High-temperature oxidation behavior of Mo–Si–B-based and Co–Re–Cr-based alloys

Improving the efficiency of aerospace gas turbine engines requires materials that can be used at increasingly higher temperatures in aggressive environments. This paper summarizes the current stage of alloy development of Mo–Si–B-based and Co–Re–Cr-based alloys regarding the high-temperature oxidation resistance. Since refractory metals, such as Mo and Re, suffer from catastrophic oxidation, the main task of research is to find alloying elements that improve the oxidation behavior of these alloys. For Mo–Si–B-based alloys, it was observed that an addition of Zr has a significant positive influence on the oxidation resistance by reducing the time necessary for the formation of a protective borosilicate layer. An addition of 0.2 at.% Y improves the viscous properties of the borosilicate increasing the protectiveness of the oxide scale. Macroalloying with Ti yields a strong positive effect on the oxidation behavior and, in addition, notably reduces the density of Mo–Si–B-based alloys. In Co–Re–Cr-based alloys, Cr is included to achieve favorable mechanical properties and to form a protective chromia layer during oxidation. As a consequence of the synergetic effect of Cr and Si, an addition of 2 at.% Si significantly improves the oxidation behavior of the alloy. Al addition further promotes the formation of the protective chromia layer at intermediate temperatures and exhibits the potential of the formation of a protective alumina scale suitable for applications at very high temperatures. The critical evaluation of the complex oxidation behavior of both metallic systems in a broad temperature range gives insight into the underlying fundamental mechanisms, reveals the potentials of particular alloying elements and, thus, guides future development of these material classes.     

Effect of Zr Addition on the High-Temperature Oxidation Behaviour of Mo–Si–B Alloys

Mo–Si–B alloys are promising candidates for structural high-temperature applications due to their excellent high-temperature mechanical properties along with high melting temperatures and oxidation resistance. After an initial period with high weight loss rates as a consequence of the volatilization of Mo-oxide, a protective borosilica (glass) layer develops on the alloy surface and steady-state oxidation is achieved. Aiming at improved mechanical properties of Mo–Si–B alloys which exhibit a continuous Mo solid solution matrix as a consequence of a powder metallurgical production route, small amounts of Zr were added. The presence of oxygen in the alloy leads to the formation of thermodynamically very stable Zr-oxide precipitates in the bulk alloy causing an enhancement of its mechanical properties. It was observed that the addition of Zr (distributed in the alloy matrix) also has significant influence on the oxidation behaviour of Mo–Si–B alloys by reducing the period for the formation of the protective and stable silica scale. Furthermore, the weight loss due to vaporization of Mo-oxides is consequently reduced. Besides this beneficial effect, Zr is harmful for the oxidation resistance at temperatures beyond 1,200 °C. This is mainly due to the increased oxygen transport through defects in the silica scale.     

Lifetime of Thermal Barrier Coatings Deposited on γ-TiAl Based Alloys Using Intermetallic Ti–Al–Cr Bond Coats with Additions of Yttrium and Zirconium

Thermal barrier coatings (TBC) of yttria stabilized zirconia were deposited on the γ-TiAl based alloy Ti–45Al–8Nb (at.%) using electron-beam physical vapor deposition. The bond coats used were 10 μm thick intermetallic Ti–Al–Cr layers with additions of the reactive elements Y and Zr produced by magnetron sputtering. Cyclic oxidation tests at 900 and 1,000 °C in air revealed excellent oxidation resistance of the Ti–Al–Cr–Y bond coat associated with the precipitation of Y-rich particles in the thermally grown alumina scale as well as in the intermetallic layer. A less protective behavior was found with the zirconium containing bond coat. Lifetimes exceeding 1,000 1-h cycles were determined for both TBC systems at 900 °C. Edge chipping of the zirconia topcoat occurred at 1,000 °C. As observed by cross-sectional examination, a continuous alumina scale was still present on the samples with Ti–Al–Cr–Y bond coat, whereas the Ti–Al–Cr–Zr layer was severely degraded and a thick mixed oxide scale formed after 1,000 cycles at 1,000 °C.     

High-Temperature Oxidation of Fe3Al and Fe3Al–Zr Intermetallics

The oxidation behavior of Fe₃Al and Fe₃Al–Zr intermetallic compounds was tested in synthetic air in the temperature range 900–1200 °C. The addition of Zr showed a significant effect on the high-temperature oxidation behavior. The total weight gain after 100 h oxidation of Fe₃Al at 1200 °C was around three times more than that for Fe₃Al–Zr materials. Zr-containing intermetallics exhibited abnormal kinetics between 900 and 1100 °C, due to the presence and transformation of transient alumina into stable α-Al₂O₃. Zr-doped Fe₃Al oxidation behavior under cyclic tests at 1100 °C was improved by delaying the breakaway oxidation to 80 cycles, in comparison to 5 cycles on the undoped Fe₃Al alloys. The oxidation improvements could be related to the segregation of Zr at alumina grain boundaries and to the presence of Zr oxide second-phase particles at the metal–oxide interface and in the external part of the alumina scale. The change of oxidation mechanisms, observed using oxygen–isotope experiments followed by secondary-ion mass spectrometry, was ascribed to Zr segregation at alumina grain boundaries.     

Bildung von Aluminiumoxid-Deckschichten auf Eisenbasislegierungen

Formation of alumina layers on iron-base alloys The formation of Al₂O₃-layers has been studied for ferritic alloys Fe-6 Al-M and austenitic alloys Fe-27 Ni-4 Al-M where M – Ti, Zr, V, Nb, W, B, Si… (concentrations in wt%). One or more alloying elements M had been added and in some cases carbon. The oxidation was performed at 1000 °C in H₂O-H₂ mixtures at PO₂ = 10^?19 bar. After ½ h oxidation the oxide layers were investigated by X-ray structures analysis, scanning electron microscopy and Auger electron spectroscopy (AES). The alloy Fe-6 Al and most doped alloys form badly adherent layers, however, on alloys with additions of 0.1 to 1% Ti, Zr, V or Y the oxide layers are fine-grained and well-adherent. The Ti-doped ferritic alloys showed very protective layers, which is caused by the formation of a Ti(C, O)-layer beneath the α-Al₂O₃. The presence of the oxicarbide induces nucleation and improves the adherence of α-Al₂O₃, according to epitaxial relations between ferrite and oxicarbide and between oxicarbide and alumina. The favourable influence of Ti and Zr on the Al₂O₃ formation is also effective on the austenitic alloys.     

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.     

Combined Al‐ plus F‐treatment of Ti‐alloys for improved behaviour at elevated temperatures

Titanium is a widely used structural material because of its low specific weight, good mechanical properties and excellent corrosion resistance at ambient temperature. As a result of increased oxidation at elevated temperatures and environmental embrittlement the maximum operation temperature of standard Ti‐alloys is only about 600 °C. The oxidation behaviour can be improved by different methods, e.g. coatings. This leads to an improvement which is, however, often limited. The combination of Al‐enrichment in the sub surface zone, so that a TiAl‐layer is formed, plus F‐treatment gives impressively good results because a protective alumina scale is formed due to the fluorine effect. This alumina scale prevents oxygen inward diffusion which causes embrittlement and protects the material against environmental attack. The procedure is applied to alloys with a very low Al‐content or even no Al at all. In the paper results of oxidation tests of α‐Ti without any treatment, with Al‐treatment and with a combination of Al‐ + F‐treatment are presented. Aluminium was diffused into the samples by a powder pack process. Fluorine was applied by a liquid phase process. The formation of an alumina scale on treated samples was revealed by post experimental investigations. The results are discussed referring to the fluorine effect model for TiAl‐alloys.     

Oxidation of Micro-Sized Aluminium Particles: Hollow Alumina Spheres

Oxidized aluminium microparticles have recently been proposed for manufacturing new, environmentally-friendly, protective coatings on stainless-steels and Ni-base alloys. The oxidation mechanisms of spherical aluminium microparticles of an average particle size of 3.5 μm were studied. Accordingly, simultaneous differential thermal analysis–thermogravimetry tests were carried out in air at different temperatures, always above aluminium melting temperature. Scanning electron microscopy and XRD were also used for the interpretation of results. Weight gain and energy results were explained in terms of the different structural changes taking place in aluminium particles. Dehydroxylation process was identified. The transformation of amorphous alumina to γ-Al₂O₃ was numerically evaluated and the alumina phase transformation (γ-Al₂O₃→α-Al₂O₃) was also studied. The temperature ranges revealed the appearance of metastable phases (θ-Al₂O₃). Complete oxidation of particles can be obtained at 1,300 °C in <1 h, although this also takes place at lower temperatures if enough oxidation time is used. Activation energy of oxidation process at high temperature was also estimated, taking a value of 334 kJ/mol. High temperature oxidation causes the formation of hollow alumina spheres, without any aluminium left inside them.     

Oxidation Properties of a Beta-Stabilized TiAl Alloy Modified by Rare Earth Elements

This paper explores the effects of adding rare earth elements (lanthanum or erbium) on the oxidation properties of Ti–43.5Al–4Nb–1Mo–0.1B (TNM) alloy. Isothermal oxidation tests were performed under air atmosphere at 900 and 1000 °C. Mass gain was measured in several steps during the oxidation test, and the oxidized specimens were characterized by XRD and FE-SEM. The results showed that while adding 0.1 at.% rare earth elements (REEs) reduced oxidation rate of the TNM alloy, 0.2 at.% REEs addition increased the mass gain of the alloys. The oxidation curves were fitted by a power-law equation; the results showed that the oxidation kinetic curves of all alloys obeyed parabolic growth kinetics (n = 2). Meanwhile, the activation energy of oxidation was in the range of 40–50 kCal/mol, thereby suggesting that the scale growth was controlled by mass transport in the TiO₂ layer. Also, the results of the scale characterization showed that addition of REEs at low level (e.g., 0.1 at.%) could reduce diffusion rate in the scale. However, addition of the higher amounts of La or Er (e.g., 0.2 at.%) due to the lower valency (+ 3) of these elements, as compared with Ti (+ 4), could lead to the increased anion diffusion, the formation of hillocks in the scale and a rise of the oxidation rate.     

Effects of Cr and Fe Addition on Microstructure and Tensile Properties of Ti–6Al–4V Prepared by Direct Energy Deposition

The effects of Cr and Fe addition on the mechanical properties of Ti–6Al–4V alloys prepared by direct energy deposition were investigated. As the Cr and Fe concentrations were increased from 0 to 2 mass%, the tensile strength increased because of the fine-grained equiaxed prior β phase and martensite. An excellent combination of strength and ductility was obtained in these alloys. When the Cr and Fe concentrations were increased to 4 mass%, extremely fine-grained martensitic structures with poor ductility were obtained. In addition, Fe-added Ti–6Al–4V resulted in a partially melted Ti–6Al–4V powder because of the large difference between the melting temperatures of the Fe eutectic phase (Ti–33Fe) and the Ti–6Al–4V powder, which induced the formation of a thick liquid layer surrounding Ti–6Al–4V. The ductility of Fe-added Ti–6Al–4V was thus poorer than that of Cr-added Ti–6Al–4V.     

High-Temperature Oxidation Behavior of Refractory High-Entropy Alloys: Effect of Alloy Composition

The high-temperature oxidation behavior of a new family of refractory high-entropy alloys (HEAs) with compositions of W–Mo–Cr–Ti–Al, Nb–Mo–Cr–Ti–Al and Ta–Mo–Cr–Ti–Al was studied at 1000 and 1100 °C. Based on these equimolar starting compositions, the main incentive of this study was to select the most promising alloy system whose properties may then be successively improved. Despite the high amount of refractory elements, Ta–Mo–Cr–Ti–Al showed good oxidation resistance at 1000 and 1100 °C. Moderate values of mass gain and complex oxidation kinetics were observed for the W- and Nb-containing HEAs. These alloys formed inhomogeneous oxide scales possessing regions with thick and porous layers as well as areas revealing quite thin oxide scales due to the formation of discontinuous Cr- and Al-rich scales. The most promising behavior was shown by the alloy Ta–Mo–Cr–Ti–Al which followed the parabolic rate law for oxide growth due to the formation of a thin and compact Al-rich layer.     

Effect of Addition of 4 % Al on the High Temperature Oxidation and Nitridation of a 20Cr–25Ni Austenitic Stainless Steel

In high temperature applications, the alumina forming austenites (AFA) have recently gained more focus. These utilise the advantageous effect of Al on oxidation resistance, and also have good mechanical properties. Two experimental alloys [20Cr–25Ni–1Mn–0.5Si–Fe (wt.%)] were prepared. To one of the alloys 3.77 wt.% Al was added. The alloys were studied in air and air/water at 700 °C and 1,000 °C, in a sulphidising/chlorinating environment at 700 °C and in a nitriding atmosphere at 1,000 °C. The time of exposure was 100 h, except for one 1,000 h exposure in air/water. At 700 °C in air and air/water, the AFA displayed lower mass gain than the reference material. After exposure in the sulphidising-chlorinating environment, the material displayed a surface alumina layer with some spallation. In air or air/water at 1,000 °C, internal aluminium nitride and alumina formation occurred, appreciably reducing the sound metal thickness. The nitridation was enhanced in the nitriding environment.     

Sulfidation–Oxidation Behavior of FeCrAl and TiCrAl and the Third-Element Effect

Short-term sulfidation–oxidation exposures were conducted under high pS₂ and low pO₂ conditions for TiCrAl and FeCrAl alloys at 600 and 800 °C. Low mass gains and submicron Al-and Ti-rich oxide scales were formed on TiCrAl at 600 °C, while high mass gains and FeS-based scale formation were observed for FeCrAl. Based on the good behavior of TiCrAl, third-element effect additions of Cr are not inherently detrimental under sulfidation–oxidation conditions. Rather, differences in the mechanistic action of the third-element addition of Cr between FeCrAl and TiCrAl alloys and its relevance to low oxygen potential sulfidation–oxidation environments were the key factors in determining whether or not a protective alumina scale was established.     

Solid solubility extension and microstructure evolution of cast zirconium yttrium alloy

Yttrium addition can improve the oxidation resistance,mitigate hydrogen embrittlement and thus enhance the mechanical properties of the zirconium alloy.To study solid solubility extension of yttrium in zirconium alloy,the lattice parameters of a-Zr phase in Zr–Y alloy were accurately determined by X-ray diffraction(XRD).Yttrium exhibits solid solubility extension in the cast zirconium alloy which forms a metastable supersaturated solid solution with solubility limit of around 3 wt%.The effect of yttrium and thermal treatment on the microstructure of the alloys was investigated by optical microscopy(OM)and scanning electron microscope(SEM).The cast Zr–Y alloy shows a normal polycrystalline structure with dispersed a-Y particles when Y content is lower than 4 wt%,while the alloy shows a eutectic structure with dendrites formation when the Y content is higher.Yttrium exhibits a strong grain refining effect on zirconium alloy and precipitates from the metastable supersaturated Zr matrix after annealing at 700 and 900 °C.     

Enhanced Oxidation Resistance of Mo–Si–B–Ti Alloys by Pack Cementation

As new high-temperature structural materials, Mo–Si–B alloys satisfy several requirements such as oxidation and creep resistance. Recently, novel Ti-rich Mo–Si–B alloys have shown an increased creep resistance compared to Ti-free alloys. However, due to the formation of a duplex SiO₂–TiO₂ oxide layer, which allows for fast ingress of oxygen, the oxidation resistance is poor. To improve the oxidation resistance, a borosilicate-based coating was applied to a Mo–12.5Si–8.5B–27.5Ti (in at.%) alloy. After co-deposition of Si and B by pack cementation at 1000 °C in Ar, a conditioning anneal at 1400 °C is used to develop an outer borosilicate layer followed by an inner MoSi₂ and Mo₅Si₃ layer. During both isothermal and cyclic oxidation after an initial mass loss during the first hours of exposure, a steady state is reached for times up to 1000 h at temperatures ranging from 800 to 1200 °C, demonstrating a significantly enhanced oxidation resistance.     

Void formation and filling under alumina scales formed on Fe–20Cr–5Al based alloys and coatings,oxidised at temperatures up to 1200 °C

During the oxidation, in laboratory air, of thin foils of Fe–20Cr–5Al based alloys, voids were formed in the substrate beneath the outer protective alumina scale after times varying from 50 h at 900 °C to 10 min at 1200 °C. Once the substrate aluminium level had dropped below a critical value (≤0.5 wt%), it no longer sustained the alumina scale formation and, as a consequence, continuing oxidation resulted in the initiation and development of a Cr‐rich oxide sub‐layer formation. At the lower temperatures, the voids filled with chromia leading to a scallop‐shaped inner layer beneath the alumina scale. In contrast, at higher temperatures, the Cr‐rich sub‐scale layer was continuous. If the Fe–20Cr–5Al based alloys are deposited as coatings, for example as a compliant layer onto a stronger substrate, there is a risk that other elements (such as silicon) from the substrate may diffuse through the coating and influence the subsequent oxidation behaviour of the coating. In order to simulate this, sandwiches of Fe–Cr–Al and a silicon rich substrate were fabricated and tested over a range of oxidation temperatures. It was then found that the silicon did indeed diffuse through the Fe–Cr–Al layer and change the oxidation mechanism. The voids formed under the alumina were now found to contain silicon oxide rather than chromia, but the void filling mechanism also appeared to be different. With chromia filled voids the filling commenced from the alumina scale, with the oxide growing inwards, while the silica rich regions grew outwards into the voids from the substrate. Scanning electron microscopy and EDX analysis were used to follow these changes and those in other more complex situations. Detailed mechanisms for void and chromia sub‐scale formation and development will be discussed in the paper.     

Long-Term Oxidation of Candidate Cast Iron and Stainless Steel Exhaust System Alloys from 650 to 800 °C in Air with Water Vapor

The oxidation behavior of candidate cast irons and cast stainless steels for diesel exhaust systems was studied for 5,000 h at 650–800 °C in air with 10 % H₂O. At 650 °C, Ni-resist D5S exhibited moderately better oxidation resistance than did the SiMo cast iron. However, the D5S suffered from oxide scale spallation at 700 °C, whereas the oxide scales formed on SiMo cast iron remained relatively adherent from 700 to 800 °C. The oxidation of the cast chromia-forming austenitics trended with the level of Cr and Ni additions, with small mass losses consistent with Cr oxy-hydroxide volatilization for the higher 25Cr/20–35Ni HK and HP type alloys, and transition to rapid Fe-base oxide formation and scale spallation in the lower 19Cr/12Ni CF8C plus alloy. In contrast, small positive mass changes consistent with protective alumina scale formation were observed for the cast AFA alloy under all conditions studied. Implications of these findings for exhaust system components are discussed.     

High temperature oxidation of FeCrAl‐alloys – influence of Al‐concentration on oxide layer characteristics

The superior high temperature oxidation resistance of FeCrAl alloys relies on the formation of a dense and continuous protective aluminium oxide layer on the alloy surface when exposed to high temperatures. Consequently, the aluminium content, i.e. the aluminium concentration at the alloy–oxide layer interface, must exceed a critical level in order to form a protective alumina layer. In the present study the oxidation behaviour of six different FeCrAl alloys with Al concentrations in the range of 1.2–5.0 wt% have been characterised after oxidation at 900 °C for 72 h with respect to oxide layer surface morphology, thickness and composition using scanning electron microscopy, energy dispersive X‐ray spectroscopy and Auger electron spectroscopy. The results show that a minimum of 3.2 wt% Al in the FeCrAl alloy is necessary for the formation of a continuous alumina layer. For Al concentrations in the range of 2.0–3.0 wt% a three‐layered oxide layer is formed, i.e. an oxide layer consisting of an inner alumina‐based layer, an intermediate chromia‐based layer and an outer iron oxide‐based layer. In contrast, the 1.2 wt% Al FeCrAl alloy is not able to form a protective oxide layer inhibiting extensive oxidation.     

Development of High Strength Ni–Cu–Zr–Ti–Si–Sn In-Situ Bulk Metallic Glass Composites Reinforced by Hard B2 Phase

In the present study, the influence of atomic ratio of Zr to Ti on the microstructure and mechanical properties of Ni–Cu–Zr–Ti–Si–Sn alloys is investigated. The alloys were designed by fine replacement of Ti for Zr from Ni₃₉Cu₂₀Zr_36?xTi_xSi₂Sn₃. The increase of Ti content enhances glass forming ability of the alloy by suppression of formation of (Ni, Cu)₁₀(Zr, Ti)₇ phase during solidification. With further increasing Ti content up to 24 at.%, the B2 phase is introduced in the amorphous matrix with a small amount of B19′ phase from alloy melt. The bulk metallic glass composite containing B2 phase with a volume fraction of ~ 10 vol% exhibits higher fracture strength (~ 2.5 GPa) than that of monolithic bulk metallic glass (~ 2.3 GPa). This improvement is associated to the individual mechanical characteristics of the B2 phase and amorphous matrix. The B2 phase exhibits higher hardness and modulus than those of amorphous matrix as well as effective stress accommodation up to the higher stress level than the yield strength of amorphous matrix. The large stress accommodation capacity of the hard B2 phase plays an important factor to improve the mechanical properties of in situ Ni-based bulk metallic glass composites.     

The Oxidation Behaviour of Alloys Based on the Ni–Co–Al–Ti–Cr System

The isothermal oxidation behaviour of a series of quinary Ni–Co–Al–Ti–Cr alloys were studied at 800 °C. Alloys with higher Cr concentrations exhibited lower mass gain after 100-h exposure, as did the alloys richest in Ni and Al for a given Cr concentration. Extensive internal oxidation and nitridation was also observed in all alloys, except those containing the highest concentrations of Ni and Al. All alloys studied generated continuous chromium oxide layers, beneath which alumina particles were observed. Compositional analysis of the subscales identified shallower Cr concentration gradients in alloys containing equiatomic levels of Ni and Co, suggesting increased availability of Cr in the alloy. Thermodynamic calculations confirmed that these alloys contained higher concentrations of Cr in their γ matrices as a result of a combination of both the elemental partitioning behaviour and the increased mole fraction of γ′ precipitates forming in the alloy.