Materials at High Temperature Vol 16, Issue 2, 1999
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The role of HCl-impurities in the environment and Y for the high temperature corrosion of CoNiCrAl(Y)-coatings on a Ni-base-superalloy under complex loading conditions
A. Jeutter and M. Schütze
Karl-Winnacker-Institut der DECHEMA e.V., Theodor-Heuss-Allee 25, D-60486 Frankfurt am Main, Germany
The influence of small concentrations of HCl in synthetic air on the high-temperature corrosion of an Y-free and an Y-modified CoNiCrAl-coating on the directly solidified Ni-base superalloy CM247LC DS was investigated at 1,000°C under isothermal and thermocyclic conditions, accompanied by acoustic emission measurements. Furthermore, thermomechanical fatigue tests were carried out. Both coatings formed an Al2O3-layer. Y had a positive influence on the scale adherence and changed the growth direction of the Al2O3-scale. Y-modified coatings showed substantial internal oxidation. Pore formation at the oxide/metal interface took place in the case of the Y-free coating during oxidation in synthetic air as well during oxidation in HCl-containing atmospheres leading to a decrease of scale adherance. No crack closure by oxidation was observed under thermomechanical fatigue conditions in the case of Y-modified coatings on the alloy CM247LC DS. For the Y-free coating in HCl-containing atmospheres scale damage could be detected by accoustic emission measurements even under isothermal oxidation.
Keywords: oxidation resistance, CoNiCrAlY-coatings, HCl-impurities, cyclic conditions
The effect of thermal ageing and mechanical exposure on low cycle creep-fatigue strength of 316 steel at 625°C
R. P. Skelton1 and C. A. P. Horton2
1Department of Mechanical Engineering, Imperial College, Exhibition Road, London SW7 2BX, UK
2Department of Computing & Information, Royal Marsden Hospital, Sutton, Surrey SM2 5PT, UK
Comparisons have been made on evolutionary and stabilised cyclic stress-strain behaviour following different prior ageing treatments in a 316 steel. Prior stress-free ageing at a temperature higher than the test temperature reduces the ability of this steel to cyclically harden and consequently the material does not achieve maximum strength. An apparent saturation of cyclic hardening at room temperature can however, be continued by further cyclic hardening at 625°C. The rate of precipitation during low cycle fatigue can be two orders of magnitude greater than that occurring during stress-free ageing at 625°C. Quantitative measurements based on an extraction technique are presented and compared with particle size measurements in the literature. Prior high temperature low cycle fatigue exposure has been shown to reduce subsequent creep rates with a concomitant reduction in ductility: the same effect is found by prestrain at room temperature. Neither cyclic stress-strain properties nor creep behaviour can be fully restored by annealing after such prior deformation has taken place. On the other hand, once the material is fully cyclically hardened, stress-free ageing appears to have little further effect on properties. Other experiments on prestrain effects are reviewed and equations derived for their prediction.
Keywords: stress-free/fatigue-induced precipitation, particle diameter, cyclic stress-strain, solution treating, prior cold/hot work, ductility
Mechanical properties and fracture behavior of interfacial alumina scales on plasma-sprayed thermal barrier coatings
J. A. Haynes1,2, M. K. Ferber1,W. D. Porter1 and E. D. Rigney2
1High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6063, USA
2Department of Materials and Mechanical Engineering, University of Alabama at Birmingham, Birmingham, AL 35923, USA
The mechanical properties and fracture behavior of Y-doped Al2O3 scales were investigated by furnace thermal cycling (to 1,150°C) of plasma-sprayed thermal barrier coatings (TBCs) with vacuum plasma sprayed (VPS) or air plasma-sprayed (APS) Ni–22Cr–10Al–1Y bond coatings. No significant alterations in Al2O3 hardness or Young’s modulus (as measured by mechanical properties microprobe) were detected as a function of bond coat type, exposure time, or number of thermal cycles. The interfacial Al2O3 scales on VPS NiCrAlY exhibited progressive increases in localized fracture, buckling, and delamination during thermal cycling. The concentration of arrayed lenticular voids in the columnar Al2O3 grain boundaries significantly increased during cyclic oxidation (as compared to isothermal oxidation), but only in scales which formed on convex surfaces, suggesting internal void growth was stress-related. The amount and frequency of scale damage was higher on convex surfaces with a relatively large radius of curvature as compared to convex surfaces with a very small radius of curvature. Although the thermo-mechanical fracture resistance of Al2O3 scales on APS NiCrAlY was superior to scales on VPS NiCrAlY, TBC lifetimes on VPS NiCrAlY were greater by a factor of 2. Apparently, severe interfacial scale damage did not rapidly degrade the adherence of the ceramic top coatings.
Keywords: TBCs, thermal barrier coatings, alumina scales, alumina fracture, plasma-sprayed coatings
Mode II creep crack initiation in 316 LN stainless steel: experiments and modelling
D. Poquillon1, M-T. Cabrillat1 and A. Pineau2
1CEA Cadarache. DER/SERSI/LECC Bat 212 13108 St-Paul-lez-Durance Cedex, France
2UMR CNRS 7633. Centre des Matériaux. Ecole des Mines de Paris BP87 91003 Evry Cedex, France
This study deals with the understanding of creep crack initiation and creep crack growth in 316 LN austenitic stainless steel. To evaluate the respective role of the creep strain and that of the main principal stress on intergranular creep damage, original creep experiments were performed on sharp notched thin wall tubular specimens in 316 LN steel tested in torsion at 600°C. The specimens contained a sharply notch perpendicular to their axis. This resulted in overall Mode II loading. In the first part of the paper, the results of four experiments are reported, then, in the second part, a damage model identified previously and based on quantitative observations of grain boundary damage is used to simulate creep crack initiation and creep crack growth. Good agreement between the experimental results and the numerical simulations is found.
Keywords: creep crack initiation, 316 LN austenitic stainless steel