Materials at High Temperature Vol 20, Issue 4, 2003

Preface

The Fifth International Conference on the Microscopy of Oxidation was held at the University of Limerick, Ireland from 26th to the 29th August 2002. Some 70 delegates attended the Meeting. This issue of Materials at High Temperatures contains 21 of the papers, the remaining 25 papers were published in issue 3. The general format of the Conference followed that of the four previous Meetings, there being sessions on the fundamentals of oxidation studies as well as chromia and iron oxide formation. Some of the issues associated with the oxidation of nuclear materials and light alloys were addressed in two further sessions and one afternoon was devoted to posters. A number of the sessions were centred on alumina formation and this reflects the continuing demand for high temperature materials that can withstand stress. The role of coatings and TBCs was also discussed and both areas continue to promote new applications of microscopy based methods for studying oxidation.

A wine reception was kindly sponsored by JEOL (UK) Ltd and FEI (UK) Ltd and we are very grateful for their generous support of the Meeting. The Conference Dinner was held in Bunratty Castle and this proved both a successful and entertaining evening.

It is planned to hold a sixth meeting in this conference series.

Simon Newcomb

Gordon Tatlock

Simulation of road salt corrosion in austenitic alloys for automotive exhaust systems

L. Antoni1, R. Bousquet2 and J.H. Davidson2

Arcelor Innovation, 1Ugine-Savoie Imphy, Centre de Recherches d’Ugine, Avenue Paul Girod, F-73403 Ugine Cedex, France

2Imphy Ugine Précision, Centre de Recherches d’Imphy, F-58160 Imphy, France

Cyclic oxidation tests in air with intermittent salt spraying have been performed to simulate the conditions of road salt (NaCl–CaCl2) enhanced corrosion in automotive exhaust systems. Tests were carried out at 600 and 700°C on three austenitic alloys, including two stainless steels currently employed for exhaust components (AISI 316 Ti and AISI 302B) and a higher nickel heat resisting alloy. The presence of salt causes internal corrosion, both along a regular front beneath an outer oxide scale and down alloy grain boundaries. An increase in temperature accelerates the corrosion rate and particularly enhances intergranular penetration. The results of micrographic and microanalytical investigations are in general agreement with an active oxidation mechanism in which NaxClx vapour species, and not only chlorine, appear to play an important role. The regions directly affected are depleted in chromium and iron and enriched in nickel. Although internal oxidation of silicon is observed, a high silicon content (2%) does not necessarily ensure effective protection against this type of attack.

Keywords: road salt corrosion, austenitic alloys, automotive exhaust systems

In situ ESEM investigations of the oxide growth on hot work tools steel: effect of the water vapour

P. Bruckel, P. Lours, P. Lamesle and B. Pieraggi

CROMeP, Research Centre on Tools, Materials and Processes, Ecole des Mines d’Albi- Carmaux, 81013 Albi, France

Tempered martensitic steel modified AISI H11 is used in forging processes where tool failure can result from the combination of thermo-mechanical and chemical damage. A better knowledge of the oxidation mechanisms in this material could be useful for a better appreciation of its service behaviour and lifetime. The low chromium content of this Fe–Cr type steel allows the development of mainly Fe2-xCrxO3 oxides with corundum structure and leads to enhanced oxidation in the presence of water vapour. In situ FEG–ESEM images show the scale microstructural modifications during high temperature exposure, as well as the lateral growth of oxide particles. Together with GIXRD, SEM/EDS and SIMS analysis, FEG–ESEM also allows assessment of the H2O effect on oxidation behaviour during high temperature exposures (600 and 700°C). Water vapour induces either pores or crystallites size increase, favours faceted oxides particles with enhanced density at the highest partial pressure. At this microscopic scale, anisotropic growth of crystallites is observed, and size expansion rates are found to be linear and characteristic of each individual particle. Temperature acts principally on oxide film microstructure. Whatever the environment, homogeneous scale growth is observed at 600°C whereas the steel surface is heterogeneously covered by oxides at 700°C.

Keywords: environmental scanning electron microscopy, oxide growth, water vapour, steel oxidation

Analysis and TEM examinations of corrosion scales grown on alloy 690 exposed to PWR environment

F. Carrette1,2, M.C. Lafont2, L. Legras1, L. Guinard1 and B. Pieraggi3

1EDF, R&D Division, Department MMC, 77818 Moret-sur-Loing Cedex, France

2CIRIMAT-UMR CNRS 5085, ENSIACET, 31077 Toulouse Cedex 4, France

3CROMEP- ENSTIMAC, 81013 Albi CT Cedex 09, France

The radioactivity in the primary circuit of pressurized water reactors (PWR) mainly comes from the activation of corrosion products in the core of PWR; corrosion products dissolve from the oxide scales developed on steam generator tubes of alloy 690. The controlling of this process, oxidation and dissolution, requires a detailed knowledge of the microstructure and chemical composition of oxide scales. Alloy 690 was studied as tubes and plates with three various surface states (as-received, cold-worked, electropolished). Corrosion tests were performed at 325°C and 120 or 155 bar in primary water conditions (B/Li(1000/2 ppm, [H2] = 30 cm3 kg–1 TPN, [O2]<5 ppb); test durations ranged between 24 and 2160 hours. Corrosion tests were carried out in a static autoclave made of stainless steel, and in one recirculating autoclave made of titanium alloy (TITANE recirculating autoclave). Characterisation of the oxide scales grown in representative conditions of the primary circuit was performed by several techniques (SEM, (X)TEM, SIMS, XPS, GIXRD, RBS). These analyses revealed the essential role of the finegrained cold-worked superficial zone of perturbed microstructure presents on as-received and coldworked specimens. They also gave some insights on the growth mechanisms of corrosion products scales.

Keywords: TEM, corrosion scales, Alloy 690, pressurized water reactors

Alumina scale growth and degradation modes of a TBC system

Catherine Guerre, Luc Rémy and Régine Molins

ENSMP, Centre des Matériaux, UMR CNRS 7633, BP 87, 91003 Evry, France

The evolution of a thermal barrier coating system was followed during exposures at high temperature (1100°C) and under various thermal and mechanical loading conditions. The TBC system is composed of an EB-PVD yttria partially stabilised zirconia topcoat (TC) and a platinum nickel aluminide bondcoat (BC) deposited on a single crystal nickel based superalloy. Depending on the kind of heat treatment (isothermal or thermo-mechanically cycled), different types of defects (pores, cracks, re-oxidation), were observed at the BC/TGO interface, the TGO/TC interface or in the alumina scale. Damage processes were identified and discussed according to the type of imposed heat treatment.

Keywords: thermal barrier coating, alumina scale growth

Stress and oxidation of nickel according to its purity

A.M. Huntz1, A. Lefeuvre1, M. Andrieux1, C. Sévérac2, G. Moulin3, R. Molins4 and F. Jomard5,

1ICMMO-LEMHE, CNRS UMR 8647, Université Paris-XI, Centre d’Orsay, F-91405 Orsay, France

2ICMMO, UPS, F-91405 Orsay, France

3Laboratoire Roberval, UTC, F-60205 Compiègne Cedex, France

4ENSMP, Centre des Matériaux, CNRS UMR 7633, BP 87, F-91003 Evry, France

5Physique des Solides et Cristallogenèse, CNRS UMR 8635, F-91195 Meudon Cedex, France

The effect of impurities on the oxidation mechanism of nickel and on mechanical characteristics of the NiO scale was studied on two industrial grades and one pure nickel. The oxidation mechanism at 800°C was clarified using kinetics approach, microstructure observations, EDX and XPS analyses, profilometry, oxygen isotopic exchange and SIMS. The mechanical characteristics were determined mainly by three point bending tests performed in a scanning microscope. Whatever the nickel grade, the oxide toughness varies with the scale thickness and tends towards the value of massive NiO. The main difference related to Ni purity consists in the fact that spalling occurs at an oxide/oxide interface for the industrial grades, while it appears at the metal/oxide interface for pure Ni. Indeed, due to the presence of impurities, internal oxidation, extrusion of metallic nickel along grain boundaries of the substrate and formation of an inner equiaxed oxide film are observed in industrial grades. This induces mechanical keying of the oxide, and therefore crack propagation in a mixed mode is easier at the oxide/oxide interface. With pure nickel, only a single oxide film is formed by outward diffusion of Ni and there is no internal oxidation. Thus crack propagation occurs along the metal/oxide interface.

Keywords: nickel, nickel oxide, oxidation

Oxidation and lifetime of PYSZ and CeSZ coated Ni-base substrates with MCrAlY bond layers

C. Leyens, U. Schulz and K. Fritscher

DLR- German Aerospace Centre, Institute of Materials Research, 51170 Cologne, Germany

IN100 blades and CMSX-4 samples with NiCoCrAlY bond coats manufactured via LPPS and EB-PVD were cyclically tested in a burner rig at 1120°C and a furnace at 1100°C. Two different TBC chemistries were deposited by EB-PVD techniques: PYSZ and CeSZ. In all cases the performance of the CeSZ coated variances was superior to the PYSZ coated counterparts. TGO growth kinetics were slightly slower for the CeSZ coated samples. The benefits in lifetime are mainly attributed to the mixed zone oxides that tolerate spinel phase formation due to their ability to “wet” each other. Pore formation in the mixed zone is hereby suppressed favoring crack blunting mechanisms.

Keywords: thermal barrier coating, EB-PVD, ceria-stabilized zirconia, yttria-stabilized zirconia

Complex oxide structures formed by oxidation of Ag(100) and Ag(111) by hyperthermal atomic oxygen

Long Li and Judith C. Yang

Materials Science and Engineering Department, University of Pittsburgh, Pittsburgh, PA 15261, USA

Silver (100) and (111) single crystals were exposed to a unique hyperthermal atomic oxygen source, which produces a high flux of 5.1eV atomic oxygen, for seven hours at 220°C. The resultant oxide and oxide–metal interfaces were characterized by optical, scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HREM). The oxide scale was more than 10 _m thick and very weakly attached to the Ag substrate. The silver oxides were complex and surprising, differ in their thickness and the oxide phases due to the orientation of the Ag single crystals. The cross-section TEM studies revealed complex microstructures with many defects, such as micro-twins, porosity and irregular shaped grains.

Keywords: oxidation of silver, hyperthermal oxygen

TEM investigations of intergranular stress corrosion cracking in austenitic alloys in PWR environmental conditions

S. Lozano-Perez and J.M.Titchmarsh

Department of Materials, Oxford University, Parks Road, Oxford OX1 3PH, UK

Analytical transmission microscopy has been used to investigate the initiation of stress corrosion cracking in Inconel 600 subjected to constant load testing under simulated pressured water reactor primary water conditions. The observations revealed that intergranular attack proceeded by the development of a zone of polycrystalline chromia along the boundary plane intersecting either the free surface or a blunted, open crack in contact with the free surface. Ni-rich metal particles were interspersed within the chromia. Conversely, open cracks were filled with nanocrystalline NiO and large compound particles of spinel and NiO, indicating a difference in potential between closed, attacked boundaries and open cracks. Open cracks appeared to have initiated by fracture of the chromia zones, such fracture being strongly dependent on boundary geometry with respect to loading direction. The observations suggest that stress corrosion crack initiation and propagation is dependent on diffusion of oxygen through the porous oxides. Dislocations and stress could enhance diffusion as chromia was observed along slip planes at the arrested tips of blunt cracks.

Keywords: transmission microscopy, stress corrosion cracking, austenitic alloys

Carbon deposition on stainless steel in oxidising environments

G.R. Millward1, H.E. Evans1, I.P. Jones1 and C.D. Eley2

1Department of Metallurgy and Materials, The University of Birmingham, Birmingham B15 2TT, UK.

2British Energy Generation Ltd., Barnett Way, Barnwood, Gloucester GL4 3RS, UK.

The formation of low-density carbon deposits onto metallic heat transfer surfaces in industrial environments of high carbon activity can have a deleterious effect on thermal efficiency. Often this deposition occurs under reducing conditions and involves carbide (typically of iron) formation as part of the process. It is possible, however, for filamentous carbon deposits to form at much higher oxygen potentials where carbide formation is not favoured thermodynamically. This study considers one such situation by examining the behaviour of a 20Cr25Ni austenitic steel used as fuel cladding in Advanced Gas-cooled nuclear reactors (AGRs). Carbon deposition has been produced in laboratory by exposure at 550°C to CO2 containing 1%CO and 1000 vppm C2H4; the dissociation of the latter additive generates carbon activities much greater than unity. The oxygen potential of this gas is sufficient to oxidise iron and chromium, of the major alloy constituents, but not nickel. The deposition takes place onto substrates that are prepared as electron microscope samples which are subsequently examined by SEM and HREM (high resolution electron microscopy) techniques. The deposits formed consist of solid carbon filaments, approximately 30 nm in diameter, with turbostratic atomic layering. Each filament contains a 10–20 nm metallic nickel particle near its tip. It is argued, and supported by HREM evidence, that such particles are produced as a direct result of the oxidation of the alloy and that prior oxidation is necessary to produce the observed carbon deposition. It is also shown that inhibition of carbon deposition occurs when the oxygen potential of the depositing gas is increased sufficiently to oxidise nickel.

Keywords: stainless steel, carbon deposits

Failure morphologies of cyclically oxidized ZrO2-based thermal barrier coatings

James A. Nesbitt1, Dongming Zhu2, Robert A. Miller1 and Charles A. Barrett1

1NASA Glenn Research Center, Cleveland, OH 44135, USA

2US Army Research Laboratory, Cleveland, OH 44135, USA

Advanced and baseline thermal barrier coatings (TBCs) were thermal cycle tested in air at 1163°C until delamination or spallation of the ceramic top coat. The top coat of the advanced TBC’s consisted of ZrO2 with various amounts of Y2O3, Yb2O3, Gd2O3, or Nd2O3 dopants. The composition of the top coat of the baseline TBC was ZrO2-8wt.%Y2O3. All top coats were deposited by air plasma spraying. A NiCrAlY or NiCoCrAlY bond coat was deposited by low pressure plasma spraying onto a single-crystal, Ni-base superalloy. The TBC lifetime for the baseline coatings was approximately 190 cycles (45 minutes at 1163°C per cycle) while the lifetime for the advanced coatings was as high as 425 cycles. The fracture surfaces and sample cross sections were examined after TBC failure by SEM and optical microscopy, and the top coats were further examined by X-ray diffraction. These post-test studies revealed that the fracture path largely followed splat boundaries with some trans-splat fracture. However, there were no obvious distinguishing features which explained the difference in TBC lifetimes between some of the advanced and baseline coatings.

Keywords: thermal barrier coatings, ZrO2–Y2O3 top coats

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

L. Niewolak1, K. Lawson2, P. Roßbach1, E. Wessel1, V. Shemet1, J.R. Nicholls2, L. Singheiser1 and W.J. Quadakkers1

1Forschungszentrum Jülich, IWV-2, 52425 Jülich, FRG

2Cranfield University, Bedford MK43 0AL, UK

_-TiAl based intermetallics possess poor oxidation properties at temperatures above approximately 700°C. Previous studies showed that protective alumina scale formation on _-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 _-TiAl–Ag alloys and coatings appear to form an _-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.

Keywords: Ti–Al–Ag alloys, alumina scale formation

Development of oxides at TBC – bond coat interfaces in burner rig exposures

N.J. Simms, P.J. Kilgallon, C. Roach and J.E. Oakey

Power Generation Technology Centre, Cranfield University, Cranfield, Bedfordshire MK43 0AL, UK

There is a desire to use gases derived from increasingly ‘dirty’ fuels (e.g. coal and biomass) in industrial gas turbines. The contaminants in these fuels have the potential to cause significant damage to the gas turbine hot gas path materials, many of which were developed and selected for natural gas fired conditions. This paper reports results of a study investigating the performance of thermal barrier coatings (TBCs) and bond coatings, applied to current industrial gas turbine materials, within clean and ‘dirty’ gas environments generated within a burner rig. The materials covered by this study included:

• TBCs based on 8%Y2O3–ZrO2 applied by both air plasma sprayed (APS) and electron beam – physical vapour deposition (EB–PVD) routes.

• Bond coats of the overlay and diffusion classes, applied by vacuum plasma spraying (VPS), electroplating (EP), chemical vapour deposition (CVD) and high velocity oxy-fuel (HVOF) spraying

• Base alloys of IN6203, CMSX-4 and Haynes 230 The required TBC/bond coat combinations were applied by commercial coating processes to cylindrical samples of base alloys manufactured for use in a burner rig.

The burner rig used in this study is designed to enable air-cooled probes of cylindrical samples to be exposed to a natural gas combustion environment. In this study, this enabled specific metal temperatures (~800 and ~900°C) to be targeted within a much higher temperature combustion gas stream (~1150°C). ‘Dirty’ fuel gas environments were simulated by introducing gaseous (SO2 and HCl) and vapour phase (Na, K, Pb, Zn) contaminants into the burner rig just upstream of the edge of the gas flame. These conditions enabled continuous tests to be performed for 1,000 hours in both natural gas and ‘dirty’ fuel environments. The relative performance of the materials was determined from cross-sections prepared after the 1000 hour exposures. These cross-sections were examined by optical and SEM/EDX to determine the thicknesses of the oxides at the TBC – bond coat interfaces, the morphologies of these interfaces and to characterise the elemental distributions in these regions.

Keywords: industrial gas turbines, thermal barrier coatings, coal, biomass

High temperature oxidation of high purity nickel: oxide scale morphology and growth kinetics

Raphaëlle Peraldi1, Daniel Monceau1, Sylvain Jean2 and Bernard Pieraggi2

1 CIRIMAT, INPT-ENSIACET, 31077 Toulouse Cedex 4, France

2 CROMEP, ENSTIMAC, 81013 Albi CT Cedex 09, France

The oxidation of high purity nickel was studied between 600 and 1200°C for scale thickness between 1 and 30 _m. At or above 1100° C, the scale growth kinetics are strictly parabolic. The scales are then compact with columnar and facetted NiO grains. A more complex behaviour is observed below 1000°C: (i) for test temperatures between 1000 and 800°C, mass gain curves cannot be fitted to a parabola, (ii) different scale morphologies and microstructures are observed depending on scale thickness and temperature, (iii) a duplex scale is formed below 800°C. In addition to the possible effect of grain-boundary diffusion, the departure of growth kinetics from simple pure parabolic kinetics could be also related to the complex scale microstructure and its large evolution during scale growth. In situ oxidation of nickel specimens in an environmental SEM equipped with a hot stage specimen holder permitted to follow the morphological evolution of NiO scales. In situ grown NiO scales show the same microstructural features as observed on Ni specimens oxidised for longer duration in pure oxygen at atmospheric pressure.

Keywords: high purity nickel, oxide scale

Oxidation of orthorhombic Ti2AlNb alloys in the temperature range 550–1000°C in air

A. Ralison, F. Dettenwanger and M. Schütze

Karl-Winnacker-Institut der DECHEMA e.V., Frankfurt am Main, Germany

Orthorhombic titanium aluminides offer the potential to substitute the current titanium alloys in aero engines, gas turbines and automotive turbo-chargers. The advantages are a wider range of thermo mechanical processing possibilities, a higher room temperature ductility, increased high temperature strength and better oxidation as well as titanium fire resistance. Nevertheless, uncertainties with regard to their oxidation behaviour presently impede their use in practical applications. In the present study a very detailed investigation of the oxide scales formed and of the resulting metal subsurface structures after oxidation for up to 1000 h was performed. The results show that both the oxide scale and the subsurface zone develop very complex structures which are responsible whether protective or non-protective behaviour is observed. In principle the oxidation behaviour of the three alloys investigated can be characterised by three types designated as type I, II or III. Each of these types has a characteristic scale structure and a different degree of protection and their occurrence depends on alloy composition and oxidation time. From a technical point of view the interstitial dissolution of oxygen in the metal subsurface zone plays an important role as this process can lead to severe embrittlement. The extent of the IAZ (interstitial affected zone) depends on the oxide scale structure. The paper presents the results from detailed metallographic and microprobe investigations which throw further light on the complex oxidation processes of this group of materials.

Keywords: orthorhombic titanium aluminides

Microstructural stability of a Ni–Pt–Al coating on CMSX-10 alloy at 950 and 1100°C

M. Reid, M.J. Pomeroy and J.S. Robinson

Materials and Surface Science Institute, University of Limerick, Ireland

Specimens of CMSX-10 alloy have been platinum aluminised to give a two phase PtAl2–(Ni, Pt)Al coating. Specimens of the coated alloy were exposed in an oxidising environment for 188, 350 and 750 hours at temperatures of 950 and 1100°C. At 950°C the two phase coating equilibrates with the substrate and this results in the development of a single phase (Ni, Pt)Al coating. After 750 hours, _–_′ transformation effects are observed but there is significant Ta present in the _′ grains. At 1100°C more rapid outward diffusion of nickel and countercurrent diffusion of aluminium facilitates more extensive _–_′ transformation and Ta substituted _′ grains are clearly visible after 188 hours. At both temperatures, refractory element rich phases form in a secondary equilibration zone adjacent to the substrate as a result of nickel depletion. The development of these phases at 1100°C is significant, such that they extend to a depth of 0.2mm beneath the diffusion layer

Keywords: Ni–Pt–Al coating, microstructural stability, CMSX-10 alloy

Residual stress measurement by Piezospectroscopy of cross sections through thermal barrier coatings

A. Selçuk and A. Atkinson*

Department of Materials, Imperial College, London SW7 2BP, UK

Piezo-spectroscopic measurements of the residual stress in the TGO have been demonstrated on cross sections through thermally cycled TBC systems with high spatial resolution (approximately 2 _ 2 _ 5 _m). The residual stress is perturbed by relaxation at the free surface, but this can be taken into account in an approximate way. This relaxation has a range approximately equal to the YSZ thickness indicating that the YSZ imposes significant mechanical constraint on the TGO despite its low modulus. The measurements have shown that the non-planar morphology of the TGO induces large deviations from the thermo-elastic equi-biaxial stress expected for a planar TGO. The mean level of compressive residual stress is reduced by relaxation due to bending of the non-planar TGO, in agreement with elastic FEM analysis of sinusoidal TGO morphology. However, the real morphology is not sinusoidal and in some locations the local curvature is extremely high. In these regions the residual stress is observed to become tensile and as high as 1 GPa. The failure mechanism is by nucleation and growth of local damaged regions caused by these tensile stresses (which are evident as low stress regions on analysis through the YSZ) into larger regions that eventually become unstable to large-scale buckling and spalling.

Keywords: Piezo-spectroscopy, thermal barrier coatings

Formation of diffusion cells in LPPS MCrAlY coatings

M. P. Taylor and H. E. Evans

Department of Metallurgy and Materials, School of Engineering, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

The formation of diffusionally isolated regions in air plasma-sprayed, APS, MCrAlY bond coat or overlay coatings has been established. The ‘diffusion cells’ experience greatly accelerated depletion of aluminium and an early onset of breakaway oxidation associated with the morphology and extent of the internal oxides. In this paper the effect of internal oxides produced in low pressure plasma sprayed, LPPS, CoNiCrAlY coatings are presented. Isothermal tests in laboratory air at 1200°C were conducted with post-test characterisation performed using scanning electron microscopy and energy dispersive X-ray analysis. A parabolic correlation was found between TGO growth rate and time at temperature. Importantly, compositional profiles of cross sections through the coatings demonstrated that no barriers to diffusion occurred throughout the body of the coating. What has been found in the LPPS coatings is a mechanism whereby diffusion cell formation is possible at sites where splat boundaries containing internal oxides intersect the outer surface. Gas access is possible at these sites and, with increasing time at temperature, formation of a continuous internal oxide layer forms. The sequence of events at these sites follows that of the diffusion cell model presented earlier and a prediction of time to failure is made.

Keywords: diffusion cells, LPSS MCrAlY coatings

The role of cementite in the metal dusting of Fe–Cr and Fe–Ni–Cr Alloys

C.H. Toh, P.R. Munroe and D.J. Young

School of Materials Science & Engineering, University of New South Wales, Sydney NSW 2052, Australia

Model Fe–25 w/o (weight percent) Cr and Fe–25 Cr–Ni alloys containing 2.5, 5, 10 and 25w/o nickel were exposed to a CO–26H2–6H2O (vol. pct) mixture at 680oC under thermal cycling conditions. The supersaturated carbon activity was calculated to be 2.9 (referred to graphite) and M3C was predicted to form on Fe–25Cr and Fe–25 Cr–2.5 Ni, but not on higher nickel content alloys. Metal dusting occurred on all alloys, accompanied by internal carburisation. Transmission electron microscopy of the dusting deposit showed that much of the carbon consisted of hollow graphite nanotubes. Small, metal-rich particles were found at the carbon filament tips. These were identified as single crystal Fe3C in the case of Fe–25 Cr, and M3C, containing low levels of nickel, in the case of Fe–25 Cr–2.5 Ni and Fe–25 Cr–5 Ni. In contrast, the particles found at the filament tips on the higher nickel, two phase, alloys were both M3C and austenitic Fe–Ni. Strong orientation relationships were determined between the graphite and cementite particles, however, no consistent and clear crystallographic relationship was deduced between the graphite and austenite particles. It is concluded that carbon deposition from the gas is catalysed by both Fe3C and austenite. Subsequent carbon nanotube growth reflects the orientation relationship between Fe3C and the graphite.

Keywords: carbide, orientation relationships, carbon nanotubes

Microscopic observations of both sides of scale-substrate interfaces after adhesion measurements of chromia-rich scales on ferritic stainless steels using the inverted blister test

F. Toscan1,2, L. Antoni3, M. Dupeux2 and A. Galerie2

1ARCELOR Innovation, Centre de Recherches d’Isbergues, BP 15 F-62330 Isbergues, France

2Laboratoire de Thermodynamique et Physico-Chimie Métallurgiques, UMR CNRS-UJFINPG 5614 BP 75, F-38402 Saint Martin d’Hères cedex, France

3ARCELOR Innovation, Centre de Recherches d’Ugine, Avenue Paul Girod, F-73403 Ugine, France

Ferritic stainless steels were oxidized cyclically at 850°C in air. Four grades were studied : one reference grade without stabilizers, one stabilized with Ti, one with Nb and one with both Ti and Nb. The mass gain evolution of these grades versus number of cycles was recorded and gave relevant information about their oxidation kinetics. Indeed, very few oxide spallation was noted (<1%). Ti appeared to accelerate kinetics significantly, mainly because of its internal oxidation. Nb seemed to have no effect on kinetics in air at this temperature. The adhesion of oxide layers was quantified by the inverted blister-test. A beneficial effect of titanium on the adhesion was noted. For Ti-containing grades, adhesion energies were about 40 J/m2 whereas for other grades, they were lower, about 10 J/m2. This effect is thought to be due to internal oxidation of titanium, playing a role of mechanical keying of the oxide scale. On the contrary, niobium is not oxidized at the interface and remains as an intergranular intermetallic ironniobium phase. Its influence on oxide adhesion is therefore negligible.

Keywords: adhesion energy, inverted blister-test, ferritic stainless steels, titanium addition, niobium addition, chromia scale

Oxide microstructure and deuterium ingress in Zr-2.5Nb CANDU® pressure tubes

O.T. Woo1, Y.P. Lin2,3 and D. Khatamian1

1Atomic Energy of Canada Limited, Chalk River Laboratories, Chalk River, Ontario K0J 1J0

2Kinectrics, 800 Kipling Avenue, Toronto, Ontario M8Z 6C4, Canada

3Present address: GE Nuclear Energy/Global Nuclear Fuel, Wilmington, NC 28402, USA

The microstructures of oxides grown on the inside surface (water-side) of Zr–2.5Nb pressure tubes removed from CANDU®* reactors were characterised by TEM and correlated with deuterium ingress. Oxide cross-sections consisted of two structurally distinct regions: a columnar oxide region next to the metal/oxide interface, and an outer coarse equiaxed oxide region. Near the metal/oxide interface, the microstructure consisted of columnar grains without overt porosity. Away from the interface, the oxide consisted of coarsened equiaxed grains with (100)m twins, grain-boundary cracks and nanopores. The oxide microstructures on various pressure tubes differ in the proportion of equiaxed grains. Electron micrographs suggest that a larger proportion of equiaxed grains is associated with higher deuterium uptake. The predominance of grain-boundary cracks in equiaxed oxides indicates that they are likely more permeable to water than columnar oxides. Energy dispersive X-ray analyses revealed substantial amounts of Fe–K_ (and Mn–K_) in the equiaxed oxide grains at the outermost surface. Energy Dispersive X-ray mapping of Fe–K_ and detection of the Mn-K_ (produced by neutron activation of 54Fe and subsequent decay of 55Fe) in the absence of external excitation, unequivocally demonstrated that the iron had accumulated in the oxide during reactor operation. The Fe concentration was highest near the outermost region, and decreased inwards towards the metal/oxide interface. These results are consistent with permeable equiaxed oxides picking up considerable amounts of Fe at the outermost region from the heavy water coolant.

Keywords: irradiated oxide microstructure, deuterium ingress, Zr-2.5 Nb

Microstructural characterization of the failures of thermal barrier coatings on Ni-base superalloys

N. M. Yanar, G. Kim1, S. Hamano2, F. S. Pettit and G. H. Meier

Department of Materials Science and Engineering, 848 Benedum Hall, University of Pittsburgh, Pittsburgh, PA 15261, USA

1Permanent Address: Department of Materials Engineering, Chungnam National University, Daeduk Science Town, Taejon 305-764, Korea

2Permanent Address: Special Steel Research Dept., Daido Steel Co. Ltd., 2-30 Daidocho, Minami-ku, Nagoya, 457-8545, Japan

Typical thermal barrier coating (TBC) systems consist of a nickel-base superalloy substrate coated with a MCrAlY or diffusion aluminide bond coat, onto which is deposited a yttria-stabilized zirconia (YSZ) TBC. The bond coats are usually deposited via diffusion aluminizing processes or low pressure plasma spray processes (LPPS). The YSZ can be deposited by air plasma spraying (APS) or electron beam physical vapor deposition (EBPVD). A layer of thermally-grown oxide (TGO), which is usually alumina, forms between the bond coat and YSZ during TBC deposition and subsequent high-temperature exposure. The conventional wisdom is that APS coatings tend to fail in the YSZ and that EBPVD coatings tend to fail at the interface between the TGO and bond coat. However, current research has shown that the situation is much more complex and that the actual fracture path can be a function of the type of bond coat, the type of high-temperature exposure, and coating process parameters. This paper describes the results of a study of the failure of state-of-the-art EBPVD TBCs deposited on NiCoCrAlY and platinum-modified diffusion aluminide bond coats. The failure times and fracture morphology are described as a function of bond coat type. The failure times were found to be a strong function of temperature for both bond coats. The failure for NiCoCrAlY bond coats was found to initiate at defects in the coating, particularly at the TGO/YSZ interface, but the fracture propagated primarily along the TGO–bond coat interface. The failure times and morphologies for platinum-modified diffusion aluminide bond coats depended strongly on bond coat surface preparation. The mechanisms for failure of the two bond coats are described. Also, the effects of modifications to the bond coats and variations in processing parameters on these mechanisms are presented.

Keywords: thermal barrier coatings, aluminide band coats, aircraft gas turbines