Materials at High Temperature Vol 21, Issue 2, 2004
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The proposal of Q* parameter and derivation of the law of creep crack growth life for a round bar specimen with a circular notch for Cr–Mo–V steel
T. Adachi1, A. T. Yokobori, Jr2, M. Tabuchi3, A. Fuji4, T. Yokobori5 and K. Nikbin6
1Faculty of Science and Engineering, Ishinomaki Senshu University, Shinmito, Minamisakai, Ishinomaki City #986-8580, Japan
2Fracture Research Institute, Graduate School of Engineering, Tohoku University, Aoba, Aramaki, Aobaku, Sendai City #980-8579, Japan
3National Institute for Materials Science, Independent Administrative Institution, 1-2-1 Sengen, Tsukuba City #305-0047, Japan
4Metallurgy Department, Research Institute, Ishikawajima-Harima Heavy Industries Co. Ltd, 3-1-15 Toyosu, Koto, Tokyo #135-0061, Japan
5School of Engineering and Science, Teikyo University, 1-1 Toyosatodai, Utsunomiya City #320-8551, Japan
6Department of Mechanical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK
Using a round bar circular notched specimen for Cr–Mo–V steel and the proposed equation predicting crack length on the basis of electric potential drop method, the creep crack growth tests were previously conducted to investigate the effect of multi-axial stress on creep crack growth rate(CCGR). In this paper, more detailed experiments and analyses on the creep crack growth were conducted and the Q* parameter which characterizes CCGR for this circular notched specimen was derived. Furthermore, using the Q* parameter, the prediction law of creep crack growth life was also derived.
Keywords: Q* parameter, creep crack growth rate, creep crack growth life, round bar specimen, circular notch, electric potential drop method, multi-axial stress
The general isothermal oxidation behavior of Cu–8Cr–4Nb
Linus U.J.T. Ogbuji
QSS, Inc., NASA Glenn Research Center, Cleveland OH 44135, USA
E-mail: thomas-ogbuji@grc.nasa.gov
Oxidation kinetics and oxide morphologies of Cu–8Cr–4Nb (‘GRCop-84’) were investigated by TGA and microscopy/microanalysis, and compared to those of ‘NARloy-Z’ (a Cu–Ag–Zr Cu alloy) between 500 and 900°C. NARloy-Z and Cu had identical oxidation behavior; but Cu–8Cr–4Nb differed strongly, with slower kinetics below ~700°C and a markedly complex oxidation behavior at all temperatures. Explanations are offered for Cu–8Cr–4Nb oxidation behavior, in the various temperature regimes, in terms of oxide types and their morphologies, with emphasis on a kinetic control at intermediate temperatures by a reservoir effect.
Keywords: GRCop-84, NARloy-Z, Cu, oxidation kinetics, oxides, reservoir effect
Comparison of thermal expansion and oxidation behavior of various high-temperature coating materials and superalloys
J. A. Haynes, B. A. Pint, W. D. Porter and I. G. Wright
Metals and Ceramics Division, Oak Ridge National Laboratory, USA
The thermal expansion mismatch between a metallic substrate and its external oxide scale generates a strain on cooling that is a primary cause of spallation of protective oxide scales. This study compares thermal expansion behavior and cyclic oxidation performance of the two major composition classes of high-temperature commercial coatings for protection of single-crystal superalloys. The thermal expansion of cast MCrAlY (M = Ni and/or Co) alloys and cast aluminides (NiAl, (Ni,Pt)Al and Ni3Al) was measured at temperatures up to 1300°C and compared to that of a single-crystal Ni-base superalloy. The tendency for scale spallation from each alloy was evaluated by cyclic oxidation testing at 1150°C. The coefficients of thermal expansion for the aluminides were lower than those of the MCrAlY-based alloys at all temperatures and scale adherence to the Hf-doped aluminides was generally superior. Scale adherence to the various compositions of MCrAlY-type alloys did not directly correlate to their thermal expansion behavior or substrate strength. For both types of materials, the presence of a reactive element (Y,Hf, etc.) had no detectable effect on thermal expansion but a major effect on scale adherence. There was no obvious influence of Al content on the thermal expansion of â phase Ni–Al compositions. The addition of Pt resulted in a lower average thermal expansion for hyperstoichiometric (Ni,Pt)Al at temperatures above 930°C, but this effect was not observed in hypostoichiometric (Ni,Pt)Al.
Keywords: thermal expansion, oxidation, coatings, superalloys
Progress in life time modeling of APS-TBC Part I: residual, thermal and growth stresses including the role of thermal fatigue
D. Renusch, H. Echsler and M. Schütze
Karl-Winnacker-Institut der DECHEMA e.V., D-60486 Frankfurt am Main, Germany
For about the last two decades there has been an effort to produce a reasonable life time model for the thermal barrier coating systems (TBC) that are used in the gas turbine industry. However, recent advances in testing technology, namely acoustic emission (AE) analysis and Raman spectroscopy, have provided many new insights into the life of TBCs. This new technology used in conjunction with more traditional testing, such as the four point bend mechanical test, has provided much needed data for the development of a life time model. Part I of this paper is devoted to the stress situation that exists in isothermally and cyclically oxidized TBC systems. The focus of the current TBC research is on the air plasma sprayed (APS) systems. However, some results for the electron beam physical vapor deposited systems (EB–PVD) are presented for the purpose of providing insight into the role of interface roughness and the stresses in the thermally grown oxide (TGO). AE analysis of isothermally and cyclically oxidized samples is used to address the role of “desk top failure” and of thermal fatigue. The possible role of intrinsic growth stress is addressed by presenting the measurement data from the oxidation of freestanding NiCoCrAlY foil. Findings are supported by thermal stress calculations and micrographs. Part II of this paper is devoted to the development of a life time model. Here TBC top coat failure and life time is considered as a two-step process, where step 1 is time to delamination macro-cracking and step 2 is time to through macro-cracking. This two-step failure mechanism comes directly from critical strain measurement data and traditional coating failure theory. Included in the model is damage accumulation due to bond coat oxidation and due to thermal fatigue. The damage terms in the model have their origins in AE data from cyclic oxidation samples. The life time model is then used in concert with measured micro-crack lengths. Finally, a full model is presented for isothermal and cyclic oxidation of APS–TBC.
Keywords: life time modeling; residual stress; thermal stress; growth stress; thermal fatigue; acoustic emission
Progress in life time modeling of APS-TBC Part II: critical strains, macro-cracking, and thermal fatigue
D. Renusch, H. Echsler and M. Schütze
Karl-Winnacker-Institut der DECHEMA e.V., D-60486 Frankfurt am Main, Germany
Here TBC top coat failure and life time are considered as a two-step process, where step 1 is time to delamination macro-cracking and step 2 is time to through macro-cracking. This two-step failure mechanism comes directly from critical strain measurement data and traditional coating failure theory. The critical strain data was measured by using a four point bend test on preoxidized samples. Included in the model is damage accumulation due to bond coat oxidation and thermal fatigue. The damage terms in the model have their origins in acoustic emission data from cyclic oxidation samples. The life time model is then used in concert with measured thermally grown oxide micro-crack lengths from isothermal oxidation samples. Finally a full model is presented for isothermal and cyclic oxidation of APS–TBC.
Keywords: life time modeling; critical strains; macro-cracking; micro-cracking; damage accumulation; acoustic emission
Carburization behaviour of Haynes 556 exposed to CH4/H2 gas mixtures
Ruchuan Yin*
Materials and Corrosion Section, Sabic Technology Center, P.O. Box # 11425 Jubail Industrial City 31961, Saudi Arabia
Email: ruchuany@sabic.com or yinruchuan@yahoo.com
The carburization behaviour of Haynes 556 has been studied after cyclic and isothermal exposures to CH4/H2 carburizing gas mixtures at high temperatures for 500h exposures. A thermodynamic analysis indicated that 1000°C was an approximate critical temperature, below which the environment should result in mixed oxidizing/carburizing behaviour, while above this temperature reducing carburizing behaviour should occur. The experimental results agree with the thermodynamic analysis. Below 1000°C Haynes 556 suffered external carburization and oxidation, typically at 800°C in 2%CH4/H2 where the external reaction products comprised Cr7C3 and Co3W3C (major phases), and (Co,Mn)(Mn,Co)2O4 and TaOx (minor phases). At temperatures in excess of 1000°C exclusive external carburization occurred, typically at 1100°C in 10%CH4/H2 resulting in the formation of Cr7C3 (major phase) and (Cr,Fe)7C3 (minor phase). Metal dusting was not experienced under highly carburizing conditions (ac >1) in this study. The morphology of an outer carbide layer is both temperature- and timedependent, while its continuity is more temperature-dependent rather than time-dependent.
Keywords: Haynes 556, carburization, reducing environment, continuity