Materials at High Temperature Vol 21, Issue 4, 2004

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Moisture-induced instability at the Al2O3/ Ni3Al(110) interface: interfacial chemistry

F. Qin, N. P. Magtoto and J. A. Kelber*
Department of Chemistry, University of North Texas, Denton, TX 76203, USA

ABSTRACT

STM, XPS, AES and LEED have been used to characterize the growth of ~ 1 nm thick Al2O3 films on Ni3Al(110), and on Ni3Al(111), and their reactivity towards H2O pressures ( PH2O) >10-6 Torr, 300 K. As-grown films on Ni3Al(110) exhibit a true oxide layer with an incommensurate interface involving both aluminum and oxygen atoms in intermediate oxidation states. Upon exposure to PH2O > 10-5 Torr, this oxide film undergoes severe reorganization and reconstruction, and the eventual loss of all long-range order. This effect is specific to H2O; similar exposures to O2 have no effect. STM measurements indicate that this effect is pressure- rather than exposure- dependent, indicating that oxide reconstruction is due to a cooperative surface reaction involving multiple H2O molecules at a single surface site. This effect is not correlated with surface hydroxylation, observed only for PH2O 2 > 1 Torr, in common with other alumina surfaces. The H2O-induced instability is also observed for the Al2O3/Ni3Al(111) films of similar thickness and composition but at much longer exposure time at a given water vapor pressure. The results suggest that the incommensurate nature of the Al2O3/Ni3Al(110) interface is important in the moisture-induced spallation of alumina scales.

*To whom Correspondence should be addressed. E-mail address: Kelber@unt.edu
Keywords: aluminum oxide; scanning tunneling microscopy (STM); water; X-ray photoelectron spectroscopy (XPS); Auger electron spectroscopy (AES); oxidation; adhesion; low energy electron diffraction (LEED)

Factors affecting the change of subgrain size of 15Mo3 steel during creep

János Ginsztler1, Soo Woo Nam2 and Peter J. Szabó1

1Budapest University of Technology and Economics, Dept. Materials Science and Engineering, Budapest, Hungary
2Korea Advanced Institute of Science and Technology, Dept. Materials Science and Engineering, Daejeon, Korea

ABSTRACT

15Mo3 type steel samples were subjected to creep loading at 550 C in order to investigate the development of microstructure and the effect of the carbides on the creep process. Electron microscopic observations have shown that relatively small subgrains are formed up to the secondary stage of creep deformation. As the creep progresses, due to the growth of carbides, the average distance between the carbides is increased. This increased the mean free path of moving dislocations, in turn the size of the subgrain is also increased. This microstructural change is also verified by EBSD technique which shows the variation of the configuration of grain boundaries.

Keywords: creep, carbides, TEM, EBSD, internal strain

Fe-base alloys in CH4/H2 carburizing gas mixtures
Ruchuan Yin

Materials and Corrosion Section, Sabic Technology Center, P.O. Box # 11669, Jubail Industrial City 31961, Saudi Arabia,
Email: ruchuany@sabic.com; yinruchuan@yahoo.com

ABSTRACT

This study involved cyclic carburization exposures of three Fe-base alloys (310SS, 800HT, and 556) at 800°C in 2% CH4/H2 and at 1,100°C in 10% CH4/H2 carburizing gas mixtures for 500h. Carburization thermodynamics, kinetics, and carburization behavior are discussed. The exposure at 800°C in 2% CH4/H2 was mixed oxidizing/carburizing, but reducing carburizing at 1,100°C in 10% CH4/H2. At 800°C Fe-base alloys suffered external carburization and oxidation, and the external Cr-rich scale layers were continuous consisting of oxides and carbides. Carburization resistance primarily depends on the protection afforded by external continuous layers rather than Ni content. At 1,100ºC, extensive external carburization occurred, and external layers became discontinuous consisting of Cr/Fe-carbides or metallic CrFe phases. Carburization resistance depends greatly on Ni (Co) content.
Keywords: Fe-base alloy, carburization, carbide, reducing carburizing