coatings on polycrystalline Al2O3 and Ubias=−50V.All temperature dependent characteristics of Ti0.42Al0.58N are improved significantly by the addition of Cerium. Wurtzite-structured AlN formation within Ti0.43Al0.55Ce0.02N can only be detected at Ta=1100°C, about 200°C higher as for Ti0.42Al0.58N. Their peak-hardness, due to spinodal decomposition of the supersaturated cubic phase is ~37.0GPa with Ta=900°C, as compared to 34.6GPa with Ta=800°C for Ti0.42Al0.58N. Additionally, even after exposure to ambient air at 950°C for 3h, still >50% of the Ti0.43Al0.55Ce0.02N coating is intact (below the ~1.2μm thin oxide scale), whereas Ti0.42Al0.58N is already fully oxidized.Based on our results we can conclude, that Ce-doping improves the deposition characteristics and mechanical properties as well as thermal stabilities (incl. oxidation resistance) of Ti–Al–N, to be used as protective coatings for a wide range of high-demanding applications.

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.

Synthesis of clean aluminide coatings on Ni‐based superalloys via a modified pack cementation process

With the aim of seeking a modified pack cementation process as a potential low‐cost alternative to form clean and reproducible aluminide coatings for laboratory diffusion coating research, four non‐contact arrangements were employed to coat Ni‐based superalloy specimens. The advantages and disadvantages of each arrangement were discussed by comparing the cleanliness of coating surface, the homogeneity of surface Al content and coating thickness, as well as the easiness of preparation. By utilizing the arrangement with the best overall quality, an Ni‐rich β‐NiAl coating with a clean surface and a low S level (0.3 ppmw) was achieved, which was comparable to the coatings previously made by an ultra‐clean laboratory chemical vapor deposition (CVD) process. Reactive element (RE) additions such as Hf, Zr, or Y were codeposited by including a small amount of the respective RE oxide in the pack mixture, and approximately 500 ppmw Hf was detected in the Hf‐modified NiAl coating.

High Temperature Oxidation and Microstructural Evolution of Modified MCrAlY Coatings

Thermal sprayed MCrAlY coatings are widely used as a bond coat in thermal barrier systems to protect the substrate from corrosion and high temperature oxidation and to improve the compatibility between the ceramic top coat and metallic substrate. In this paper, the high temperature oxidation resistance of MCrAlY coatings with modified compositions was evaluated; in particular, the effect of the addition of reactive and refractory elements (Ta, Re, Si, and Hf) was investigated. MCrAlY coatings were obtained by high velocity oxygen fuel spray and vacuum plasma spray techniques; samples were exposed to air at 1423 K (1150 °C) and the oxidation kinetics were evaluated by measuring the thickness of the thermally grown oxide (TGO) scale at several exposure times. Experimental data confirmed that the oxidation resistance of MCrAlY coatings is strictly related to the amount of the reactive and refractory elements in the starting powders and that a thorough understanding of the microstructural modifications taking place during oxidation is essential for controlling TGO growth and thermal barriers’ durability.

Oxidation studies of Fe10CrAl–RE alloys exposed to Pb at 550 °C for 10,000 h

Five experimental FeCrAl–RE alloys have been exposed up to 10,000h in stagnant liquid Pb at 550°C. The test matrix consisted of three 10wt.% Cr alloys, with an Al content ranging from 4 to 8wt.% (10Cr–4Al, 10Cr–6Al and 10Cr–8Al), one alloy without additions of reactive elements (RE) (10Cr–6Al), and one reference alloy with 21wt.% Cr and 5wt.% Al (21Cr–5Al). The evaluation showed a clear difference in oxidation properties, and it was possible to divide the alloys into two distinct groups. A critical Al concentration in the interval of 4–6wt.% at the given RE content was required to form a thin protective oxide. However, the absence of RE addition in one of the two 10Cr–6Al alloys resulted in a significant reduction in oxidation resistance, comparable with 10Cr–4Al. None of the alloys were severely corroded, however Pb penetrated to a relatively large extent into the porous oxide of the low performing alloys. A 100nm thick oxide scale, partly consisting of alumina (Al2O3), was observed for the high performing 10Cr–6Al alloy. The Fe10CrAl–RE alloys showed overall very good corrosion resistance and are hence a promising new alloy category for liquid Pb applications.

A Study on Effect of Reactive and Rare Earth Element Additions on the Oxidation Behavior of Mo–Si–B System

Mo–9Si–8B–1Ti, Mo–9Si–8B–1.8Ti, Mo–9Si–8B–0.2La and Mo–9Si–8B–0.4La2O3 (at.%) alloys were prepared using mechanical alloying followed by hot isostatic pressing and field assisted sintering. XRD, SEM and EBSD analysis confirmed the formation of Mo solid solution, A15 and T2 phases in the alloys. Isothermal oxidation behavior of the specimens was studied in the temperature range from 750 to 1,300 °C for up to 100 h. Both the Ti and La containing alloys showed superior oxidation behavior compared to unalloyed Mo–Si–B at 900 °C at the initial periods of oxidation. Ti-added alloys suffered higher rate of weight loss at higher temperatures (1,000–1,300 °C) due to the formation of non-protective low viscosity SiO2-TiO2-B2O3 scale. La-alloyed Mo–Si–B showed superior oxidation resistance at intermediate temperatures (900 °C) as well as at higher temperatures. Enrichment of La at the oxide/alloy interface was found to be the reason for improved oxidation behavior of La-alloyed Mo–Si–B. Amongst the four materials studied, the La2O3 containing alloy showed the best oxidation resistance at 900 °C.

Effects of reactive elements on the structure and diffusivity of liquid chromia: Anab initiomolecular dynamics study

Effects of minor addition of reactive elements (Ce, Hf, La, Y, and Zr) on the structural and dynamical properties of liquid chromia were investigated by the ab initio molecular dynamics approach. The calculation results show that minor doping of reactive elements has a significant effect on the diffusivity of chromium and oxygen ions. It is observed that Hf and La are the most efficient elements in retarding the diffusion of chromium and oxygen, Ce and Y are secondarily efficient, while Zr is the least efficient. The relative bonding strength between doping elements and oxygen was found to be in descending order of Hf-O > Zr-O > (Ce,Y)-O > La-O, which could be estimated from the major peak intensities of pair correlation functions or the values of mean residence time. Our calculation results are in satisfactory agreement with those obtained from experimental measurement and thermodynamic database. The mechanism regarding the effects of reactive elements on diffusivity was further analyzed in view of possible factors: ionic size, bonding strength, and ionic weight. It turns out that no sole factor could be responsible for the relative efficiency of reactive elements, and a novel evaluation parameter combining these factors was proposed.

The Future of Alumina-Forming Alloys: Challenges and Applications for Power Generation

Alumina-forming alloys have been studied for over 50 years and are now needed for high efficiency power generation applications operating at higher temperatures. Especially in the presence of water vapor, alumina-forming alloys outperform conventional chromia-forming alloys above 1000°C. However, alloy mechanical behavior is a significant issue and alumina-forming alloy development has been limited. The opportunity for alloy development is discussed as well as thefactors that limit oxidation resistance, including alloy thermal expansion and optimizing reactive element additions. Finally, lifetime modeling is discussed for thick section components together with the need to address performance in more complex environments.

Synthesis of Hf-modified aluminide coatings on Ni-base superalloys

With small additions of Reactive Elements (RE), such as hafnium (Hf), to aluminide coatings applied on aircraft turbine engine components, the adherence of the protective oxide scale to the coatings and the oxidation resistance of the coatings at high temperatures can be significantly improved. At SIFCO, Hf-modified aluminide coatings, Hf–Pt-modified aluminide coatings on Ni-base superalloys and the industrial-scale production of Hf-modified aluminide coatings on different aircraft turbine engine components were successfully developed. Surface morphology and cross-section microstructure of the developed coatings were inspected and compared by using a Scanning Electron Microscope (SEM) equipped with energy dispersive spectroscopy (EDS), and Glow-Discharge Mass Spectrometry (GDMS). Experimental results showed that Hf was successfully incorporated and precipitated Hf-rich particles were observed on the coating surface, along grain boundaries within the additive layer, and at the interface between the additive layer and the interdiffusion layer. Finally, Hf-rich particles as well as Hf solubility are discussed.

Microstructural Observations on Oxidized Ni-Based Superalloy TMS-75 and PGM-Doped Derivatives

The isothermal oxidation behavior of experimental third and fourth generation Ni-based superalloys was studied at temperatures between 900 and . It was found that the lower Cr concentration in third and fourth generation alloys compared to second generation alloys negatively affects the ability of the alloy to form and maintain a protective alumina scale especially at lower temperatures . An extensive internal reaction zone comprised of oxide and nitride phases is observed underneath a nonprotective external oxide scale. At higher temperatures, a continuous alumina scale forms underneath (Ni,Co)O and spinel layers. Due to the low level of reactive element (RE) additions in these experimental alloys, scale adhesion is poor. An increase of Cr content is necessary to ensure formation of a protective alumina scale at lower temperatures. Addition of Y and/or Zr is recommended for improved oxide adhesion, resulting in better overall oxidation performance.

A finite element analysis of the reactive element effect in oxidation of chromia-forming alloys

In many situations, failures such as spallation or cracking occur when chromia-forming alloys are subjected to high-temperature thermal treatment. On the other hand, a small amount of reactive element addition can remarkably increase the adherence between chromia scales and alloys. A two-dimensional finite element model has been developed to analyze the effects of reactive elements on the selective oxidation of chromia scale-forming binary alloys. The quantitative relation between the diffusivities of chromium ions and the activities of reactive element (yttrium) has been derived for the first time and has been incorporated with a continuum thermodynamic model accounting for stress–diffusion interaction in the oxidation of Cr–Fe alloys. The model predicts that the diffusivities of Cr ions are affected by the activities of reactive element compounds, which eventually leads to a decrease of the interfacial spallation driving force, as well as an increase of the interfacial adherence.