Hot corrosion of two nickel-base superalloys under alkali sulfate deposition conditions

Abstract

The single crystal MD2 (Ni – 8Cr – 5Al –5Co– 2Mo– 8W–6Ta – 1Ti) and the polycrystalline Inconel 792 (Ni –13Cr – 3Al –9Co– 2Mo– 4W–4Ta– 4Ti) were exposed at 850 and 900°C in an severely sulfidising environment comprising synthetic air with 8% water vapour and 2000 ppm SO₂, and a deposit of 20 mol% Na₂SO₄+80 mol% K₂SO₄ applied every 160 hours.

Results were evaluated in terms of mass change, which indicated the existence of an incubation time before the onset of extensive attack, and post-test metallography to evaluate metal loss. Metallographic studies are presented showing the complex external oxide scale formation and internal oxidation and sulfidation.     

Mechanical and chemical properties of Ni3Si and Ni3(Si,Ti) alloys multiphased by chromium addition

Abstract

The alloying behaviour and microstructure of Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys were first characterised by optical microscopy, X-ray diffraction, and scanning electron microscopy with electron probe analysis. The microstructures of the Ni–Si–Cr ternary alloys consisted of large dispersed Ni₅Si₂ phase and finely precipitated Ni₃Si phase in nickel solid solution, while the Ni–Si–Ti–Cr quaternary alloys consisted of finely precipitated Ni₃(Si,Ti) phase and nickel solid solution. Then, the high temperature mechanical properties, bend strength, and oxidation and corrosion properties of the alloys were investigated. The Ni–Si–Cr ternary alloys showed significant strengthening over a wide range of temperatures, and also large compressive plastic deformation at high temperatures. The strength and fracture toughness at ambient temperatures were correlated with the volume fraction of Ni₅Si₂ phase. The Ni–Si–Ti–Cr quaternary alloys did not show increased yield strength, but exhibited improved tensile ductility and plasticity over a wide range of temperatures. Both Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys showed substantially improved oxidation resistance in air at 1173 K, compared with Ni₃Si and Ni₃(Si,Ti) alloys. Also, the Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys showed corrosion resistance comparable to that of the Ni₃Si and Ni₃(Si,Ti) alloys.     

Oxidation of TiAl based intermetallics

The high temperature oxidation behaviour of the binary and ternary alloys of the Ti–48Al system was studied at different temperatures. The primary objectives of this work were the establishment of the activation energies, the migration tendencies of the alloy species, mechanism of oxidation and chemistry of the oxide scales. The ternary additions were Cr (1.5 at 19%), V (2.2 at%), W (0.2 at%) and Mn (1.4 at%). The addition of ternary additions did not play a significant role in the oxidation behaviour at 704°C. At 815°C the alloys with Cr and V exhibited linear oxidation behaviour with large weight gains while the base Ti–48Al alloys exhibited the best behaviour. At 982°C the Mn-containing alloy was the worst, exhibiting a linear oxidation behaviour while the alloy with V and W and the base alloy with 400 p.p.m. oxygen exhibited the best oxidation behaviour. At 982°C the outermost oxide layer in contact with air is always near stoichiometric TiO₂. In all the alloys a layer of porosity is created just below the outer TiO₂ layer by the Kirkendall mechanism due to the rapid outward diffusion of Ti atoms. The addition of trivalent atoms like Cr in small amounts appear to be detrimental to the oxidation process as they can generate additional oxygen vacancies while the addition of atoms with valence of 5, such as V, and 6, such as W, appear to have beneficial effect on the oxidation behaviour at 982°C by tying up oxygen vacancies. This revised version was published online in November 2006 with corrections to the Cover Date.     

The effect of Ni additions on the oxidation behaviour of Co–Re–Cr high-temperature alloys

Abstract

The present study focused on the influence of Ni on the microstructure and oxidation behaviour of Co–Re–Cr-based alloys. Alloys with three different Ni contents were tested in laboratory air at 800–1100 °C. A refinement and a reduction of the σ phase volume fraction as well as a change in the matrix microstructure were observed. Thermogravimetric measurements showed that the alloys with higher Ni contents possess a better oxidation resistance when exposed to higher temperatures. All alloys suffered from continuous mass loss during oxidation at 800 °C due to the formation of porous oxides scales, consisting of Co₃O₄, Co(Ni)O and Ni-doped CoCr₂O₄, which allow the evaporation of Re-oxides. At 900–1100 °C, only the alloy with 25 at. % Ni showed parabolic oxidation kinetics after a short period of transient oxidation. This is a result of the fast formation of a protective Cr₂O₃ layer. It was also found that exposure to air at 1000 °C leads to a phase transformation of the bulk material; an oxidation-induced formation of fine hexagonal close-packed (hcp) grains was observed near the oxide scales. It is supposed that the improved oxidation resistance of Ni-containing Co–Re–Cr alloys is a result of enhanced Cr diffusion caused by the Ni addition. The extensive formation of the fcc phase in the alloy matrix had a detrimental effect on the oxidation behaviour of the Ni-containing Co–Re–Cr-based alloys.     

A study on the influence of Mo, Al and Si additions on the microstructure of annealed dual phase Cr–Ta alloys

The effects of additions of 5 at.% Mo, Al and Si on the long-term annealed microstructures of a two phase Cr–Cr₂Ta alloy have been studied. Following 200 h at 1300 °C, the lamellar eutectic constituent of all the alloys disintegrated into discrete particles of the Laves phase embedded within a Cr-rich solid solution phase, along with the formation of fine Laves phase precipitates. One of the predominant differences between the three alloying additions was the extent of the C14 to C15 polytypic transformation of the Cr₂Ta-based Laves phase. With Mo and Al additions, the Cr₂Ta Laves phase transformed from C14 to either C15 or intermediate hexagonal polytypes following 200 h annealing at 1300 °C. In contrast, Si additions stabilised the C14 polytype, with no transformation to other polytypes observed after prolonged annealing at 1000, 1100 and 1300 °C.     

Phase evolution and crystallography of precipitates during decomposition of new “tungsten-free” Co(Ni)–Mo–Al–Nb γ–γ′ superalloys at elevated temperatures

This article reports the microstructural stability and consequent phase decomposition including the appearance of topologically close-packed (TCP) phases at high temperature of recently discovered tungsten-free γ–γ′ alloys of base composition Co–10Al–5Mo–2Nb with or without the addition of Ni and Ti. On prolonged aging at 800 °C of the Co–10Al–5Mo–2Nb alloy, needle-shaped DO₁₉-ordered precipitates with stoichiometry of Co₃(Mo, Nb) start appearing in the microstructure. In addition, growth of cellular domains from the grain boundaries featuring a three-phase composite lamellar structure could be observed. These phases are fcc γ-Co with composition different from the original matrix, CoAl with B2 ordering and Co₃(Mo, Nb) with DO₁₉ ordering. All the phases exhibit well-defined crystallographic orientation relationships. The decomposition of the alloys depends on the solvus temperature of the γ′ phase. The Ni-containing alloy exhibits no phase decomposition until 100 h of aging at 800 °C without any significant effect on γ′ volume fraction (76 %). However, at 950 °C, the alloy decomposes leading to the appearance of four different phases including TCP phases: a Cr₃Si-type cubic phase, a hexagonal Laves phase, rhombohedral μ phase, and solid solution of Co phase. The γ–γ′ microstructure in the Co–10Al–5Mo–2Nb and Co–30Ni–10Al–5Mo–2Ta alloys is not stable at 800 and 950 °C, respectively, on long-term aging. This shows that the measured solvus temperatures (i.e., 866 and 990 °C) are metastable solvus temperatures. We also report that the Ti-containing alloy exhibits superior stability with no evidence of either TCP phase formation or any other decomposition of γ′ precipitates, even after aging at 950 °C for 100 h.     

Influence of sol–gel derived Al2O3 film on the oxidation behavior of a Ti3Al based alloy

Al₂O₃ thin films were deposited on a Ti₃Al based alloy (Ti–24Al–14Nb–3V–0.5Mo–0.3Si) by sol–gel processing. Isothermal oxidation at temperatures of 900–1000 °C and cyclic oxidation at 800–900 °C were performed to test their effect on the oxidation behavior of the alloy. Results of the oxidation tests show that the oxidation parabolic rate constants of the alloy were reduced due to the applied thin film. This beneficial effect became weaker after longer oxidation time at 1000 °C. TiO₂ and Al₂O₃ were the main phases formed on the alloy. The thin film could promote the growth of Al₂O₃, causing an increase of the Al₂O₃ content in the composite oxides, sequentially decreased the oxidation rate. Nb/Al enriched as a layer in the alloy adjacent to the oxide/alloy interface in both the coated and uncoated alloy. The coated thin film decreased the thickness of the Nb/Al enrichment layer by reducing the scale growth rate.     

Transient oxidation of alumina forming Ti–Al–Ag-based alloys and coatings studied by SEM,AFM, XPS and LRS

Abstract

γ-TiAl based intermetallics possess poor oxidation properties at temperatures above approximately 700°C. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. Recently, this type of materials has therefore been proposed as oxidation resistant coatings for high strength TiAl alloys. In the present study, a number of cast Ti–Al–Ag alloys and magnetron sputtered Ti–Al–Ag coatings were investigated in relation to transient oxide formation in air at 800°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques, such as SEM, ESCA, AFM and LIOS-RS. The γ-TiAl–Ag alloys and coatings appear to form an α-Al₂O₃ oxide scale from the beginning of the oxidation process, in spite of the relatively low oxidation temperature of 800°C. The formation of metastable alumina oxides seems to be related to the presence of Ag-rich precipitates in the alloy matrix.     

The influence of chromium on early high temperature corrosion of ferritic alloys under SO2 atmosphere

Comprehensive insights into the early stages of corrosion mechanisms provide fundamental knowledge to further understand and model long time material behaviour. The present work studies the early stages of combined oxidation and sulphidation of ferritic model alloys for time scales up to 250 h at 650°C to observe the influence of chromium during the corrosion under SO₂. Model alloys were used to focus on the reaction of the intended elements: Fe, Cr, S, and O. Pure iron simultaneously forms magnetite and iron-sulphide in an early stage of corrosion, covered by a pure oxide layer after 100 h. Iron with 13 wt-% Cr shows hematite and mixed Fe–Cr-oxides first, before sulphides nucleate in the inner corrosion zone. With increasing ageing time a magnetite layer is observed below the hematite layer. Quantitative phase fractions of all corrosion products observed were determined from cross section images. Characterisation of the Fe13Cr corrosion scale by FIB revealed a highly porous structure in the inner corrosion zone where Cr-rich (Fe, Cr)-sulphides are present, and caused the scale to spall easily.     

Hot corrosion behavior of porous nickel-based alloys containing molybdenum in the presence of NaCl at 750 °C

The hot corrosion of porous Ni-23Cr-xMo (0%, 4.5%, 9.0%, 13.5%, mass fraction) alloys tested at 750 °C under cyclic procedure was investigated in order to elucidate the effect of Mo addition on hot corrosion in the presence of NaCl. The hot corrosion experiments were performed at 750 °C in air with 4 mg cm^− 2 NaCl deposit. The performance of the alloys was evaluated by the results of weight change kinetics. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were used to characterize the corrosion products. The results indicate that NaCl accelerated the oxidation of the alloys by chloridized elements Mo and Cr. Among the porous Ni-23Cr-xMo alloys, Ni-23Cr-9Mo alloy exhibited the best hot corrosion resistance due to the formation of NiO-NiCr₂O₄-Cr₂O₃ oxide scales. Furthermore, these oxide scales were confirmed more effective to protect the alloys after adding of Mo.     

Aging behaviour of cobalt free chromium containing maraging steels

Abstract

This paper reports an investigation of the aging behaviour of two Co free Cr containing maraging steels (Fe–1·0Si–11·2Cr–1·3Mo–9·1Ni–1·2Al–1·0Ti and Fe–0·8Si–17·2Cr–6·1Ni–0·4Al–0·9Ti, all at.-%), using hardness measurements, electron microscopy of replicas and thin foils, atom probe field ion microscopy (APFIM), and thermochemical calculations. Two different families of intermetallic phases (Ti₆Si₇Ni₁₆G phase and η Ni₃Ti) have been found to contribute to age hardening. The composition and morphology of these precipitates were studied in deformed and undeformed alloys after aging at 420–570°C for various times. In addition, reverted austenite has been found in the aged structure. Results obtained using APFIM are compared with equilibrium thermodynamic calculations and previous APFIM studies of conventional Cr free low Al and Si maraging steels having higher Mo contents.

MST/1558     

Role of rare earth oxide coatings on oxidation resistance of chromia-forming alloys

Rare earths (RE) have been used to increase high temperature oxidation resistance of chromia and alumina forming alloys. The RE can be added as elements (or oxides) to the alloys or applied as oxide coatings to the alloy surface. This paper presents the effect of different RE oxide coatings and lanthanum chromite coatings on the high temperature oxidation behavior of Fe20Cr and Fe20Cr4Al alloys. The oxidation resistance of the Fe20Cr alloy increased with increase in ionic radius of the RE element in the coating. The RE oxides decreased chromia growth rate more than alumina growth rate. In extended cyclic oxidation tests that were carried out from peak temperatures of 900 °C, 1,000 °C and 1,100 °C to room temperature at cooling rates of 300 °C/s and 1,000 °C/s, the La₂O₃ coating increased cyclic oxidation resistance of the Fe20Cr alloy significantly more than the Pr₂O₃ coating. The role of RE in increasing overall oxidation resistance of chromia forming alloys is discussed.     

Oxidation behaviour of an intermetallic Ti–Al–Cr–Zr bond coat on a γ–TiAl based TNB alloy with 7YSZ thermal barrier coating

Abstract

Intermetallic titanium aluminide alloys are attractive light-weight materials for high temperature applications in automotive and aero engines. The development of γ-TiAl alloys over the past decades has led to their successful commercial application as low pressure turbine blades. The operating temperatures of γ-TiAl based alloys are limited by deterioration in strength and creep resistance at elevated temperatures as well as poor oxidation behaviour above 800 °C. Since improvement in oxidation behaviour of γ-TiAl based alloys without impairing their mechanical properties represents a major challenge, intermetallic protective coatings have aroused increasing interest in the last years.

In this work, a 10 μm thick intermetallic Ti–46Al–36Cr–4Zr (in at.-%) coating was applied on a TNB alloy using magnetron sputtering. This layer provided excellent oxidation protection up to 1000 °C. Microstructural changes in this coating during the high temperature exposure were extensively investigated using scanning and transmission electron microscopy. The coating developed a three-phase microstructure consisting of the hexagonal Laves-phase Ti(Cr,Al)₂, the tetragonal Cr₂Al phase and the cubic τ-TiAl₃ phase. After long-term exposure the three-phase microstructure changed to a two-phase microstructure of the hexagonal α₂-Ti₃Al phase and an orthorhombic body-centred phase, whose crystal structure has not yet been definitely identified. On the coating, a thin protective alumina scale formed. Applying this intermetallic layer as bond coat, thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on γ-TiAl based TNB samples using electron-beam physical vapour deposition. The results of cyclic oxidation testing (1 h at elevated temperature, 10 min. cooling at ambient temperature) revealed a TBC lifetime of more than 1000 h of cyclic exposure to air at 1000 °C. The ceramic topcoat exhibited an excellent adhesion to the thermally grown alumina scale which contained fine ZrO₂ precipitates.     

Protective coatings on orthorhombic Ti2AlNb alloys

Abstract

The oxidation behaviour of an orthorhombic Ti–22Al–25Nb alloy, bare and with protective coatings, was investigated at 750°C in air under quasi-isothermal and thermal cycling conditions. As found by post-oxidation analysis, the uncoated substrate material was severely degraded by formation of spalling oxide scales and ingress of oxygen and nitrogen causing nitride precipitation, internal oxidation and interstitial embrittlement. Metallic Ti–51Al–12Cr coatings as well as nitride coatings based on Ti–Al–Cr–Y–N, either monolithically grown or with superlattice structure, provided an effective diffusion barrier against oxygen. The excellent oxidation resistance of the TiAlCr coatings was associated with the ternary Ti(Al,Cr)₂ Laves phase promoting the formation of a protective alumina scale. The different intermetallic phases formed in the interdiffusion zone caused neither cracking nor spallation of the protective coating. Both, monolithically grown TiAlCrYN and superlattice TiAlYN/CrN coatings, exhibited slow, but nearly linear oxidation kinetics at 750°C in air. In the subsurface region of the substrate a niobium rich phase and the α₂-phase formed. At the coating/substrate interface pores and a thin, fine-grained TiN layer were found.     

Sintering behaviour of Nb–16Si–25Ti–8Hf–2Cr–2Al alloy powder fabricated by a hydrogenation–dehydrogenation method

The present study investigated the sintering behaviour of Nb–16Si–25Ti–8Hf–2Cr–2Al alloy powders. The alloy powders were produced using ahydrogenation–dehydrogenation method and featured an irregular morphology. Powders sintered at 1500°C and 1600°C exhibited pores in their microstructures, while powders sintered at 1700°C for 4?h were fully densified and poreless. The cast ingot and powders were composed of three phases: Nb_ss, Nb₅Si₃, and Nb₃Si. However, the Nb₃Si phase was not observed, while HfO₂ oxides formed in the sintered compact. The hafnium and oxygen reacted to form an HfO₂ oxide during the high-temperature sintering process. From the result of the thermodynamic calculation, Hf oxide formed after sintering because Hf has the highest driving force for oxidation among the elements constituting the alloy.     

The oxidation of Fe3Al–(0, 2, 4, 6%)Cr alloys at 1000 °C

Thermomechanically treated Fe₃Al–(0, 2, 4, 6at.%)Cr alloys were isothermally oxidized at 1000 °C in air, and their oxidation characteristics were studied using thermogravimetric analyzer, X-ray diffractometer, scanning electron microscope, electron probe microanalyzer, and TEM/EDS. It was found that Cr decreased the oxidation resistance of Fe₃Al alloys to a certain extent. The oxide scales that formed on the unalloyed Fe₃Al alloys consisted primarily of α-Al₂O₃ containing a small percentage of dissolved iron ions. Less than 1% of dissolved chromium ions was additionally present in the oxide scale formed on the Fe₃Al–Cr alloys. An Al-free, Fe-enriched zone was formed beneath the oxide scale, owing to Al consumption to form the oxide scale. The oxide scale on all alloys had poor adherence.     

Oxide adherence of Fe-20Cr-4Al alloys with small amounts of sulfur and reactive elements (Y,Hf)

Abstract

Oxide adherence of Fe–20Cr–4Al alloys with small amounts of sulfur, yttrium and hafnium was studied in air for 360 ks at 1,373, 1,473 and 1,573K by mass change measurements, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. After oxidation at 1,373K, spalling of oxide scales on 7ppmS, 53ppmS and 1,300ppmS alloys was recognized. However, spalling of oxide scales on the other alloys was not observed. After oxidation at 1,573K, spalling of the oxide scales on the alloys with sulfur increased roughly with increasing contents of sulfur, and spalling of oxide scales on the alloys containing yttrium was scarcely recognized, however, oxide scales on all of the alloys containing hafnium spalled at the entire surface. Oxide adherence on the alloys may relate to morphologies of oxide scales and oxide–alloy interface, size and distribution of chromium sulfide, Y₃Al₅O₁₂ and HfO₂ particles at the oxide–alloy interface and temperature of oxidation.     

Design of near-α Ti alloys via a cluster formula approach and their high-temperature oxidation resistance

The multi-component composition characteristics of high-temperature near-α Ti alloys were investigated in the present work by means of a cluster formula approach. The uniform cluster formula [CN12 cluster](glue atom)₃ for the hexagonal close-packed α solid solution was first obtained based on the Friedel oscillation theory, with a total atom number in the formula of Z = 16. Then it was analyzed that the Z values in the cluster composition formulas of typical near-α Ti alloys are within the range of Z = 16.00 ˜ 16.30, being perfectly consistent with the ideal Z = 16. Based on it, a series of new alloys with Z = 16 and with Nb/Ta substitution for Mo in Ti1100 alloy were designed, suction-cast into φ 6 mm rods, and then heat-treated with solid solution and aging. It was found that the alloy with co-addition of Mo, Ta and Nb has a high strength and good ductility at both room and high temperatures. More importantly, the additions of Nb and Ta can contribute to the formation of continuous and compact Al₂O₃ scales, resulting in an obvious improvement of oxidation resistances at both 923 K and 1073 K. The effects of Mo, Ta and Nb on the oxidation behaviors of the designed alloys at 1073 K were further discussed.     

Oxidation protective coatings for γ‐TiAl – recent trends

Engine designers show continued interest in γ‐TiAl based titanium aluminides as light–weight structural materials to be used at moderately elevated temperatures. Although alloy development has made significant progress in terms of mechanical properties and environmental resistance, protective coatings have been developed that help to extend the lifetime of these alloys significantly. The major challenge of coating development is to prevent the formation of fast growing titania. Furthermore, changes of coating chemistries at high temperatures have to be considered in order to avoid rapid degradation of the coatings due to interdiffusion between substrate and coating. The paper describes recent work of the authors on different coatings produced by means of magnetron sputter technique. Thin ceramic Ti‐Al‐Cr‐Y‐N layers tested at 900 °C exhibited poor oxidation resistance. In contrast, intermetallic Ti‐Al‐Cr, Si‐based and aluminum rich Ti‐Al coatings were tested at exposure temperatures up to 950 °C for 1000h resulting in reasonable and partially excellent oxidation behaviour.     

Microstructure and mechanical properties of Al–Si cast alloy with additions of Zr–V–Ti

The microstructure and tensile properties at temperatures up to 300 °C of an experimental Al–7Si–1Cu–0.5Mg (wt.%) cast alloy with additions of Ti, V and Zr were assessed and compared with those of the commercial A380 grade. The microstructure of both alloys consisted of Al dendrites surrounded by Al–Si eutectic containing, within its structure, the ternary Al–Al₂Cu–Si phase. Whereas the Al₁₅(FeCrMn)₃Si₂ phases were present in the A380 alloy, Ti/Zr/V together with Al and Si phases, Al(ZrTiV)Si, were identified in the experimental alloy. As a result of chemistry modification the experimental alloy achieved from 20% to 40% higher strength and from 1.5 to 5 times higher ductility than the A380 reference grade. The role of chemistry in improving the alloy thermal stability is discussed.